This door minder electronic project uses an IR beam to monitor door & passage-ways or any other area. When the IR beam is broken a relay is tripped which can be used to sound a bell or alarm. This door minder electronic project is suitable for detecting customers entering a shop, cars coming up a driveway, etc. Because the IR beam is very strong distances over 25 yards can be monitored with electronic circuit . This circuit must be powered from a 12volt DC supply .
The transmitter circuit consists of two square-wave oscillators, one running at approx. 250Hz and the other running at 38kHz. The 38kHz frequency acts as a carrier wave and is required by the IR receiver module on the receiver board.
The oscillators are made by using two 555 timer ICs set up as astable configuration multivibrators.
Another 555 timer ( IC2) is used for the 38KHz oscillator. Resistors R4 and R5 and capacitor C3 set the frequency.
Diodes D1 and D3 are used to create a symmetrical output. Normally the external capacitor C1 (C3) charges through resistors R1 and R2 (R4 and R5) and discharges through R2 (R5). Without the diodes this output waveform would have a longer “high” time than the “low” time.
The output from the IC1 is coupled via diode D2 and resistor R3 to the trigger input of IC2. When the IC1 output is low it stops IC2 from running and IC2’s output is forced high (no IR LED current). When IC1 output is high, IC2 runs and the IR LED is pulsed at 38KHz.
The receiver module consists of an IR receiver module that detects the incoming beam from the transmitter. The IR signal is used to keep a capacitor charged which in turn holds a relay operated. When the beam is broken the capacitor discharges and the relay releases.