Locker Guard


Protect the Home Locker with this simple Alarm. When somebody opens the Locker, it sounds an alarm till the locker is closed again.

Idea of the circuit is simple. When the circuit is placed inside a locked locker, darkness in  its inside increases the resistance of LDR, So, Zener diode will not get its breakdown voltage and remains off. The buzzer driver Transistor BC 547 also will be off. When the door of locker opens, light enters and LDR gives current flow to charge the 100 uF capacitor, When its voltage increases above 5 volts, Zener conducts to switch on the transistor. Buzzer then sounds .

A natural doubt will arise. What will happen if the locker is opened in the night? Its answer is simple. User needs room light to use the locker. A Burglar uses a torch to search for valuables. In both cases, LDR gets light and alarm sounds. So it works day and night.

Multi Siren Generator


A single low power IC UM 3561 can make four different siren tones just changing its pin connections. Its pin 5 is Vcc and needs only 3 volts from two pen cells. Maximum Vcc should be 4.5 volts. It is a ROM IC with pre recoded tunes.  The 220 K resistor between its pins 7 and 8 determines the frequency of tone. So do not change its value. Pin 2 is ground pin. Pin 3 gives feeble output which can be amplified by a single transistor that drives an 8 ohm speaker.

The diagram shown is an Ambulance siren by connecting its pin 6 to ground. By connecting pin 6 to Vcc pin 5, IC generates Fire engine sound. By keeping pin 6 not connected, IC gives Police siren. By connecting pin1 to Vcc pin 5, IC gives Machine gun sound.

Battery charging Indicator


A simple solution to confirm whether a battery is charging or not. You can add this to the Car battery charger.

It confirms 1. Battery is properly connected 2. Current is flowing from the charger to the battery.

When the circuit is connected to the charger, LED remains off. But when the battery is connected, LED turns on. At first, the LED brilliantly lit indicating that current is flowing to the battery. Once the battery is fully charged LED turns dim.

There is a resistor R2 ( R Sense ) between the Emitter and the base of the PNP transistor BC 557 . Its value is selected as 27 Ohms to drop 24 mA current .When a current higher than 24 mA passes through the R Sense resistor, LED brightness increases. Circuit gives around 100-150 mA current for slow charging. Use a transparent Red LED.

Motion Sensing Bell


In many homes, Wireless Door bell is used. It has a Bell unit that gives around 24 types of tunes and a  Bell switch unit  placed outside the home on gate pillar or near the front door. We can convert this bell into a Motion sensing Door bell. So somebody move in front of the Bell switch , Door bell automatically turns on for few seconds and then turns off. Again triggers in another movement.

You need a PIR Motion Sensor Module, BD 139 transistor, two 1K resistors and one LED to modify the Door bell into Motion sensing Door bell. Power supply can be 12 DC.

Open the Bell switch unit and remove the 12 volt battery in it. Below the Press button, there are two contact points that make contact when the button is pressed. Short it. Connect 12 volt power supply. Tap 12 volt for the Bell switch unit from the Emitter of BD 139 transistor and ground. Connections over. Enclose the set up in a box with the Lens of PIR sensor and LED exposed from the box.

Switch on the Door bell and move in front of the Door bell switch Unit. Bell will sound. In the night, the tone of the Bell can be changed to Alarm mode. So it works like a Burglar alarm.

TV Remote tester


TV Remote has an Infrared LED in its front. It emits invisible Infrared light that pulsate at 38 kHz. This tester has an Infrared Sensor Module that senses only the Infrared rays pulsating at 38 kHz. The IR sensor present in the TV senses the coded infrared pulses from the Remote and functions accordingly.

We can check the Remote using this tester whether it is working or not. Just focus the Remote on to the IR sensor of the tester and press any key. LED in the tester will flash and confirms the working of Remote.

