This versatile dark sensor is made up of two transistors. It is a versatile module, because you can use it for testing several electronic components, such as, relay, reed switch, light dependent resistor (LDR), electret microphone, phototransistor, photodiode, etc. Besides, you can use this module for making simple clap switch, light operated switch, water level indicator, etc.
Dark Sensor Circuit
When you block the light falling on LDR, the transistor Q1 switches off and transistor Q2 switches on, that drives the LED D1. Change the value of R3 to change its sensitivity.
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