This circuit was originally developed in 1989 to allow the triggering of loads from audio signals generated by an MSX microcomputer. However, it can be used for the same purpose to serve as a control shield for Arduino or other microcontrollers. With it, the audio signal generated by the microcontroller can trigger one of 4 relays, according to its frequency, determined by the double T. The circuit can be powered by voltages from 6 to 12 V according to the relay and the trigger signal can be 3, 3 to 5 V.
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