Bluetooth Speaker


Make your own Bluetooth speaker using the Bluetooth Module. When paired, it will wirelessly play music from your Mobile phone.  It is powered by a Mobile charger. Stereo output from the Bluetooth Module goes to a 3 watt Stereo Amplifier Module. Two small 8 Ohm speakers are connected at the output of the Amplifier Module. Whole set up can be enclosed in a small box. You can use two small computer speaker boxes for that.

Power the circuit, and see the Bluetooth connection in the Mobile phone. The Icon of Bluetooth Module will appear in the Mobile phone. If not, scan the nearby devices. Once detected, you can pair the Bluetooth Module with your phone and play music.

Mini Emergency Light


Tiny Emergency light that turns on when power fails. A handy match box sized emergency light useful during travel. 4 Volt battery in it charges via a Mobile charger. Red LED indicates that the battery is charging and mains power is available. During that time, PNP transistor BC 557 remains off due to the positive bias in its base through 4.7 K resistor. White LEDs remains off. When power fails, base of BC557 turns negative and it switches on to light the White LEDs. Keep switch S1 in On position to work it as Emergency light and keep it in Off position when the light is not needed.  Battery can be a small 4 volt rechargeable battery.

Unusual LED Flasher


    This LED Flasher gives brilliant flashes. It is not a conventional transistor design and looks absurd. But still it works properly. I am using this flasher continuously as a Mock flasher for the last two years. Still its performance is excellent.

Transistor BC 547 is using as a reverse biased diode without connecting its base to anywhere. Charge and discharge of 470 uF capacitor determines its flash rate. With this value, it is one flash in one second. Use a 5 mm Red coloured transparent LED. Circuit works only above 11. 5 volts.

 It can be an ideal indicator for 12 volt battery chargers. When the battery attains 12 volt, LED starts flashing. When the battery full voltage reaches 13.8 volts , Flash rate increases. Try it and see the result. Remember; use only 12 volt power supply. It will not work in 9 volt battery.

Water Tank Over flow Alarm


This simple circuit beeps loudly when the water tank start to over flow. So no more wastage of water. N – Channel MOSFET IRF Z44 N is used as a switch. Buzzer and LED are connected in series with 100 Ohms resistor to the Drain (D) of the MOSFET. Source (S) of MOSFET is connected to ground. Gate (G) is connected to the Source through 1 M resistor to prevent latching of MOSFET so that buzzer will turns off when the water level drops.

Fix the probes (with 1 cm gap) just below the rim of water tank. Power and switch on. When the water level touches the probes, buzzer sounds and LED turns on. When the water level drops, probes loose continuity and buzzer stops beeping. 9 Volt battery or 12 volt adapter can be used as power supply.

Solar Garden Light


Simple but efficient LED light for Garden. Turns on in the evening and turns off in the morning. 6 Volt Solar panel charges the battery in sunlight. 4 volt battery power the LED in night. During day time, PNP Transistor BD140 remains off due to the positive state of its base through 4.7 K resistor. When the current from the solar panel decreases in the evening, base of BD140 will be pulled to ground via 1 M resistor and the transistor turns on to light the 1 watt White LED. In the morning, the condition reverses and the LED turns off.

AC Lamp Flasher


A Warning lamp for places where machineries are operating. It flashes continuously to remind that there is high voltage and heavy machinery is running. It can also be used as a warning lamp in roads where road work is progressing.

Lamp flashes due to the charge and discharge of 470 uF capacitor and the switching of Diac DB3. When the capacitor charges fully, Diac DB3 fires and Triac BT136 conducts. This turns on the Lamp. When the capacitor discharges, Diac turns off to switch off  the Triac. This turns off the lamp. This cycle repeats and the lamp flashes continuously. By lowering the value of capacitor, the flash rate can be increased. Flash rate can be slowed down by increasing the value of the capacitor.

Caution: AC Circuit that can give fatal shock when handled carelessly. Do not test or troubleshoot when the circuit is connected to mains.

