touch free timer switch circuit

Posted on Feb 6, 2014

This type of infrared proximity circuit is widely used as an electric switch where physical contact is not desired for hygiene purpose. For example, we commonly see use of infrared proximity sensors on public drinking fountains and in public washrooms. The simple circuit presented here can be operated by moving your hand in front of it. This is ac

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hieved by detecting the infrared light reflected by your hand onto a receiver device. Fig. 1 shows the circuit of the touch-free timer switch. It has two sections: transmitter and receiver. The IR transmitter is built around timer LMC555 (IC1), which is wired as an astable multivibrator. The multivibrator produces 38kHz pulses (at low duty cycle) that drive an infrared LED (LED1). This frequency can be tuned using a 10-kilo-ohm preset (VR1). A 220-ohm series resistor (R3) ensures that the current consumption of the IR transmitter is not out of arrangement. The receiver section is built around IR receiver module TSOP1738 (IRX1), timer LMC555 (IC2) and a few discrete components. The TSOP1738 is an integrated miniaturised receiver for infrared remote control systems. Everything required for IR signal processing, including the PIN diode and preamplifier, are assembled on a lead frame and the epoxy package is designed as an IR filter. When a short IR burst is received by IRX1 (as you wave your hand in front of the switch), the demodulated pulses are fed to the trigger input (pin 2) of the second LMC555 (IC2). This, in turn, triggers the monostable wired around IC2 and its output pin 3 goes high for a period determined by the 2. 2-mega-ohm potentiometer and capacitor C5. This turns off the standby indicator (LED1) and transistor T1 conducts to drive the 5V relay (RL1). LED1 enables you to locate the switch in the dark. AC mains...

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