This door
monitor project uses an infrared beam to monitor door & passageways or any other area. When the beam is broken a relay is tripped which can be used to sound a bell or alarm. Suitable for detecting customers entering a shop, cars coming up a driveway, etc. The IR beam is very strong. Distances over 25 feet can be monitored. A 12VDC supply is required to power the circuit. A 12V wall adaptor is fine. Provision has been made so that only one power supply needs to be used to power both units. The relay is rated to switch mains voltages.
Door Monitor Transmitter Board
The door monitor transmitter board 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. This carrier wave is “ANDed” or modulated by the 250Hz frequency to produce an output signal that contains bursts of 38kHz at a rate of 250Hz. This signal is used to drive an infrared LED. The oscillators are made by using two 555 timer ICs set up as “astable” (free running) multivibrators. IC1 is used for the 250Hz oscillator. Resistor R1 and R2 and capacitor C1 set the frequency. Another 555 chip, IC2, is used for the 38KHz oscillator. Resistors R4 and R5 and capacitor C3 set the frequency. Notice the diodes D1 and D3. These are provided 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 diode bypasses resistor R2 (and R5) when the capacitor is charging, so that it is only charged via R1 (or R5). This gives the same charging and discharging time and so the output waveform has equal high and low times. The charge time (output high) is given by:
THIGH = 0.693 x R1 x C1 (or 0.693 x R4 x C3)
The discharge time (output low) is given by:
TLOW = 0.693 x R2 x C1 (or 0.693 x R5 x C3)
The output frequency = 1 / (THIGH + TLOW)
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 Waitrony IR LED is driven directly from the output of IC2. Resistor R6 sets the maximum LED current. With a 12VDC supply the current is about 45mA (the LED drops 2V across it when conducting). Lowering the value of R6 will increase the current through the LED thus boosting the signal strength. This may be necessary if the kit is used outside in direct sunlight or if you need “very long range”. Keep in mind that the maximum current that the 555 can handle is 200mA
If the distance to be monitored is less than about 10 yards then you will need to fit the 5mm shrink tubing over the IR LED. This narrows the radiating angle of the IR beam and makes it much more directional. The IR output is strong. It can easily bounce off walls etc to give false readings.
FOR FURTHER READING VISIT www.efy.com