Automatic Night LED


White LED turns on in the dark and turns off in the light. When there is light, LDR offers less resistance and conducts. Transistor BC547 will not get sufficient base bias and remains off. White LED connected to its collector also remains off. In dark, resistance of LDR increases and transistor gets base bias and conducts. LED then turns on.

Automatic Outdoor Light


It turns on in the evening and turns off in the morning. Triac BT 136 is used as a switch to control the lamp. LDR senses light intensity. When the LDR conducts during day time ,  Diac DB3 will not get sufficient threshold voltage ( 34 V ) to fire. So Triac BT136 remains off. Lamp also will be off. When the resistance of LDR increases in the night, it conducts less and Diac fires to switch on the Triac. Lamp turns on.

Triac BT 136 can handle up to 250 watts load with a heat sink. Use ordinary bulb or LED bulb.

Caution : AC Circuit that can give fatal shock when handled carelessly. Do not test or troubleshoot when the circuit is connected to mains.

Power On Indicator Flasher


Here is a Simple Flashing LED indicator for the switch boards. One end of it can be connected to the point of switch going to the load and the other end to the neutral. When the switch is on, LED starts flashing. So it confirms that the load is switched on. This is useful to monitor the working of battery chargers and other devices that are continuously switched on.  Alternately, it can remind to switch off high current devices like water heater, electric iron etc after the use.

Working of the circuit is simple. 47 K, 1 watt resistor drops 220 volt AC and 1N 4007 diode rectifies it. 47 uF capacitor charges slowly. When the voltage in the capacitor increases above the threshold voltage ( around 34 volts ) of Diac DB3, it conducts and capacitor discharges. This lights the LED. When the capacitor discharges, LED turns off. This cycle repeats and LED turns on /off alternately .

LM 317 Variable Power Supply


Simple Variable supply using Variable voltage regulator IC LM317. If the input voltage is 32 volts, output voltage can be adjusted from 1.2 volts to 28 volts by turning 10K Pot. Use a Heat sink for LM317 to dissipate heat.

Minimum Input voltage – 4.2V

Maximum output voltage – 40V

Minimum Adjustable voltage – 1.2V

Maximum adjustable voltage – 37V

Maximum output current – 1.5A

Digital Volt Ampere Meter and its connections



Digital Volt Ampere meters are now available to measure DC voltage and Current. It can be connected to the output of the Power supply. So the output voltage and current to the load are displayed in the meter. If a battery is connected to the power supply for charging, we can see the current consumed by the battery during charging. When the battery is fully charged, current drops to zero which indicates termination of charging. If the project is connected, the voltage and current consumed by the project are displayed in the meter.

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Lead Acid and Tubular battery charger



Here is an efficient Car battery charger for fast charging the Car battery. It is a 5 Ampere charger with Digital Volt and Ampere Meter to display the charging voltage and current. Since the charger provides maximum 5 Amps current, battery can take high current it needs to charge very fast. As the charging process completes, current flow reduces to zero and will be displayed in the meter. So it is easy to confirm whether the battery is fully charged or not.

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Simple Line Tester



This simple circuit can indicate the condition of Phase, Neutral and Earth wiring in the domestic power supply. If Phase, Neutral and Earth lines are intact, Red, Yellow and Green LEDs glow. If Earth line is brocken, Green LED remains off. If either Phase or Neutral line is brocken, all LEDs remains off. The circuit can be enclosed in a Three pin plug. Just plug it in the socket and check the conditions of home wiring.

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Five stage Water level Indicator – School Project 1


A Simple Water level indicator circuit indicating 5 stage water levels in the tank. LED1 glows when the water tank is empty. LED 5 glows when the tank is full. LED 2 indicated 40%, LED 3 indicates 60% and LED 4 indicates 80% water in the tank. LEDs glows one by one when the water tank fills and turns off one by one when the water level drops.

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LED in AC


It is easy to light an LED by directly connected to 220 volt AC. It can be indicator in AC devices or Bed lamp if White LED is used. If used as a Bed lamp, the circuit can be enclosed in a plug  with LED projecting out.

