LDR Light Detector (LED Turns On in Light)
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A simple light detector with an LDR (photoresistor) and a BC547 transistor. When light hits the LDR, its resistance drops, the transistor turns on and the LED lights up. Turn the potentiometer to set how much light it takes.
Parts: 9V battery, LDR, 100 kΩ potentiometer, 1 kΩ resistor, 470 Ω resistor, BC547 NPN transistor (or a 2N3904) and an LED.
Full explanation: LDR Circuit Diagram on Build Electronic Circuits.
LDR Light Detector (LED Turns On in Light) Schematic
The LDR and the potentiometer R2 form a voltage divider. In the dark, the LDR's resistance is high and the voltage at the base of Q1 stays too low to turn it on. In light, the LDR's resistance drops, the voltage rises, and Q1 turns on the LED. R3 limits the base current, and R1 limits the LED current to about 15 mA.
LDR Light Detector (LED Turns On in Light) Parts List
How to Build the LDR Light Detector (LED Turns On in Light)
The LDR, potentiometer, transistor and LED stand in row E, and the two resistors lie flat below them. The bottom rails carry the battery's plus (red) and minus (black). If the LED never turns on, turn the potentiometer, then check that the transistor's flat side faces the way it does here.
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