Photoresistive light control switch

Probe domestic switch needle KG-300K, needle head diameter is 3.0mm, normally open switch needle

Photocoupler

The photo-control switch circuit uses a photoresistor as the core component and includes relay control output. There are various configurations, such as self-locking bright excitation, dark excitation, and precise bright or dark excitation. Below are several typical circuits. Figure (2) shows a simple dark excitation relay switching circuit. The working principle is as follows: when the ambient light level drops to a pre-set value, the resistance of the photoresistor increases, causing the transistor VT1 to turn on. This allows the excitation current to flow through VT2, which activates the relay. As a result, the normally open contact closes while the normally closed contact opens, enabling control over the external circuit. The next figure illustrates a precise dark-excitation time-delay relay switching circuit. When the light level decreases to the set point, the resistance of the photoresistor increases, raising the voltage at the inverting input of the operational amplifier IC. This triggers the output of the IC, turning on the transistor VT. The excitation current from VT then activates the relay, closing the normally open contact and opening the normally closed contact to control the external circuit. This type of circuit is commonly used where a delay is needed for lighting control or automation systems.