A solution for using more than 6 channels in a tone-modulated system that uses LM567 as decoders, is to use sequential tones. The application of the figure shows how to do the recognition of two sequential tones using two NE567 in parallel. In this circuit, the time constants of the circuits are calculated so that the output of the first one remains at the low level when the second tone reaches the second , in order to achieve the simultaneous low levels on the NOR gate that provides the external signal. Obviously, in this case, the use of harmonic tones should be avoided so that any other blocks that operate in the same system do not erratically activate.
