This guard uses an operational amplifier µA741 and a photoresistor (LDR). The operational amplifier µA741 is used as a sensitive voltage comparator. LDR1 and resistor R1 are connected to the inverting pin 2 of IC1. LED1 and LDR1 are installed on the opposite side of the entrance, and the light from LED1 will fall on LDR1.
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