LDR Dark Detector (LED Turns On in the Dark)
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A dark detector with an LDR (photoresistor) and a BC557 PNP transistor. When it gets dark, the LDR's resistance rises, the transistor turns on and the LED lights up. Turn the potentiometer to set how dark it must be.
Parts: 9V battery, LDR, 100 kΩ potentiometer, 1 kΩ resistor, 470 Ω resistor, BC557 PNP transistor (or a 2N3906) and an LED.
Full explanation: LDR Circuit Diagram on Build Electronic Circuits.
LDR Dark Detector (LED Turns On in the Dark) Schematic
The LDR and the potentiometer R2 form a voltage divider. A PNP transistor turns on when its base is pulled about 0.7V below its emitter. In the dark, the LDR's resistance is high, so R2 pulls the base down and Q1 turns on the LED. In light, the LDR pulls the base up close to 9V and Q1 turns off. R3 limits the base current, and R1 limits the LED current.
LDR Dark Detector (LED Turns On in the Dark) Parts List
How to Build the LDR Dark Detector (LED Turns On in the Dark)
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); the transistor's emitter goes to plus. 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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