Light control switch SCR circuit diagram

As shown in the picture, it is a light-controlled electronic switch. 555 is used to control the on or off of SCR , and a photoresistor is used as a light sensor.

The circuit is as shown in the picture, the LDR is the light dependent resistor and the 555 works as a hysteresis comparator. When the light intensity shining on the LDR reaches a certain level, the voltage on the LDR is reduced to one-third of the power supply voltage, triggering 555 to make the output of pin 3 jump to high level, triggering the SCR to turn on through C3, and the load (Photometer, etc.) is powered on, when the light weakens to a certain value, the voltage on the LDR rises to two-thirds of the power supply voltage, the 555 output jumps to low level, and 3 CD is instantly turned on through C4, so the SCR The anode is cut off because there is no current .

R1 is used to adjust the light control sensitivity, and R3 is used to adjust the output voltage (5V) of the voltage regulator W317. The currents of D1, D2 and SCR should be selected according to the load (photometer) current, and the withstand voltage only needs to be 35V or more. 3 The maximum allowable current of CD is greater than the load current . If necessary, multiple PNP power transistors can be used in parallel, or germanium PNP transistors can be used instead. In addition to the photometer, the load can also be other things such as electric lights, motors, etc.