An Electronics Hobbyist needs to know the symbols of the electronic components in the Circuit diagram. Before starting the construction of a circuit, it is necessary to study the circuit diagram first. It has a number of symbols with the values marked. There are different symbols for representing the components. So it is necessary to study the symbols generally appearing in circuits.

The value of the components is directly represented along with the symbol. For example resistor values are represented as K for Kilo Ohm and R or E for Ohms. Capacitors are represented as C , Diodes and LED as D, Variable resistors as VR, Thermister as TH, Coil as L, Transistors as T or Q, Relay as RL, Potentiometer as POT or P, Transformer as TR etc. Some components are represented using their names itself.

This is the Procedure before starting a Circuit construction

1. First study the circuit diagram well and identify the components and their values.

2. Make a list of components such as resistors, capacitors transistors etc with their value and type.

3. Enquire in the shop about the rare components in the circuit. If they are available, the other components can be purchased.

4. If you are going to construct the circuit on Bread board or Common PCB, first connect resistors, then capacitors, ICs, transistors etc.

5. Finally connect the power supply.

6. Touch the components to check whether there is any heating of components due to shorting. If so immediately disconnect the power supply and test the error.

Fig. 1 and 2 shows the symbols of the Electronic components and Fig.3 the circuit diagram of a Lamp chaser.

Fig.1

Fig.2

Fig.3

Sometimes you need a simple circuit to protect the Electronic gadgets like LCD TV from voltage fluctuations. Normal domestic power supply is 230 volts at 50Hz in most countries. But this voltage may not remain steady and may go as high as 250 volts or as low as 180 volts. Complaints in the distribution transformer, operation of heavy current appliances like flour mill, welding machine etc in the nearby places can cause severe voltage fluctuations in the lines. A good stabilizer with Buck / Boost facility can solve the problem, but still it is interesting to make this circuit and it costs less than Rs.100.It cut off the power supply to the gadget connected when the voltage increases above 230 volts and decreases below 200 volts.

You need the following components
1. 0-12 volt 500 Milli Ampere transformer- 1No
2. IN4007 Diodes- 2 Nos
3. 470 uF 25 volt Electrolytic capacitor-1No.
4. 10 uF 25 volt Electrolytic capacitor-1No.
5. 10 K Pot / Preset – 2 Nos
6. LED Red – 5 mm -1No
7. LED Green-5 mm -1No
8. 1K ¼ Watt resistors – 4 Nos
9. 470 Ohms ¼ Watt resistor – 2 Nos
10. BC 548 Transistor – 2 Nos
11. 12 volt 200 Ohms, 2 Amps Relay – 1 No.
12. 2 Amps Fuse – 1 No.
13. Common PCB- Small Size-1No
14. Connecting Wires – 1mm gauge electric wire, Thin plastic wires.
15. LED Holder – 5 mm- 2 Nos
16. Plastic Switch box with 3 Pin Socket provision- 1No
Circuit working

Power supply to the circuit is derived from a 0-12 volt 500 mA transformer. Note that, only a half wave rectification using Diode D1 is done which is essential to achieve the result. That means, when the input voltage in the transformer Primary increases or decreases, a corresponding change occurs in the Secondary also which reflects in the circuit power supply. Capacitor C1 smoothes the power supply from ripples. Green LED indicates the Power On status. Circuit works on the basis of the switching action of T1 and T2. When the voltage level is normal (200 – 230 Volts), Transistor T1 remains off (as adjusted by VR1). This allows T2 to conducts because, it gets base bias through R4. The relay then triggers and connects the power to the load.

When the voltage rises above 230 volts, more current goes to the base of T1 through R2 and VR1which switches on T1. Since the base of T2 is connected to the Collector of T1, it goes to the ground potential and T2 turns off. Relay then turns off and power to the load cut off.

When the voltage decreases below 200 volts (as adjusted by VR2), both T1 and T2 turns off and relay cut off to break the power to the load.

How to set

You need a Variable AC power supply for precise high and low voltage settings. Don’t worry you can make a crude setting. Before connecting the Phase and Neutral lines adjust VR1 and VR2 to set the voltage. Assuming that the domestic supply is between 200-230 volts, just turn VR1 just the Relay and Red LED turns On. So when the voltage rises above this level , relay will turns off. Similarly, adjust VR2 also till the relay and Red LED turns On.
If a variable power supply is available, provide 240 volts in the transformer primary and adjust VR1 till relay and Red LED turns off. Then provide 180 volts and adjust VR2 till Relay and Red LED turns off.

Connections

Phase line from the Primary of transformer should go to the Common point of Relay. From the NO (Normally Open) contact of the relay, solder a wire which should go to the Right pin of Socket. Take the Neutral wire from the second primary wire of transformer which should go directly to the Left pin of socket. So when the relay triggers circuit completes. Ground can be connected to the Screw of Transformer.

Relay

You can use a 12 volt PCB relay to make the unit compact. It has 5 pins. On one side ,there are three pins. Middle pin is Common for connecting AC phase. The pins on either side of it are DC connections. You can connect either way round because it is the coil connections. Two pins on the other side are NC (Normally Connected- To the Common pin) and NO (Normally Open- Connects with Common pin when Relay energize. You can easily identify the NC and NO pins through Continuity test using Multimeter. Connect one probe to the Common pin and touch the other ones with second probe. The pin that gives continuity is NC. So use the other NO pin to connect with the Socket.

12 Volt Relay Pins

Casing
If you have an old Stabilizer case, it will give enough space to enclose the circuit and transformer. Moreover, you will get AC cord and Socket attached to it.

Various kinds of Lightning arrester circuits are available, but here describes one with Multi-level protection so any one of the component may be damaged if a strong lightning occurs. Thus it leaves the gadget without damage. Usually one Surge protector like MOV, GDT, Bidirectional Diode etc is used in surge protector circuit. But here a combination of all the three is used to give multi level protection. The circuit can be hooked up in Telephones, Cable TV lines, Modem, Dish antenna cable etc.

The circuit is straight forward. The input is the incoming lines of Telephone or cable Modem. Output goes to the Telephone instrument or Cable modem. In between the lines, there are components like a 2 Amps fuse, a 2 KV capacitor, GDT, MOV and a VDR. So each component can give protection from surge from lightning.

