Dark sensor using transistor, phototransistor and photodiode

Description: 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.  Experiment 1: Dark sensor using Light dependent resistor(LDR). THIS CIRCUIT WORKS WITH 9V ALSO. 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. EXPERIMENT 2: DARK SENSOR USING PHOTOTRANSISTOR For testing a phototransistor, just replace the LDR with a phototransistor, it simply works. EXPERIMENT 3: DARK SENSOR USING PHOTODIODE For testing a photodiode, just replace the LDR with a photodiode, it simply works. Anode of photodiode should be connected to base of transistor Q1(as shown in Figure below). Schematic made using SchemeIt. WATCH THE VIDEO: EXPERIMENT 4: RELAY TEST  Make dark sensor using relay.