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1	this circuit makes use of a small speaker which is used as a sound sensor
2	principle of operation : sound comes in through the 0.01 microfarad on the left. the operational amplifier is configured as a very high impedance, high gain, inverter amplifier. the negative feedback resistor from the output to the inverting input is intended to peg the quiescent direct-current output voltage to ground potential. a configuration of this type sometimes results in a momentary op-amp latch-up, on account of the capacitor and high resistance combination. the two diodes configured as a +/- 0.6-volt clamp prevents this latch-up. go to top    next
3	principle of operation : the amplified sound from the first stage is accepted at the input of an inverting amplifier with a gain of (negative) Rf/Ri, 100/10, or 10. a potentiometer is useful for adjusting the gain of this circuit. the left node of the R & C combination is kept at ground potential, in order to peg the quiescent dc output voltage at ground. an input voltage below -50 millivolts will produce an output higher than +0.5 volt. this positive voltage is sent back to the non-inverting input via the diode to force the op-amp to latch-up with its output about 1.5 to 2.5 volts below the positive rail. the resistor will keep the non-inverting input at 0.6 volts below the output voltage. in time, the input capacitor will charge and the inverting input will be pulled up by the 100k resistor towards the output voltage. the inverting input crosses the voltage level of the non-inverting input as it traverses up to the output voltage. this fact will force the op-amp to un-latch and go back to its quiescent output state at ground potential, to wait for the next trigger. go to top    prev
4	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.