Lightning Arrester Circuit

Components Description

680 pF 2KV Capacitor

It is high voltage ceramic capacitor that protects the instrument from very high intensity spikes.

680 pF 2 KV Ceramic Capacitor

GDT
GDT or Gas Discharge Tube, is similar to MOV (Metal Oxide Varistor) in function. It diverts the extra current from the hot line to the ground using an inert gas as the conductor between the two lines. It can be incorporated between the two lines of Telephone or between the Core (Central conductor) and the Shield (Copper clad aluminium braid shield conductor) of the coaxial cable.

GDR       H230 LB

GDT functions as an on/off switch with high impedance in the normal voltage conditions. Under surge condition, its internal gas ionizes to turn the device to a low impedance state called “Arc mode or Short circuit”. In this state the GDT conducts the high voltage transients from hot line to the ground. Once the transient passes to the ground, the GDT returns to its high impedance state.

When the voltage through the cable is at a certain level, GDT will not conduct. When the voltage increases above this normal level due to a surge, the electrical power is strong enough to ionize the gas inside the GDT which act as a conductor to pass the high current to the ground until the voltage returns to the normal level. Again the GDT becomes a poor conductor.

MOV

Metal Oxide Varistor (MOV) is a Kind of VDR (Voltage Dependent Resistor) that contains a ceramic mass of Zinc oxide grains, in a matrix of other metal oxides such as small amounts of bismuth, cobalt, manganese etc. sandwiched between two metal plates which forms the electrodes. The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks.

MOV          431 KD 14

down due to a combination of thermionic emission and electron tunnelling, and a large current flows. Varistor can absorb part of a surge. The effect depends on the equipment and details of the selected Varistor.

The Varistor remains non-conductive as a shunt mode device during normal operation when voltage remains well below its “clamping voltage”. If a transient pulse is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. This will blow up the Fuse and protect the device.

TPA 200

It is a Bidirectional Diode from Trisil which is generally used in Telephones for Bidirectional Crowbar Protection. Its voltage range is from 62 V to 270V. It is the common lightning arrestor device used in telephones which protects sensitive components in the circuit from transients voltages induced in lines from lightning.

TPA 200 Bidirectional Diode

Fuse Clip

2 Amps Fuse           10 Ohms 1 Watt Resistor

Assemble the circuit on a common PCB or on a etched PCB and enclose inside the device. Alternately it can be enclosed in a small box with suitable telephone sockets and can be fixed outside.

Data Sheets
GDT 230 LB DATA SHEET
TPA200 DATA SHEET

You can use a Solar panel to power electrical appliances directly during day time if the Sunlight is very good, direct and continuous. So you have to install a Solar panel in a well lit area over the roof top in the East- West direction with a slight angle facing the Solar panel to the sky. So when the solar panel gets sunlight, it generates voltage and current which flows through the cables connected to it. You can have two options. The current can be stored in a battery to power the appliances at night through an Inverter or you can directly use the solar power using an Inverter without battery. Solar power is DC voltage and the Inverter converts this DC into AC to power the AC appliances. So let us see the requirements.

What Solar panel you needs?
A 12 volt solar panel will give an “Open circuit voltage” of around 20 volts in bright sunlight. But it drops to 12-15 volts when connected to a load. No problem, a 12 volt Solar panel is sufficient. But you have to consider its “Wattage” which is the power delivered by the panel.
Suppose you need to run 100 Watts appliances total for three hours every day.
Now multiply the total watts using with the hours to run = 100 x 3 = 300 Watts.
So you needs total 300 Watts for 3 hours.
Suppose the Sunlight is full only 6 hours in a day. Then divide the total watts with the hours of sunlight.
300 Watts / 6 Hours = 50 Watts. That is ,the panel should give 50 Watts per hour.
So you need a 50 Watts solar panel for your needs.
Now select a 12 volt 50 Watts solar panel, so that you can run the 100 Watts appliances for 3 hours daily with Solar power directly. If you use a 100 Watts solar panel, then you can increase the usage hours to 6 hours.
What type of Inverter you need?
Inverter is an electronic device that converts DC (Direct Current) to AC (Alternating Current). This DC can be from the Solar panel or a Battery. Most of the Inverters are 12 volt types and its power rating is in “VA (Volt Ampere)”. That means, the Voltage using x Current (Ampere) use of the Appliance. Inverters like 300 VA, 500 VA, 800 VA, 1KV, 2 KV, 5 KV etc are available. But we need only a small type since the “Wattage of load “is only “100 Watts”. So you can select a “300 VA Inverter”.
How to connect the Solar panel, Inverter and Load?

Solar panel is just like a battery that generates current when sunlight falls on its. Numerous “semiconductor stripes” are on the panel and each unit is called a “Solar Cell”. As a rule, each “10 Watts” solar panel gives “1 Ampere “current. But it depends on the sunlight. The Solar panel has two “terminals” marked as + and –. Connect a good cable to the positive and negative terminals and note the wires for the polarity. If two coloured wires are used, it is easy to identify the polarity. It is better to use a high gauge wire to handle current. You can use 1 mm electric wire. Place the Solar panel on the roof top in the East – West direction at an angle with the panel facing the sky. So all time, the panel gets light when the sun moves from east to west.

Now bring the Cable to the house where Inverter is placed. The Inverter has Inputs for solar panel and output for the load. Connect the cables to the inputs of Inverter observing the polarity. It also has + and – markings near the connectors. Load should be connected to the output. Since it is AC sometimes no marking is present but usually L and N markings will be present. The loads must be Bulbs, Fan, TV, etc, but maximum 100 Watts. Here you can see the wattage of some appliances.
CFL – 5,8,11,18,22,36 Watts.
Bulbs- 15, 40, 60,100 Watts.
Fan – 80-120 watts
TV – LCD 40-100 Watts, CRT- 100-300 Watts.
Cost of the unit – Maximum rates in India
Solar panel- Generally Rs.100 maximum per Watts. So 50 Watts Panel costs Rs.5000
Inverter – 300 VA- Rs.4000
Cable – For 10 + 10 meters – Rs. 200
So the Total cost will be below Rs.9500
If battery is also used, cost will hike.
45 AH Car Battery – Rs. 3500
100 AH Tubular battery – Rs. 12,000
150 AH Tubular battery – Rs. 14,000
Maintenance
Solar panel needs periodic maintenance to increase its life and efficiency. Usually a Solar panel will work well up to 20 years. So once in a month, clean the surface of solar panel with moist cloth and clean the terminals to remove rust and reconnect the wires with clean tips.

