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).
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]