When it’s dark and the photoresistor value is high, the output from an unconnected voltage divider would be around 4.5V.īut since the output of the voltage divider is connected on the base of the transistor, the voltage will be limited by the forward voltage of the base-emitter connection (around 0.7V). When it’s light and the photoresistor value is low, the output from the voltage divider is around 0.5V, which is not enough to turn on the transistor. What Are the Voltages out From the Voltage Divider? That means the LED is also on and will light up. That means the voltage divider gives a high output voltage which turns on the transistor. When it’s dark, the photoresistor will have high resistance. So the transistor is off and cuts off the current to the LED. When there is a lot of light, the photoresistor will have low resistance, which means the voltage divider gives a low output voltage. The photoresistor and the 100 kΩ resistor make up a voltage divider. I recommend using a breadboard since it’s quick and you can easily reuse components.īelow you can see how I connected this circuit on a breadboard: How The Night Light Circuit Works There are many ways to connect this circuit. Photoresistor (around 5kΩ in light, 200kΩ or more in dark).
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