One of the main problems in Solar Inverter system is poor charging of Battery during rainy seasons and in cloudy days. The high current Tubular battery requires more than 1 Ampere current for proper charging. To solve this problem, I have designed a” Hybrid Solar charger” and the circuit was published in Electronics for You magazine September 2013 issue. So the charger has two sides. One side gets power from Solar panel and the other side from a Step-down transformer. If the voltage from Solar panel reduces below 9 volts, the charger shifts to AC mode and battery charges via the current from transformer.

You can see the working details of the circuit in the pdf file given. Normally the 12 volt 100 Ah Tubular battery will last around 5 years if the battery is properly charged and discharged. That means both the charging and discharging cycles maintain the life of  battery. Otherwise, the cells of the battery becomes weak and Suphation occurs which damage the cells permanently. Suphation is the accumulation of Sulphur deposits on the lead plates, which prevents the release and acceptance of charge electrons. In ordinary Lead-Acid battery, this is very high but in Tubular battery it is less. Still it needs proper charging.

So the Hybrid Solar charger keeps the Inverter battery in top condition so that you can keep the battery unattended.
HYBRID SOLAR CHARGER

This is my latest Circuit published in the June 2015 issue of Electronics For You Magazine. It is a device that measures the intensity of Radiation in the room. Lot of Electromagnetic radiations are present in home emitting from WiFi, Mobile phone, Computer and other electronic devices. So the device shows the level of RF energy in a meter. Meter shows full deflection when the RF radiation is very high. It also gives a simultaneous LED indication.

See the working of the circuit
Radio Frequency Detector

You can easily make this low cost LED light for your Garden. It automatically turns on in the evening and turns off in the morning. It uses 1 watt White LED so sufficient light will be available from the lamp. The circuit uses a 6 Volt 300 mA Solar panel to charge a 4 volt 1 A h battery.

In bright sunlight, the 6 Volt 300 mA solar panel gives 300 mA current for fast charging the battery so that the battery will be fully charged. The 1 watt White LED requires 3.5 to 4 volts and around 200 mA current for full brightness. The automatic switching of the lamp is done through a PNP transistor BD 140 which drives the LED. So here the Solar panel itself acts like an LDR for automatic working of the circuit.

During day time, battery charges from the Solar panel through the forward biased diode D1. At the same time, the base of T1 turns high by the current through R1 and it remains off since it is a PNP transistor. LED thus remains off during day time. When the light falling on the solar panel reduces in the evening, battery charging stops and current to the base of T1 ceases. T1 then conducts to turn on the White LED.

Assemble the circuit on a common PCB and enclose in a water proof transparent box or glass case. Solar panel should be fixed on the top of the box to get direct sunlight. LED should be fixed so as to direct the light towards the side or bottom.

Components used in the circuit

You can make a simple Current regulated Charger for your Solar Panel. It safely charges the 12 volt battery with a constant current of 540 Milli Amperes. It needs only a few components and can be hooked between the Solar panel and the battery. It also has an LED charge indicator and a revere polarity protection diode.

The circuit is designed for 12 volt 10 Watts Solar panel. In bright sunlight, the 12 Volt Solar panel gives an Open circuit voltage up to 20 volts but it drops to 14-16 volts when connected to the battery. For a 12 volt Lead Acid battery, we need more than 14 volts for charging. Usually this voltage will be available only during the bright sunlight hours between 11 am and 3 .pm. The current also varies proportional to the voltage from the solar panel. The 10 Watt Solar panel gives the following current at different voltages theoretically.

12 V – 833 mA

14 V – 714 mA

16 V – 625 mA

18 V – 555 mA

20 V – 500 mA

So any way, the current from the solar panel swings between 500 mA and 800 mA. So we need a constant current charger to give stable current for charging. This circuit gives a constant of 540 Milli Ampere current for charging irrespective of the voltage changes.

What the circuit doing?

The circuit uses a medium power NPN transistor BD 139 to maintain a constant current through the battery. The positive line from the battery goes directly to the Battery + Ve through Diode D1 which gives polarity protection and also prevents the discharge of battery to the solar panel at night. The Negative terminal of the battery goes to the Collector of the NPN transistor. An LED is provided at the base of T1 to make the Base- Emitter junction voltage around 1.08 volt. Two 1 Ohm resistors R2 and R3 are connected serially from the emitter of T1 to the negative rail and give a total 2 Ohms resistance. So the current across the battery allowed by T1, R2 and R3 is
1.08 / 2 = 0.54 Amps = 0.54 x 1000 = 540 Milli Amps.

Anyway it gives 500 mA constant current if the Solar panel voltage lies between 12 volts and 20 volts.C1 acts as a buffer for the voltage from solar panel.

LED lights when the current passes through the battery and turns off when the charging completes and at night. If the battery is not charging due to damage or clips not properly connected, the LED will not light. So it acts as a good indicator for the flow of current through the battery. Now this is the course of current flow.

Solar panel Positive – DI – Battery + Ve – Battery – Ve- Collector of T1- Emitter of T1- R2 – R3 – Negative of Solar panel.

How to connect?

Assemble the circuit on a Common PCB and enclose in a small plastic case with a hole for LED. Connect the circuit with the Battery using Crocodile clips. You can Tap the battery voltage from the battery terminals to power inverter or charging devices such as Mobile phone.

Use 1 mm Copper wire to connect the Solar panel with the Battery.

Components used

Presenting here three circuits to make efficient Battery chargers. The 12 volt Lead Acid Car battery requires high current for fast charging. If we provide high current, the battery will charge within 1-2 hours fully. Usually a Lead Acid battery takes high current initially, which drops to 500 mA or less after 10-20 minutes. As a rule, it is advisable to charge the battery with a current that is 10 times lesser than the battery capacity. The Car battery is 32 Amps or 45 Amps so it requires 3.2 Ampere and 4.5 Ampere current respectively. So the charger should provide 14 volts with a current of 5 Amperes.

