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2015-09-25 17:13  
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A sensitive electret microphone picks up the sound and feeds the signal to a two-stage amplifier circuit, consisting of Ula and Ulb. The amplified output of Ulb is fed ?·to a voltage-doubler circuit (comprised of Dl, D2, C4, and C5). The output of the doubler is input to the gate of Ql.

When the de voltage reaches the gate`s threshold level, Ql switches on, starting the recorder. Resistor R6 sets the circuit`s sensitivity and should be experimented with to obtain the optimum adjustment.

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