The Photo Cell Block contains a component commonly called a photoresistor or an LDR (light dependent resistor). As more light strikes the surface of the photo cell, the resistance of the component decreases. Like the potentiometer, the photo cell controls things over a range. The Photo Cell Block works best with LEDs and also the buzzer. (With the buzzer, you'll need a bright light source, such as a flashlight, that you can move fairly close to the surface of the photo cell.)

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