Photosensitive Electronic Organ

Project Number: 111

OBJECTIVE: To show how resistor values change the frequency of an oscillator.

Description: Use the circuit from Project #110 above. Replace the 10K ohm resistor (R4) with the photoresistor (RP). Turn on the slide switch (S1). The speaker (SP) will sound and the LED (D1) will light. Move your hand up and down over the photoresistor and the frequency changes. Decreasing the light on the photoresistor increases the resistance and causes the circuit to oscillate at a lower frequency. Notice that the LED flashes also at the same frequency as the sound. By using your finger, see if you can vary the sounds enough to make this circuit sound like an organ playing.