Circuit 1- Crude Battery charger

This Charger circuit is simple that provides “Raw DC” for charging. It is better to charge the Lead Acid battery with “Pulsed DC” so that the charging process will be efficient. The “Pulsating DC “ agitates the electrolyte so that electrons pass more efficiently. Usually a “Smoothing capacitor” is provided after the rectifier bridge to make “Clean DC”. Even though the diodes converts low volt AC to DC, the DC will have some fractions of AC. This DC is called as “Dirty DC”. So to remove the ripples from the DC, a high value “Electrolytic Capacitor” is used. Here the capacitor is omitted and the dirty DC is used for charging.

A 14-0-14, 5 Ampere “Step-Down transformer” converts 230 volt AC to a low volt AC around 18 volts. The 10 Ampere diodes D1 and D2 convert the low volt AC to DC around 16 volts which is used for charging. A 12volt Car bulb (one used in tail lamp) is provided in series with the positive line. This bulb restricts the charging current and also functions as a “Charging indicator”. The brightness of the bulb indicates how much charge is flowing into the battery. When the battery becomes fully charged, lamp turns off. If the lamp is staying on with full brightness for more than 30 minutes, it indicates that the battery is dead and is not accepting charge.

The Bulb is a very good indicator for charging status. It is connected in series with the positive output rail so that current flows through the bulb into the positive terminal of the battery. From the positive terminal, current passes through the battery chemistry into the negative terminal and then returns into the transformer.

The current flowing through the bulb depends on how much charge is using by the battery. The indications of bulb when connected to battery are
1. Bulb ON- Battery requires charging
2. Bulb OFF – Battery dead or clips not properly connected
3. Bulb glow brightly – Battery is discharged to around 50% and is charging
4. Bulb first brightly and then gradually dims and finally the filament appears red hot – Battery charging normally and finally charging completed.
5. Bulb stays ON after 30 minutes without reducing the brightness- Battery dead and not charging.

Circuit. 2. Battery Charger with Volt meter and Ammeter
The charger circuit is similar to the above one but a “Volt meter” and “Ammeter” are connected to get charging voltage and current. Ampere meter (Ammeter) is connected in series with the Positive line and the Volt meter is connected across the Positive and Negative lines.
You can use either Analogue meter or Digital Meter. Now Digital Volt and Ampere meter in one pack is available, so it will be more suitable and you can observe minute changes in voltage and current easily.

Circuit. 3. Voltage Regulated Charger

This charger can be used for charging NiCad, NimH, Li -Ion battery and also Mobile phone battery. Here a “Smoothing capacitor” (C1) is added to get “Clean DC”. A Voltage regulator IC (78XX) is provided to give stable voltage for charging. Output current from the IC is maximum 1 Ampere and voltage depends on the IC type. Capacitors C2 and C3 reduces noise and transients in the output.

78XX series IC s are Positive voltage regulators and 79XX series are Negative voltage regulators.

7805 – Output 5 V
7806 – Output 6 V
7808 – Output 8 V
7809 – Output 9 V
7812 – Output 12 V

Most of the CMOS circuits need a well regulated power supply for proper working. Regulated power supply is the DC source that gives constant Voltage and Current in its output irrespective of the input changes. Various types of power regulation methods are available which depends on the use and type of the circuit. Here explains three Regulated power supply designs.

The most simple type of Regulated power supply is the Zener regulated one. A 9 volt Step-down transformer is used to drop 230 volt AC to low volt AC which is rectified by the full wave rectifier comprising D1 through D4. The resulting DC is smoothed by capacitor C1. LED indicates the power on status. The 9 volt DC is regulated to 5 volt DC using a Zener diode rated 5.1 volt.

Zener is special kind of diode that breaks down and conducts current when the voltage increases above its rated voltage called “Break down voltage or Avalanche voltage”. For example, if the Zener voltage is 5.1, it conducts when the voltage into it increases above 5.1 volts. Then it gives a constant 5.1 V output till the input voltage reduces below 5.1 volts.

Zener requires a current limiting resistor to restrict the current through it. The value of resistor R can be calculated using the formula

R = Vin – Vz / Iz

Where V in is the input voltage, Vz output voltage and Iz current through the Zener.

In most circuits, Iz is kept as low as 5mA. If the supply voltage is 9 V, the voltage that is to be dropped across R to get 5 V output is 4 volts. If the maximum Zener current allowed is 30 mA, then R will pass the maximum desired output current plus 5 mA. That is 35 mA. So the value of R appears as

R = 9 – 5 / 35 mA = 4 / 35 x 1000 = 114 ohms

So the nearest available value 120 Ohms is selected.
Power rating of the Zener is also an important factor to be considered while selecting the Zener diode. According to the formula
P = IV.
P is the power in watts, I current in Amps and V, the voltage.

So the maximum power dissipation that can be allowed in a Zener is the Zener voltage multiplied by the current flowing through it. For example, if a 5.1V Zener passes 5.1 V DC and 35 mA current, its power dissipation will be 175 Milli Watt. So a Zener diode rated 400 mW is sufficient.

Circuit.2 .Regulated power supply using Voltage Regulator IC

Some what robust type of Regulated power supply is the one using Voltage regulated IC. There are two types of Voltage regulator IC. Positive voltage regulators comes in the series 78XX and Negative voltage regulators as 79XX series. They provide a constant output voltage based on their voltage rating. For example 7805 gives 5 volts, 7812 gives 12 volts etc. Its output voltage remains constant until the input voltage drops below its rated voltage. Regulator IC requires 2 or more volts higher than its rated voltage for proper working. For example 7805 requires 9 or 12 volts input and 7812 requires 14 volts ot more as input. Regulator IC gives maximum 1 Ampere current from its output.

The circuit uses 7805 Regulator IC to produce 5 volts DC from a 9 volt transformer power supply. Capacitor C2 and C3 removes transients and noise from the output voltage. Diode D5 protects the IC if a short circuit occurs in the output. This diode by passes the short circuited current and prevents it from entering into the IC.
Capacitor C4 acts as a buffer for the output voltage.

Circuit 3. High Current Regulated power supply.

