Sound-activated-tape-switch

Description: This circuit can cause a cassette recorder to automatically turn on and record when a sound or noise is present. Another use is when the sound-activated switch is used to turn on a cassette player, allowing it to function as a burglar alarm detector and sounder. Operational amplifiers U1a and U1b are connected in tandem to amplify the sounds picked up by the detector's microphone. The amplified audio voltage, output at pin 7 of U1b, is fed to a voltage doubler circuit consisting of D1 and D2. The elevated voltage from the doubler circuit is input to the positive input of operational amplifier U1c, which operates as a simple comparator circuit. The other input of U1c is connected to a voltage divider that sets the switching point for the DC signal voltage, triggering the circuit when the signal level exceeds approximately 1.5 V. When the comparator activates, its output at pin 8 becomes positive and supplies a forward bias to turn on D3 and Q1, which in turn starts the recorder. The RC combination of C4 and R9 determines the cassette's run time after the input sound has ceased, preventing the recorder from stopping or cutting off between closely spaced sounds or words detected by the microphone. The delay time is approximately 6 to 8 seconds. Resistor R11 sets the circuit's gain. A low-impedance cassette microphone should be connected to the amplifier's input, and the output of Q1 should be connected to the cassette's remote input or to the internal input, with the recorder set to the record position. The amplifier's gain can be adjusted with R11 for the desired sensitivity.

This circuit is designed to provide automatic activation of a cassette recorder in response to ambient sound levels, making it suitable for applications such as security systems. The dual operational amplifier configuration ensures effective sound amplification, allowing for reliable detection of noise. The voltage doubler circuit enhances the output voltage, ensuring that the comparator receives a sufficient signal to trigger the recording function.

The comparator, configured with a voltage divider, establishes a threshold that determines when the circuit activates. This setting is crucial for filtering out background noise and ensuring that only significant sounds, such as speech or alarms, will trigger the recorder. The inclusion of a delay mechanism through the RC time constant formed by C4 and R9 prevents premature stopping of the recording, allowing for a seamless capture of sounds that occur in quick succession.

Additionally, the gain adjustment feature provided by R11 allows for fine-tuning of the circuit's sensitivity, accommodating various environments and sound levels. The recommended use of a low-impedance microphone ensures optimal performance, as it minimizes signal loss and maximizes the clarity of the captured audio. The connection of the output from Q1 to the cassette's input enables direct control of the recording process, streamlining the operation of the device.

Overall, this circuit represents a practical solution for sound-activated recording, with versatile applications in both personal and security-related audio capture scenarios.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.

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.