When the phototransistor comes on in the circuit, this element passes through the conduction transistor BC547. When BC547 is transmitted, relay is activated and lamp lights up. When the light from the phototransistor is turned off, the relay returns to the first state. The value of the desired intensity of illumination can be adjusted by operating the R1 potentiometer. As can be seen, the circuit is sensitive to daylight. We need to close the lens of the phototransistor with a dark transparent plastic, as this system only needs to be sensitive to infrared rays. When we do this, the phototransistor only detects the rays emitted by the infrared diabetic donors.
For example, let’s think about what we need to do to set up an alarm in a room. When this is done, a mini infrared transceiver circuit is mounted on one side of the room. The circuit in figure 3.22 is placed in the wall opposite this transmitter. When an object enters between the two circuits, the infrared rays coming from the phototransistor are cut off. This causes the phototransistor to cut through the transistor BC547. The cutting BC547 changes the position of the relay contacts and the burning lamp goes out.