The Zener regulated and IC regulated power supplies give only a current less than 1 Ampere. If high current is needed, a Transistor based circuit is ideal. Here a high current NPN transistor 2N 3055 is used to give maximum current from the transformer. Here a 14 volt 2 Ampere transformer is used. The 12 volt Zener diode at the base of T1 regulates the output voltage from T1 to 12 volts. The High power transistor passes maximum current from its Emitter. 2N3055 requires a heat sink, since it becomes hot when high current passes. TIP 3055 can also be used.

This is a simple but useful LED flasher that can be used as power on indicator or along with alarms. Simply you can use it as a Mock Alarm in the gate or door to baffle the intruders. Do this very simple circuit and it can be an evening project for you that take only 10 minutes to complete.

The circuit is a simple Relaxation oscillator using two complementary transistors BC 547 and BC 557. Capacitor C1 is doing the trick and it gives the positive feedback to start the oscillation of T1 and T2. R1 and C1 form the timing components of the oscillator and with a 330 K resistor (R1) and 10 uF capacitor (C1) the LED blinks around 1 Hz rate. By varying the value of either R1 or C1, the blinking rate of LED can be changed. That means, lower value gives fast blinking and higher value gives slow blinking.

What happens in the circuit is like this. When power is applied, C1 starts charging via R1. When C1 fully charges, T1 conducts and pulls the base of T2 to ground potential and it also conducts. The collector of T2 gives power to LED and it lights. At the same time C1 discharges through R3. So T1 and T2 turn off. This turns off LED. Again C1 starts charging and the cycle repeats making the LED blinking continuously.

Flashing LEDs are available now which needs only a current limiting resistor. If it is not available, you can use this circuit. Anyway, it is a simple circuit for the beginner to enjoy its fun.

Bread board assembly of Flasher Circuit

Smallest Flasher ever seen

Circuit build through point to point soldering

You can use any type LED. The 3 volt White and Blue LEDs also work well. Transparent LEDs give bright flashes than opaque LEDs. High bright LEDs like the one used in Traffic signals is the best  to give long range light.

Usually we use a battery level indicator along with rechargeable battery to monitor the voltage level. If the Lead Acid battery is deeply discharged below a particular voltage level ,Memory effect develops and the battery never attains full charge later. So it is necessary to monitor the safe voltage level in the battery. A 12 volt Lead Acid battery shows 13.8 volts in fully charged condition and it should not deep discharge below 9 volts. If it happens, immediate charging is required to maintain the health of  battery.

So the most simple battery level indicator is an LED based circuit that lights an LED when the battery voltage is above the set level, say 9 or 10 volts and below that ,the LED extinguishes. But this kind of circuits has a drawback. The LED is always on,so it consumes around 2 volts which is a waste of power and the battery voltage drops even if the load is not running. So here is the solution to prevent that. This circuit lights the LED only when the battery voltage is below 9 volts. The circuit consumes very little power in the standby mode and as long as the battery voltage is between 9 and 13 volts, LED remains off. When the battery voltage drops below 9 volts, LED lights indicating the need for charging. You can also add a buzzer if audible warning is required. The circuit can be attached to any 12 volt battery like Inverter battery, Car battery etc.

How the circuit works?

Two NPN transistors (T1 and T2) act as a voltage controlled switch for the LED. The transistor switch is controlled by a Zener diode. Zener diode is a special kind of diode that conducts only when it gets a voltage more than its rated voltage. For example, a 7.5 Volt Zener diode used in the circuit requires more than 8.5 volts for its smooth conduction. That means, the Zener requires + 1 or 1.5 volt excess than its rated value for proper conduction. Remember this when you select Zener value. For a 3.1 volt Zener, input voltage should be above 4 volts, for 5.6 V Zener input voltage should be 7 volts and so on.

So here the 7.5 volt Zener is connected between the positive rail and the 1K Preset (VR 1). Resistor R1 limits the current through the Zener. To the junction of R1 and VR1, the base of T1 is connected. So T1 will conduct when it gets base voltage from the Zener. When the battery voltage is above 9 volts, Zener conducts and T1 switches on. As a result, the collector voltage drops and it switches off T2 because the base of T2 will be at ground potential. So LED remains off. That means, the battery voltage is above 9 voltage and the battery condition is good.

When the battery voltage drops below 9 volts, Zener switches off. T1 also turns off. At this time, the collector of T1 becomes high through R2 and T2 gets base bias and it turns on. The LED connected to the collector of T2 turns on to indicate low battery level. VR1 adjust the exact point of the LED switching.

Assembled circuit on Common PCB

Soldering side

How to set?

For proper setting ,you need a variable power supply. Connect it to the power supply and provide 9 volts. If LED is off, turn the wiper of VR1 till LED turns on. Then increase the voltage to 10 volts. LED will turns off. The circuit is now ready to use. If you do not have a variable power supply, use this method. Connect a new 9 volt battery and adjust VR1 till LED turns off. You can then connect the circuit to 12 volt battery. LED will turns off. You can also add a buzzer to get audible warning.

Bread board assembly- LED Off at 11.8 Volts

Bread board assembly – LED on at 8.7 volts

Note: In the Prototype, low watt Zener and Resistors are used. If the battery is high current one, use 1 Watt Zener and 1 Watt resistors. You can also use higher value resistors for other levels of battery voltage indication. For example, use 9 volt Zener for 10 volt indication and 10 volt Zener for 11 volt indication.

This is the simplest type of Voltage Monitor you can build in 5 minutes. No need for a PCB and just solder lead to lead and hook up in the Emergency lamp. As long as the battery voltage is above 5 volts, LED lights to indicate a healthy battery. When the battery voltage drops below 4.7 volts, LED turns off to indicate the time for charging. It is a must for emergency lamp since deep discharge  permanently destroys the battery. Let us see what is happening in the emergency lamp battery.

The emergency lamp battery is 6 volt and 4.5 Ah. That means it can deliver 4.5 Ampere current in 1 hour. A new fully charged battery shows 6.2 volts which drops when the lamp is running and needs recharging again.

What usually happens if we keep the battery under charge for long time? Battery over charges and the heat developed inside it will damage the battery permanently. What happens if the battery is not properly charged ?  Deep discharge causes “Memory effect” in the battery and it never attains full charge later. In short, both over charging and deep discharging permanently destroys battery.

