Schematics Proximity Detector Circuit Using NE555 PCB | Electronic Circuits

Schematics Proximity Detector Circuit Using NE555 PCB

Circuit
 This post shares a proximity detector circuit comprising mainly NE555 IC. This circuit can be used for burglar alarms based on beam interruption, with the added advantage that the transmitter and receiver are housed in the same enclosure, avoiding any wiring problems.

Circuit diagram of 555 Proximity Detector : Click on image to enlarge

This proximity detector is constructed using an infrared diode detector. Infrared detector can be used in various equipment such as burglar alarms, touch-free proximity switches for turning on a light, and solenoid controlled valves for operating a water tap. Briefly, the circuit consists of an infrared transmitter and an infrared receiver (such as Siemens SFH506- 38 used in TV sets).

The transmitter part consists of two 555 timers (IC1 and IC2) wired in astable mode, as shown in the figure, for driving an infrared LED. A burst output of 38 kHz, modulated at 100 Hz, is required for the infrared detector to sense the transmission; hence the set-up as shown is required. To save power, the duty cycle of the 38kHz astable multivibrator is maintained at 10 per cent.

The receiver part has an infrared detector comprising IC 555 (IC3), wired for operation in monostable mode, followed by pnp transistor T1. Upon nreception of infrared signals, the 555 timer (mono) is turned  ‘on’ and it remains  ‘on’ as long as the infrared signals are being received.
 
When no more signals are received, the mono goes  ‘off’ after a few seconds (the delay depends on timing resistor-capacitor combination of R7-C5). The delay obtained using 470kilo-ohm resistor and 4.7µF capacitor is about 3 seconds. Unlike an ordinary mono, the capacitor in this mono is allowed to charge only
when the reception of the signal has stopped, because of the pnp transistor T1 that shorts the charging capacitor as long as the output from IR receiver module is available (active low).
 
This setup can be used to detect  proximity of an object moving by. Both transmitter and receiver can be mounted on a single breadboard/PCB, but care should be taken that infrared receiver is behind the infrared LED, so that the problem due to infrared leakage is obviated.

An object moving nearby actually reflects the infrared rays from the infrared LED. As the infrared receiver has a sensitivity angle of 60 degree, the IR rays are sensed within this lobe and the mono in the receiver section is triggered. This principle can be used to turn  ‘on’ the light, using a relay, when a person comes nearby. The same automatically turns  ‘off’ after some time, as the person moves away.

The sensitivity depends on the current-limiting resistor in series with the infrared LED. It is observed that with in-circuit resistance of preset VR1 set at 20 ohms, the object at a distance of about 25 cms can be sensed.

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