When the phototransistor is in the circuit, this element transmits and transmits the BC547 transistor. When the BC547 is switched on, the relay pulls and the lamp illuminates. When the light from the phototransistor is turned off, the relay returns to its original state. With the R1 trimpot, the value of the desired illuminance intensity can be adjusted. As shown, the circuit is sensitive to daylight. If we want this circuit to be only sensitive to infrared rays, we need to cover the lens part of the phototransistor with dark transparent plastic. When we do this, the phototransistor detects only the rays emitted by transmitters with infrared diodes.
For example, let’s think about what to do to set up an alarm in a room. A
mini-infrared diode transmitter circuit is installed on one side of the room during this operation . The circuit on the wall opposite this transmitter is placed in Figure 3.22. As soon as an object enters between the two circuits, the infrared rays from the phototransistor are cut. This causes the phototransistor to interrupt the BC547 transistor. The BC547 that goes to the cut changes the position of the contacts of the relay and the illuminated lamp goes out.