So what is the ideal voltage in the battery? It should be between 4.5 volts and 6.2 volts. If the lamp is run on a deep discharged battery, the tubes still light with low light. But the ends of tubes darken and gradually the tube damages. So the lamp should be switched off when it starts dimming.

What this circuit indicates?

The circuit indicates the low voltage level of battery below 4.7 volts. When the battery holds more than 5 volts, LED lights. If the voltage goes below 4.7 volts, LED turns off. So you can charge it again. If properly maintained, the battery will last 3 to 4 years.

How the circuit works?
Simple and nothing complicated. A Zener is doing the trick. Zener is a special kind of diode that conducts only when the voltage into it is above its rated value. For example, the 4.7 volt Zener conducts only if it gets 5 volts or more. So here the 4.7 volt Zener, an LED and a current limiting resistor (R1) are connected in series. Note that, the Zener is connected in the reverse order. This “reverse biasing” is the method of Zener connection. The end of Zener having a black ring is Cathode and it should go to the positive side. When the voltage of the battery is above 5 volts, Zener conducts and LED gets current and it lights. When the battery voltage drops below 5 volts, Zener switches off and LED turns off. That much is the theory in it. The Red LED takes 1.8 volts but its current consumption is low around 16 Milli Amperes.

Red LED has the forward voltage drop of 1.6 volts. So the voltage at the cathode of LED is 1.6 volts. Current limiting resistor is 100 Ohms. So just apply Ohms law
I = V / R = 1.6 / 100 = 0.016 Amps or 0.016 x 1000 = 16 Milli Amps
How to set?

Solder lead to lead and connect the cathode of Zener to the point of lamp switch that goes to lamp circuit board. The wire from it is going to the circuit board. So when the switch is on, both the circuit board and the LED indicator gets power. Drill a 5 mm hole on the case and fix the LED there. That is all. Enjoy its simplicity and use.

See the bread board assembly of the circuit

Prototype powered by a discharged 9 volt battery. Input voltage is 5.36 volts. LED is on

Prototype powered by a discharged 9 volt battery . Input voltage is 3.19 volts – LED off

No need for any adjustments. Simply hook it. It will do its job.

What will do if you want a low voltage less than 3 volts as reference voltage for a circuit? Usually we use a low volt Zener to give low voltage to a particular section of the circuit. But Zener below 3.1 V is not available. So do this simple method. The LED can act a Zener to give a reference voltage. Each type of LED has a forward voltage drop that range between 1.7 V to 3.3 V. This forward voltage drop slightly increases when the input voltage increases. This is due to the increase in the luminosity of the LED. So let us exploit the LED to do the job of the Zener.

Nothing complicated. Connect the LED with current limiting resistor and tap the voltage from the junction of resistor and the Anode of LED. You will get the reference voltage. Reference voltage from three typical LEDs are shown in the figure. You can use other LEDs for different reference voltages. Images below show the forward voltage drop of all LEDs now available. The current limiting resistor 470 Ohms limits the LED current to around 10 mA.

Note – The reference voltage measured here is using a 5 volt regulated power supply. If the input voltage is 9 or 12 V, the forward voltage drop will increase 0.1 volt to 0.3 volts. For example, here the Red LED gives 1.7 V at 5 volt input. It will be 1.8 V if the input voltage is 9 or 12 V.

Images showing the forward voltage drop of different LEDs at 5 volt regulated power supply

Here is a solution to protect your circuit assembly on Bread board. Usually we test the working of a circuit first on bread board before making the PCB assembly. Sometimes due to mistake in wiring, valuable components may destroy through short circuits. With the power on , we still continue the trouble shooting which may further destroy many components. Only after sensing heat or burning, we realize the problem. So here is the simple solution to assemble the circuit on bread board safely. If there is any short circuit in the bread board, the power to the board will cut off and Red LED lights. So immediately we can detach the board from power supply.

The circuit shows the following indications.
1.  Green LED On and Red LED Off– Out put power available and the bread board assembly is perfect.

2. Green LED Off and Red LED On – Board shorted and No output power.

Hook up this circuit in the bread board and give input power from power supply. Ideal input voltage is 5- 12 volts at 500 mA. Take its output and connect to the bread board. That is all to do.

How it works?

It is the trick of two complementary transistors T1 and T2. T1 is the general purpose NPN transistor BC 547 and T2 is the medium power PNP transistor SK 100. It is also available as CK 100 and BEL 100 P. At power on, T1 conducts and pulls the base of T2 to ground potential. Since T2 is a PNP transistor, it conducts when its base becomes negative. So full power is available at the collector T2 which is used to power the bread board. Green LED lights at this time to indicate the availability of output power and no short in the bread board.SK 100 can deliver maximum 800 mA current so almost full supply voltage ( less 0 .6 to 0.8 volts due to D1 and T2) is available in the board.

If the bread board power is ok, Red LED remains off since its cathode is connected to the output and in a high state. When a short  circuit occurs in the board, Green LED extinguishes and Red LED lights. Short circuit in the output blocks the collector current of T2 so the cathode of Red LED gets the current path and it lights. Green LED remains off due to the absence of current from T2.

A Red Flash LED gives more visual attraction so use it if available. You can also connect an Yellow LED with 1K resistor at the right end of bread board across the supply rails to get the indication of continuity of power. Use a heat sink for T2 since it becomes hot during short circuit.

Circuit assembled and tested on bread board

Circuit attached to bread board and showing normal and shorted conditions

To reduce size, and cost, Transformer less power supply is now used. If it is a well regulated SMPS power supply, output will be stable. But simple Transformer less power supply is not ideal for many circuits as observed in my experience. I have designed different types of Transformer less power supply but none of them is showing good performance. It can be used to power simple circuits like LED lighting. If sensitive semiconductors are present in the circuit, this kind of power supply creates many problems including erratic response. This is very high in sensor based circuits and oscillators. So to get a perfect output voltage, it is ideal to use a Transformer based power supply or SMPS. These are the problems I noticed in the Transformer less power supply.

