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, 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 the 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 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.

Project Related Downloads

Schematic of contactless DTMF switch


TL071 - Low-Noise JFET-Input General-Purpose Operational Amplifier
TL074 - Quad Low-Noise JFET-Input General-Purpose Operational Amplifier
TL081 - JFET-Input Operational Amplifier
TL084 - JFET-Input Operational Amplifier
LM324 - Quad Operational Amplifier
MT8870 - CMOS Integrated DTMF Receiver
74LS154 - 4-Line to 16-Line Decoder/Demultiplexer