Help interfacing DTMF 8870 decoder with an audio source

Thread Starter


Joined Oct 2, 2015
Hi all,
I'm using DTMF audio decoder based on 8870 chip. It's a breakout board that I bought from Ebay like one of these:

The decoder works fine if I connect it instead of a speaker of a desktop phone's receiver. However, the decoder doesn't work if I connect it instead of (or in parallel to) an answering machine's speaker.
I don't have an oscilloscope and can't see the difference in audio signals between a phone receiver and an answering machine's speaker.
Can somebody point out why one works and another doesn't?

Thank you.

Thread Starter


Joined Oct 2, 2015
I'll try to explain again since there were no answers to my question.....

DTMF decoder works if I take the audio from desktop phone's handset (remove speaker from the handset and use those leads to take audio).
However, DTMF decoder doesn't work if I use answering machine instead (remove speaker from the answering machine and use those leads to take audio).
Is it a difference in voltages between headset speaker voltage and answering machine speaker voltage that fails the DTMF decoder?
I don't have oscilloscope, only multimeter, and can't really see what's going on with those two audio sources.
Thanks all.


Joined Sep 22, 2013
There is a big difference in microphone level and speaker level. IF I understand you correctly.

Have your got a print on the decoder board?


Joined Nov 29, 2005
Read its data sheet and check the permissible audio input level. A phone earpiece should be around -9dBu (0.25V), the answering machine can be 100 times louder at its speaker level, deafening the 8870.

Try a much lower volume or resistors to attenuate the speaker level; or pick the signal at its volume control wiper instead of at its speaker.