1. No Galvanic isolation – 230 Volt AC is directly connected so if the power supply fails, that destroys the circuit. More over, shock hazard is very high if handled carelessly.
2. Low current – Maximum current available from a transformer less power supply is 75 milli amperes. This varies depending on the input voltage fluctuations.
3. Unstable – Even if a Zener diode is used at the output, it will not provide stable output (see images).
4. Voltage fluctuation– Output voltage from the capacitor fluctuates according to the fluctuations in AC lines.
5. High input DC after rectification – Most of the AC capacitors give 25 to 40 volt DC after rectification through the Bridge rectifier. So using a Zener to get 6-15 volt output is risky, since the input voltage to the Zener is as high as 40 volts. Zener may destroy easily. Even if Voltage regulator is used, this much high input voltage can damage the device. I think this is the major reason for the instant damage of many low cost gadgets available in the market.
6. Noise and transients – Since the power supply is directly connected to AC, Noise and transients in the AC lines will reflect in the output unless a filter is used. This cause problems in SCR and Triac controlled circuits and show false triggering.

How the circuit works?
As you know, the mains AC is 230 Volt at 50Hz. We can’t use it directly for an electronic circuit because all semiconductors need DC for working. So we have to reduce the high volt AC to a low volt AC. A resistor can do that, but it causes wastage of energy through warming. So AC rated capacitors ( X Rated ) are used to reduce the AC voltage. These capacitors reduce 230 volt AC to around 15 – 40 volts AC. Note that, this voltage is based on input voltage. If it drops below 230 volts, a corresponding drop will appear in the output of the capacitor.

A simple Transformer less power supply is shown below.

It uses a 0.47 uF 400 Volt AC capacitor with a 100 Ohms resistor in series. This resistor protects the capacitor from high inrush current at power on. An 1 Meg resistor (R2) is provided across the capacitor. This “Bleeder resistor” is very important to discharge the voltage from the capacitor after the switch off. Note that, the AC capacitor can store more than 600 volts even if the power supply is unplugged. This high voltage may remain many days. So the bleeder resistor immediately discharges the capacitor at power off. Its value can be between 470 K to 1 M.

The low volt AC then passes through the 1 Amps Bridge rectifier to convert it in to DC. So around 15 -40 volts DC will be obtained. This voltage depends on the type of capacitor used. The DC is then made ripple free using the Smoothing capacitor (C2). Then a 9 volt 1 Watt Zener diode with the current limiting resistor (R3) is used to regulate the output voltage to 9 volts.

After making the same circuit, I found the following problems. You can see it in the images.

1. First I used 0.47 uF capacitor. Its output DC before Zener regulation is 19.3 volts. Here comes the problems. If an LED is used here as power on indicator, this much high voltage burns LED. If a high value resistor above 1.5 K is used, LED will not light properly. If the LED is used at the output after regulation, it drops around 2 volts, and the output voltage and current reduces.

2. Second thing is the fate of Zener diode. I used 1 Watt Zener which is the one easily available. A high voltage of 19 volt is flowing through the 9 volt Zener. So I think it will not survive many hours or days.

3. Output current is around 40 mA. So it can’t be used for a circuit because most of them require around 100 mA minimum. No relay works in this low current. If an LED is used, it takes 20 mA so balance only 20 mA is available for all the other components.

So I changed the value of capacitor to 225 K. It is 1 uF capacitor that can give maximum current of 75 mA. So I found
1. Output voltage after rectification is 34.6 volts. It is too high for the Zener.
2. Zener is not working properly and giving 9.9 volts instead of the regulated fixed 9 volts.
3. A relay based circuit is working on it since current is around 75mA. But the working of the circuit is not satisfactory. It is showing false triggering with the mains voltage fluctuations.

So finally I decided to use these kinds of circuits to power LEDs in series so that the high DC will not make problems. For all other circuits, it is better to use Transformer based or SMPS power supply.

Caution: The circuit given above is highly lethal and can give a fatal shock. Do not construct it unless you are experienced in handling high volt AC. Take all precautionary measures while handling the circuit. Do not trouble shoot the circuit when it is connected to mains. Do not leave the powered testing circuit on the work bench when you leave the place.

High current White LED requires constant current for giving maximum brightness. Connecting 1 Watt LED to the DC source using a current limiting resistor gives poor performance and the LED will be either too bright or too dim. More over, the low value resistor (10 Ohm or less) will heat up since the LED consumes around 200 to 350 milli ampere current . So here is a simple LED driver chip AMC 7135 that gives constant current (CC) to LED to give maximum brightness. It also increases the life of LED. It is too simple and enjoy the project.

AMC 7135 CC Chip

It is an SMD chip that includes the circuit for providing constant current (CC) for the LED. It has 350 mA constant current sinking capacity and Low drop out voltage. Its supply voltage range between 2.7 V – 6 V and with an Output short / Open circuit protection. The IC costs Rs. 25.

Pin connection

AMC 7135 Chip has three pins. From the facing side, these are In, Ground and Vcc. Cathode of LED is connected to the “In pin” and Anode to the Vcc pin. 2.7 V to 6 V DC is provided to the Vcc pin and Negative is connected to the middle ground pin. The ground pin also has a Tab for soldering which also act as a heat sink.

A single chip drives one 1 Watt LED with constant current of 350 mA. So the DC source should be 2.7 to 6 V at 500 mA current. As the input voltage increases, current consumption in LED reaches its maximum 350 mA. For example between 4.5 volt and 6 volts. But below 4 volts, the chip delivers less than 200 mA current to LED so its brightness drops. So the ideal voltage is 4.5 volts.

1 Watt LED
This LED is available in many types. Some have markings for Anode and Cathode as + and –. If not ,identify using a 9 volt battery with 100 ohms series resistor. Touch two leads with resistor tip and negative of battery. If LED lights, the lead in contact with the resistor is Anode. If not reverse. You can identify the leads easily. See image below.

How to set?

Since AMC 7135 is an SMD chip, it takes time to solder on a common PCB because its pins are very short. If you have an SMD soldering iron, the job will be easy. First solder the Tab of Chip to one point of the common PCB. Then slightly turn and solder the Vcc pin (3rd pin). To avoid shorting, use two thin cut resistor leads to solder with other two pins. Carefully solder the pins without shorting the pins. Now you can connect the 1 watt LED with correct polarity of Anode and Cathode.

