One of the light control switch circuits controlled by flashlight

You can use a flashlight to control lights or household appliances on and off within 6-10m. (1) One of the light-controlled switch circuits controlled by
flashlight.     The circuit is shown in Figure 2-102. It uses the phototransistor VTi as the detection element, the three-terminal fixed integrated voltage regulator A as the working power supply, and the triode VT4 as the switching tube.     When the phototransistor VTi is not illuminated by the flashlight, the relay KA is released. When VTi is exposed to the flashlight for the first time , KA is engaged and the load is turned on; when the flashlight shines on VTt again, KA is released and the load is cut off.     Adjust the potentiometer RP so that the light control distance is far away and will not be affected by indoor light and cause malfunction. Adjusting the resistance of resistor R3 can change the triggering sensitivity of thyristor V, that is, the sensitivity of the device.