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Fire Alarm Using a Thermistor   

This project can effectively be used as a temperature-sensing fire alarm. Here, an NTC thermistor is used as the temperature sensor. Resistance of the NTC thermistor decreases with an increase in temperature.

Door-Opening Alarm Using Hall Sensor   

This simple door-opening alarm uses a hall sensor to produce sound when someone opens the door. The circuit can be installed at a workshop, office or home for security purposes. The 3-terminal hall sensor is easily available in the market.

Device for Maintaining Car Batteries   

When we do not drive our cars for extended periods of time, the abandoned rechargeable batteries become fully discharged. Sometimes, these get damaged, too. Presented here is the simple device that can maintain two 12V rechargeable batteries.

Over-Heat Detector   

An NTC thermistorís sensitivity to temperature changes, even in small increments, enables the device to be used in temperature-sensing/control applications. This project for the over-heat detector uses a 10k NTC thermistor.

Four Frequencies Generator   

This circuit produces fixed frequencies of 4MHz, 6MHz, 10MHz and adjustable square-wave with variable frequencies from around 10Hz to more than 100kHz.

Simple Adjustable Staircase Waveform Generator   

Staircase signals find many applications in the field of electronics, such as in television systems, telemetry, and analogue and digital communications. Here is a circuit illustrating the basic principles of staircase waveform generator.

Mini Offline UPS   

Here is a circuit of an offline UPS, which a hobbyist can make at a reasonable cost. The capacity of the UPS is 350VA, so it can be used for any equipment with a load below 350VA. The UPS can be upgraded to 1kVA by making just a few changes.

Letterbox with Letter-Counting Facility   

Here is a circuit that starts counting when you insert a letter in the letterbox at your home or office. It is designed to save your time from going to the letterbox to check if there are letters inside. The number of letters present in the box is indicated by a seven-segment display.

Low-Cost 6-Bit DAC   

For controlling the load of appliances such as cooling fans, low-wattage heaters, thermostats, low-wattage light sources, small electrical toys and test benches for loudspeakers, we need a power source whose voltage can be controlled in small steps and is capable of providing current of more than 1A. For that, we require low-resolution digital-to-analogue converters (DACs) with three to seven bits.Here is the circuit capable of setting output voltage between 1.25V and 15V in 64 steps.

Automatic USB-Controlled Power Switch   

Here we present a circuit that can control mains power of an LED TV and a DTH set-top box (STB).

Dual Audio-Signal Tracer   

The circuit described here is of a dual audio-signal tracer. An audio-signal tracer is a simple device used to troubleshoot for audio signals in radio and other electronic circuitry.

Simple Light and Sound Indicator for Mains Power Supply   

While repairing or installing electrical machines in a building, the AC mains power supply is switched off from the mains electrical switchboard installed outside the building. There is a chance that someone who is not aware on the mains from outside. This poses a great danger for the technician working inside. Hence, an indicator like the one described here, which can be plugged into a nearby mains wall socket, might prove very useful for the technician.

Smoke, Alcohol and LPG-Detection Alarm   

Presented here is a circuit for raising an alarm on detecting smoke or LPG cooking gas leakage, or even alcohol vapours in breath. This is achieved by using a basic unit with different sensors for smoke, LPG and alcohol. So, different alarms can be made by simply changing the input sensor.

Museum Watchdog   

This self-contained alarm widget provides a flexible, cost-effective solution for museums, art galleries, historic houses, heritage properties and corporate offices, among others.

Wireless Baby Monitor   

You can now monitor your sleeping baby with this wireless baby monitor. It is a radio system used to remotely listen to sounds made by an infant. Simply place the circuit near your sleeping baby and listen through an FM receiver from any other room in the house.

12V Battery Absorb and Float Charger   

Most battery chargers stop charging the battery when it attains its maximum charging voltage set by the circuit. This circuit charges the battery at a particular voltage, that is, absorption voltage, and once the maximum charging voltage is attained, the charger changes the output voltage to float voltage for maintaining the battery at that voltage. Absorption and floating voltages are dependent on the type of battery.

Multichannel Visual Thermometer   

Temperature of some electronic components used in a device may rise unexpectedly and these might get damaged. This multichannel visual thermometer can indicate the problem through glowing LEDs. Since transistors are very cheap and widely available, this circuit uses these as low-cost temperature sensors and amplifiers.

Glass Break Alarm   

This circuit can be used for sounding an alarm to detect the breaking of a glass window by an intruder, even when the intruder ensures there is no sound of the shattered glass.

Laser Based Security Lock   

Presented here is a security system that uses an inexpensive laser torch which is usually available with a key ring. The advantage of this security lock is that you can use any laser torch or pointer that is easily available in toy shops.

Bicycle USB Charger   

This circuit has been developed to add a USB charging port to a bicycle for charging a mobile phone.



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