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Sound to Light

Zero-crossing detector

Transistor: general purpose pnp

This circuit gives out a short pulse each time the 220v supply passes through zero.

N.B. There is only one cut on the copper side of this board; you should be able to see where it must be!

Modulator

The pulses are used to set (start) a monostable (made using a CMOS 555 timer). When the timed period is finished, the monostable resets and gives a pulse which causes the triac to start conducting. The time period of the monostable is varied using a rectified and smoothed voltage taken from the music source. Opto-isolator: MOC3020 or similar. Op-amp: 081.

The capacitor C determines the speed of response of the system; try about 1µF.

If a low pass filter is connected to the input, the light will be modulated by the bass notes in the music. With a high pass filter, the light will respond the treble notes.

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