Caution : The circuit is directly connected to high voltage that can give a fatal shock if handled carelessly . Do not test or troubleshoot when connected to mains. 

Simple Power Failure Alarm


Beep once when power fails. 12 volt relay is used as a switch. When power is available, relay pulls and capacitor charges via diode D1 and NO and Common contacts of relay. When power fails, relay turns off and capacitor discharges via Common and NC contacts and buzzer. Buzzer gives a short beep at that time. If more timing for beep is needed, connect one more 4700 capacitor parallel to C1. 12 Volt DC for the circuit should be from a 12 Volt adapter or transformer power supply.

Alarm for Water Heater


It was a big problem in my house that, sometimes the Water heater left on for many hours after the use. It has a Thermostat to cut off but when the water cools, again it switches on. This causes hike in electricity bill. Finally I found a solution. Just connected a Timer based alarm with the heater AC socket so that it will also switches on when the heater turns on. After 30 minute, it gives a loud alarm to remind that the heater is On. Alarm continues for another 30 minutes or till the heater is switched off. So that alarm reminds to switch off the water heater. This can be used for Electric Iron also.

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Flicker Free Automatic Light


When we use Triac and LDR to make Automatic Lights, a major problem arises . Lamp starts flickering during the light transition period in the  evening and morning. Lamp fully turns on only when there is complete darkness and turns off only when day light is full. This simple design solves the problem. Light instantly turns on / off without any flickering. You can drive CFL, LED bulb etc which turns on automatically around 6 pm and turns off around 6 am. See the simple design.

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Simple Tranformerless Power supply


It is very easy to convert 220 Volt AC to DC for powering electronic circuits. Any DC voltage from 3 volt to 36 volt can be generated from 220 volt AC using appropriate value of X rated capacitor and Zener diodes. The X rated capacitor drops 220 V AC to low volt AC. This can be rectified to DC by a Rectifier diode and smoothed by a Filter capacitor. By selecting a required value Zener, regulated DC can be obtained. But the only drawback of the Tranformerless power supply is its low current, so only light loads can be used. Here explains the design of a 4.7 Volt 4.6 Milli Ampere Tranformerless power supply to power PIR sensor .

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AC Powered PIR Light


A Simple design of PIR Light using Tranformerless Power Supply and Triac for driving the load. Too compact and can be enclosed in a small case. The light turns on when the PIR sensor detects human movement and turns off after the preset time. Re triggers in another movement. Its range is 30 cm to 10 meters and time delay, 30 seconds to 3 minutes both can be adjusted using the presets in the PIR sensor Module.

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Fast Charger for Car battery



Here is an efficient Car battery charger for fast charging the Car battery. It is a 5 Ampere charger with Digital Volt and Ampere Meter to display the charging voltage and current. Since the charger provides maximum 5 Amps current, battery can take high current it needs to charge very fast. As the charging process completes, current flow reduces to zero and will be displayed in the meter. So it is easy to confirm whether the battery is fully charged or not.

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Wireless Electricity Generator


Wireless Electricity Generator
Here is a High Efficiency Wireless Electricity Generator project. Usually we make a simple wireless electricity generator to light 1.8 volt Red LED. But here the efficiency of the Wireless Electricity Generator is increased to light 1 Watt White LED Bulb that requires 3.6V. It generates voltage and current even to light a Strip LED. Try it and see the wonder of LED lighting without a battery connection.

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PIR Sensor Alarm


PIR Sensor Alarm

This simple alarm can be used to detect the movement of human beings at a distance of 10 meters. It works both day and night and beeps shortly when there is a human movement through its detecting range. It is ideal to monitor the entry passages, stair case, corridors etc. It is highly useful to monitor the human movements in Flats, Multi storied buildings, Shops etc. A relay can also be connected for activating lights, electric alarms etc. It can be used as a Personal alarm during journey to protect luggage.

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