 Caution : Do not increase the voltage above 6 volts . If so, both LED and Driver IC will burn.

If a 3 Watt LED is used, two chips should be cascaded as shown below.

If more  1 Watt LEDs are used, chips should be cascaded as shown below. That means, each LED requires one chip.

The Driver IC works very well in two pen cells that gives 3 volts. Voltage is not a problem for 1 Watt LED but current should be high. The 1.5 Volt pen cell gives around 1.5 Amps current.

Current consumption of 1 Watt LED with Driver chip AMC 7135 at different volatages

SMD Soldering Iron 12 Volt DC

Here is a simple Mini Inverter circuit to light 11 Watts CFL from 12 volt battery. It is an ideal gadget to check the vehicle engine or electrical circuit in night. It can be hooked in the vehicle’s battery and the CFL will light. It can also be used in home as emergency lamp using the 12 volt UPS battery. An AC socket provision is also added to charge Mobile phone or other low watt gadgets while travelling. The 12 volt DC can be tapped from the Cigarette lighter socket of the car. The Inverter has maximum 18 Watts power.

It is a Mini Inverter that converts 12 volt DC to 230 volt AC. Two high power NPN transistors
T1 and T2 acts as a simple oscillator to generate the frequency. The oscillating pulses are fed to the inverter transformer and from its secondary winding, AC will be available.

The 12 volt DC from the battery first passes through the 330 uH choke that eliminates noise from the circuit during the switching of the inverter transformer. Resistor R1 biases T1 directly and T2 through the winding of transformer. So with C1, Transistors T1 and T2 oscillate and the oscillations are fed to the winding of transformer with a center tap that gets DC from the battery through the choke. So the oscillations of T1 and T2 induce AC current in the secondary of transformer which lights the CFL.

Assemble the circuit on a common PCB. Use ready made inverter transformer and 330 uH inductor with ferrite rod. Enclose in a shock proof plastic case with a holder for CFL and AC socket for charging purpose.

15 Watts CFL Lights in 12 volt battery using the circuit

Pin connection of CTC 880. Its equivalent is D 880

Inverter Transformer

Caution: The circuit generates high volt AC at the output. So do not touch on the PCB when it is connected to the battery, Do not trouble shoot when the circuit is powered. It can give a fatal shock if handled carelessly.

Transformer is used to convert high volt AC to low volt AC (Step down) or low volt AC to high volt AC (Step up). So step-down transformer is generally used to convert 230 Volt AC to low volt AC which can be used to make a DC power supply after rectification and smoothing. But a Non polarized capacitor can act like a transformer to step down the high volt AC to low volt AC. Say, 230 V AC to 40 VAC. Let us see how the capacitor is doing this.

See the above image. When a capacitor (C) and a resistor (R) are connected to AC lines, a constant current can be maintained through the resistor (R) so long as the “Reactance” of the Capacitor is greater than the “ resistance” of the Resistor. The current flow depends on the value of the capacitor assuming that V1 (input voltage) is greater than V2 (Output voltage). Thus the current through the resistor is

So theoretically, a 105 K 400 V capacitor (1 uF) gives 70 mA current through it when it gets 230 Volt AC. This value changes when the value of the capacitor changes. That is , high capacitance gives more current and lower value less current. Anyway, the maximum current available will be between 70 mA and 150 mA in the capacitors now available. So these capacitors can only be used to make power supply for less current applications.

Reactance of the capacitor and its ability to deliver current are directly proportional. Reactance of the capacitor is expressed as

The image below shows a simple Capacitor power supply using 105 K 400 V capacitor. It gives 30-40 volts DC with a current of 60-70 mA.

So after calculating the Reactance of the capacitor, it is easy to find the current delivering capacity. It is important to note that, the current from the capacitor depends on the input voltage. As you know, the AC voltage will not be steady normally. So if it drops to 200 V or less, a corresponding reduction in the voltage and current will reflect in the output of the capacitor.

The table below shows the tested Voltage and current from different types of X rated capacitors now commonly used.

The table below shows the Tolerance codes of the X rated capacitor. Commonly, the capacitors are available as K, J etc after its EIA code. For example 105 K means 1 uF capacitor with + or – 10 % Tolerance. So in actual use, its value may show a 10 increases or decrease.

Caution: The power supply based on the Capacitor is dangerous since it is directly connected to high volt AC without any galvanic isolation. So do not construct this kind of circuit unless you are competent and experienced to do that. A simple carelessness can give a fatal shock or burst a fire. Do not test or trouble shoot the circuit when connected to mains. Take all precautions while testing such circuits like wearing gloves and foot wear.

For bread board assembly of Micro controller and Arduino projects ,we need  well regulated  5 V and 3.3 V DC power supply. Here is the simple but efficient regulated power supply that gives four stage power outputs  – 12 V unregulated, 9 V, 5 V and 3.3 V regulated DC .

It is a Transformer based power supply using a 12-0-12 volt 200 mA Step-down transformer. The low volt AC is rectified by diodes D1 and D2. The high value smoothing capacitor C1 removes ripples and gives a clean 12 volt DC .This 12 volt DC is unregulated which can be used to drive back end components like Relay, Buzzer, Lamps etc. Regulator IC 7809 (IC 1) takes the 12 volt unregulated DC and gives 9 volt regulated DC. IC 7805 (IC2) takes 9 V DC from IC1 and gives 5 V regulated DC. Capacitor C2 removes noise from the power supply. Green LED with the current limiting resistor R1 is the power on indicator. Diodes D3 , D4 and D5 are used to reduce the 5 V DC to 3 to 3.8 V DC. Silicon diodes usually have a forward voltage drop between 0.4 V to 0.7 V. This may vary based on the make of the diode. So three  IN 4007 diodes drops 1.2 – 2.1  Volts. Therefore, the output from D5 will be around 3 – 3.8 V.  But the forward voltage drop of the diode will not be accurate in all makes and may vary between 0.4 to 0.7 volts. Anyway a mid rage of 3.5 volt can be expected from D5.

Click on the image for enlarged view

Assemble the circuit on a common PCB. Transformer is 200 mA only and the regulator ICs gives around 100 mA output current which is sufficient for the project. Use suitable sockets and pins to tap the 12V, 9V, 5V and  3.3V outputs.