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 Â·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.