Dual-Tone (Frequency) Decoder (Detector)

The output of this circuit will be activated when a mix of two tone/frequency is detected on the input. The frequency f is determined by the values of R1 and C1, where f=1/(1.1 * R1 * C1).

Dual Tone Decoder circuit schematic diagram

For security, using dual tone is more secure compared with using only single frequency. With single frequency, someone can easily find the secret frequency by scanning the input with a variable frequency function generator. Using dual frequency, the scanning process will be harder. [Circuit’s schematic diagram source: Philips Semiconductors Application Notes]

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