Electronic Cicada

Project Number: 112

OBJECTIVE: To show how capacitors in parallel change the frequency of an oscillator.

Description: Use the circuit from Project #110 above, replace the photoresistor (RP) back to the 10K ohm resistor (R4). Place the 0.02uf capacitor (C1) above the whistle chip (WC). Turn the slide switch (S1) on and adjust the adjustable resistor (RV). This circuit produces the sound of the cicada insect. By placing the 0.02uf capacitor on top of the whistle chip, the circuit oscillates at a lower frequency. Notice that the LED (D1) flashes also at the same frequency. It is possible to pick resistors and capacitors that will make the pitch higher than humans can hear. Many animals, however, can hear these tones. For example, a parakeet can hear up to 50,000 cycles per second, but a human can hear up to 20,000.