The presented is a system that can recognize a tone that is superimposed on the voltage of the power grid. The circuit is shown in the figure. This circuit needs an input signal from 50 mV to 500 mV so that, when recognizing the tone, the relay is activated. An application for this circuit is in a remote-control system via the network or in the remote sensing of the functioning of a machine using the power network to transmit the information. The T1 transformer can be wound on a ferrite rod consisting of 200 + 200 turns of enameled wire 32 AWG or thinner. The values of the other components are selected for a carrier frequency of 100 kHz. This same configuration can be connected to the output of an IR or radio receiver to obtain a tone-modulated remote control. It must be considered that the fact that harmonics can cause random activation greatly limits the number of channels that make up a remote-control system. Thus, in practice, adjusting the frequencies is already critical when the 6-channel limit is exceeded. In this case, one channel can already interfere with the other, both due to selectivity problems and the recognition of harmonics.
