555 Touch Switch

This touch switch built based on famous 555 timer IC. When the plate is touched the 555 timer is triggered and the output on pin 3 goes high turning on the LED and the buzzer for a certain period of time. The time that the LED and the buzzer is on is based on the values of the capacitor and resistor connected to pin 6 & 7. The 10M resistor on pin 2 causes the the circuit to be very sensitive to the touch.

IR Remote Switch

Imagine the convenience of selecting TV channels using your remote and then pointing the same remote to your switchboard to switch on/off the fan or the tubelight. Here is a simple circuit to remotely switch on/off any electrical device through a relay using the normal TV/VCR/VCP/VCD remote control unit. It works up to a distance of about 10 metres. The circuit is built around a 3-pin IR IC receiver (Siemens SFH-506-38 or equivalent) that can detect 38kHz burst frequency generated by a TV remote. (This IR receiver module has been covered earlier in many projects published in this blog.) The output pin of IR sensor goes low when it detects IR light, triggering the monostable (1-second) built around timer NE555. The output of the mono toggles the J-K flip flop, whose Q output drives the relay through SL100 npn transistor (T1).

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Gas Leakage Alarm

 LPG gas leakage detector with audio visual indicator

LPG gas is supplied in pressurized steel cylinders. As this gas is heavier than air, when it leaks from a cylinder it flows along floor and tends to settle in low spots such as a basement. This can cause fire or suffocation if not dealt with.

Here is a circuit that detects the leakage of LPG gas and alerts the user through audio-visual indications. Fig. 1 shows the circuit of the gas leakage alarm. The circuit operates off a 9V PP3 battery. Zener diode ZD1 is used to convert 9V into 5V DC to drive the gas sensor module. The SEN-1327 gas sensor module from RhydoLABZ is used in this circuit. Its output goes high when the gas level reaches or exceeds certain point. A preset in the module is used to set the threshold. Interfacing with the sensor module is done through a 4-pin SIP header. Pin details of the gas sensor module are shown in Fig. 2.

Random Number Generator Game

This electronic game is simulation of one-arm bandit game. Electronics hobbyists will find it very interesting. When toggle switch S1 is in ‘run’ position, all segments of 7-segment displays (DIS1 through DIS3) will light up. On turning toggle switch S1 from ‘run’ to ‘stop’ position, displayed digits will continue advancing and the final display is unpredictable. Thus the final number displayed in DIS1 through DIS3 is of random nature. The speed with which the number in 7-segment display keeps changing on flipping switch S1 from ‘run’ to ‘stop’ condition slowly decays before stopping with a random number display. To play this game, one has to obtain three identical numbers in displays DIS1 through DIS3.

The contestant would score 1 (one) point if he manages to get a final display of ‘000’, 2 points for getting ‘111’ display, 3 points for ‘222’,… and so on—up to ten points for ‘999’. He should try to score maximum possible points in fixed numbers of attempts (say, 20 to 25 attempts). Apart from using this circuit as a game for entertainment, one can use it as random number generator for any other application as well. The decay time with the given component values is around 15 seconds before the display could stop at a final random number.

Circuit diagram:

High / Low Voltage Cutoff With Timer

This inexpensive high / low voltage cutoff circuit can be connected to an air-conditioner/fridge or to any other sophisticated electrical appliance for its protection. Generally, costly voltage stabilizers are used with such appliances for maintaining constant AC voltage. However, due to fluctuations in AC mains supply, a regular ‘click’ sound in the relays is heard. The frequent energisation/de-energisation of the relays leads to electrical noise and shortening of the life of electrical appliances and the relay/stabilizer itself. The costly yet fault-prone stabiliser may be replaced by this inexpensive high-low cutout circuit with timer.

The circuit is so designed that relay RL1 gets energised when the mains voltage is above 270V. This causes resistor R8 to be inserted in series with the load and thereby dropping most of the voltage across it and limiting the current through the appliance to a very low value. If the input AC mains is less than 180 volts or so, the low-voltage cut-off circuit interrupts the supply to the electrical appliance due to energisation of relay RL2. After a preset time delay of one minute (adjustable), it automatically tries again. If the input AC mains supply is still low, the power to the appliance is again interrupted for another one minute, and so on, until the mains supply comes within limits (>180V AC).

Circuit diagram:

High / Low Voltage Cutoff With Timer Schematic

Fire Alarm Using Thermistor

This small and simple fire alarm circuit uses thermistor as the heat sensor. When temperature increases, its resistance decreases, and vice versa. At normal temperature, the resistance of the Thermistor (TH1) is approximately 10 kilo-ohms, which reduces to a few ohms as the temperature increases beyond 100 C. The circuit uses readily available components and can be easily constructed on any general-purpose PCB. You can used this circuit as Home-Security purpose.


Circuit Diagram:

Fire Alarm Circuit Diagram