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this is a very simple circuit and is very much related to the one described for the 555, earlier.
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| 2
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principle of operation :
- the 555 is configured as a variable frequency oscillator.
- the resistive and capacitive circuit parameters are chosen to provide audio frequency.
- sound may be picked up from pins 3 or 7, as the designer so chooses.
- a simple audio driver is provide in another circuit.
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| 3
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principle of operation :
- the input to the variable frequency generator above is one that prevents oscillation.
- this function can be done many ways,
- bring pin #4 (the reset pin of the 555) to ground
- provide a diode to clamp the capacitor voltage low or high
- provide an npn transistor whose collector is at the positive terminal of the capacitor to discharge it or allow it to charge
- provide a diode with its anode at the positive terminal of the capacitor also discharging it or allowing it to charge
- provide another 555 with its pin #7 (open collector output) at the positive terminal of the capacitor also discharging it or allowing it to charge
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| 4
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in summary, sound which crosses the threshold of the amplifier produces the op-amp to latch high. this level must be sensed by another circuit as a pre-determined signal to implement an action.
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