This circuit can cause a cassette recorder to automatically tum on and record when a sound or noise is present. Another use, is when the sound-activated switch is used to tum on a cassette player so that it operates as a burglar-alarm detector and sounder. Op amps Ula and Ulb are connected in tandem to amplify the sounds picked up by the detector`s mike. The amplified audio voltage, output at pin 7 of Ulb, is fed to a voltage`doubler circuit, consisting of Dl and D2. The elevated voltage from the doubler circuit is input to the positive input of op amp Ulc, which is operating as a simple comparator circuit.
Sound-activated-tape-switch - schematic

The other input of Ulc is connected to a voltage divider that sets the switching point for the de signal voltage, to turn on when the signal level is greater than about 1.5 V. As the comparator switches on, its output at pin 8 becomes positive and supplies a forward bias to tum on D3 and Ql, which in tum, starts the recorder. The rc combination of C4/R9 sets the cassette"s run time after the input sound has ceased, preventing the recorder from chopping-up or turning-off between closely spaced sounds or words picked up by the mike. The delay time is roughly 6 to 8 seconds. Rll sets the circuit"s gain. Connect a low-impedance cassette mike to the amplifier"s input, and connect the output of Ql to the cassette"s remote input or to the internal input and set the recorder to the record position. Talk and adjust the amplifier"s gain with Rll for the desired sensitivity.

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