Contactless DTMF switch

This is contactless DTMF (Dual Tone - Multi Frequency) switch to control 16 channel relay boards. This module can turn ON/OFF relays based on DTMF singles which captured from its microphones.

This project is building using 4 commonly available ICs which including TL084, TL081, CM8870, and 74LS154. TL084 is used as microphone preamplifier and LPF. TL081 act as combiner and CM8870 is used to decode the DTMF inputs. 74LS154, the demultiplexer is used as line selector. In this circuit TL084 and TL081 and replace with any compatible operational amplifier IC. During our prototyping, we replace TL084 with LM324, TL074, and TL081 with TL071 and got identical results.

For the microphones, it is highly recommended to use omnidirectional condenser microphones. In our tests, we get good results when both the microphones are in the 90⁰ plane.

This circuit is design to work with single rail 5V DC power source and its output terminal (SV1) is compatible with most of the 16 channel relay boards in the market.

To get maximum performance it is essential to build this circuit on PCB with proper shielding. As per our observations regulated power supply is vital for this circuit and AC ripples on power lines may leads to erroneous results.

Schematic of this contactless DTMF switch is available to download at google drive.

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