這是利用可見光作監視的門衛電路。發射器改用白色發光二極管，接收可見光束的傳感器件也改用光敏電阻(LDR)，控制電路采用uA741運算放大器作電壓比較器。預置電位器VR1提供基準電壓至uA741(IC1)的同相輸入端③，而光敏電阻LDR1和電阻R1則連接至IC1的反相輸入端②。

LED1和LDR1分設在大門進口的兩側，這樣，在大門關上時，從LED1射出的可見光束可落到LDR1上面。

在有LED光束射至LDR1時。LDR1的電阻相比R1變小，因此IC1的輸出腳⑥變低電位，這時LED2開始閃爍而蜂鳴器PZ1則停止嗚叫，這表示大門是關著的。

當有人打開大門進入或走出時，從LED1照射到LDR1的光線被阻斷，LDR1的電阻變得極高。結果IC1的輸出腳⑥變高電位，這時LED2停止閃爍而蜂鳴器PZ1則發出鳴叫，這表示大門已經被人打開。

google translate:

Using visible light to monitor guard
This is the use of visible light for monitoring the guard circuit. Using white light-emitting diode transmitter, the receiver can use photosensitive resistor (LDR), control circuit uA741 op amp for voltage comparator. Preset potentiometer VR1 to provide reference voltage to uA741 (IC1) of the same phase input ③, and photosensitive resistance LDR1 and resistance R1 is connected to the inverting input of IC1 ②.
LED1 and LDR1 situated at the entrance on both sides of door, so that when in close the door, the receiver can receive the signal.
when LDR1 receive the signal. the resistance of LDR1become smalle, so the output of IC1 (⑥ )goes low potential , then LED2 will start flashing and the buzzer PZ1 stop, which means that the door is closed.
When someone opens the door to enter or get out when the light from LED1 to LDR1 is blocked, LDR1 resistance becomes extremely high. Results IC1's output pin⑥ becomes higher potential, then LED2 will stop flashing which means that the door has been opened.