Infrared Light Gate has is ON/OFF by a transmitter and receiver. This IR light gate describes an infrared light source and detector, which can be used in a wide variety of applications ranging from intruder alarms to automatic garage door openness. When the light beam the infrared source interrupted, the receiver circuit detects this and energizes a relay.
The use of an infrared light source is an obvious choice for this type of application. The circuit described here uses the infrared emitter and photodiode.
For a given range the transmission system should be as efficient as possible and the receiver must have high gain. Such a high gain can be achieved from AC modulated light beam and AC coupled receiver.
Transmitter circuit :
- The simple transmitter circuit is shown in figure. It consists of a 555 timer connected as a stable multi-vibrator, driving an output transistor which switches the IR emitter ON & OFF. The duration of the transmitted light pulses is about 10ms and the repetition rate is just less than 1 KHz. The average current drawn by the circuit is about 12mA and the peak current through the IR diode is around 700mA.
- The power supply for the transmitter is not critical provided the output voltage is not greater than 9V as this could result in the maximum current rating of IR diode.
Receiver circuit :
- The receiver circuit is shown in figure. An infrared photodiode is operated in the reverse bias mode. The leakage current of this diode varies with the light received from the transmitter, which causes a varying voltage to appear across resistors of the transistor T1. The signal appearing at the collector T1 is fed to IC1 (TBA120), which is used as an amplifier and limiter. P1 varies the sensitivity by altering the reverse bias voltage of the diode D1.
- When light pulses are being received from the transmitter, a negative going pulse train with amplitude in excess of 1V peak-to-peak appears at the output of IC1 (pin8). This turns T2 ON & OFF continuously, charging up C11. T3 is thus always turned ON, T4 is turned OFF and the relay is not energized.
- When the light beam between the transmitter and received is interrupted, the amplitude of the pulse train from the output of IC1 will fall. T2 will be cut off, C11 will discharge, T3 will turn OFF and T4 will turn ON. This energizes the relay. Once the light beam is restored the relay will, of course, drop out again but can be made to hold in for several seconds after the light beam has been restored by adding the components shown dotted. R12 and C12 are designed according to holding time. Alternatively latching arrangement may be used that will hold the relay on until a reset button is pressed.
About TBA120 :
The TBA120 is produced by several manufactures and several different versions are available. All of these should function satisfactorily in the receiver circuit. However, in some cases it may be necessary to omit R6 or connect it to ground instead of +V, to obtain the best signal to noise ratio. Care should be taken when alerting R6 not to disturb the relative positions of the receiver and transmitter, as this could give false results.