I'm trying to build a DTMF decoder using the CM8870 DTMF decoder IC. I made the project on a breadboard. But the decoding never works!! On powerup, it displays a random BCD output on it's 5 output pins. And then that Outputs stays high till I turn the power off. I used the exact circuit here + a few more decoupling capacitors too.

I'm dead sure about the connections of the circuit. And the clock ( 3.579545 MHz ) is working fine too.. What maybe the problem here? Anybody could help?

 

The circuit looks fine, as it is the same one taken straight from the data sheet.

You can do the following checks:

a. check that pin 4 is approximately half the supply voltage
b. increase the resistor between pin#2 and pin#3(not the one connects to 100nF) from 100K to 220k, 330K or 470K. This increases the input gain from 1 to 2.2, 3.3 or 4.7 to suit your input signal strength.

 

Thank You! Increasing the value of GS resistor solved the problem.. Now it's a 300K resistor..

 

techadiya can you please send me encode circuit of dtmf at <SNIP>

 

techadiya can you please send me encode circuit of dtmf at <SNIP>

Why not share it with everyone else!

 

techadiya can you please send me encode circuit of dtmf at <SNIP>

 

On powerup, it displays a random BCD output on it's 5 output pins.

There is no 'power on reset' state with this device. See here for one way round the problem:

http://forum.allaboutcircuits.com/showpost.php?p=217156&postcount=2

 

techadiya can you please send me encode circuit of dtmf at <SNIP>

I do not have a encode circuit here. I just used the normal telephone and the DTMF encoder software here ( http://www.polar-electric.com/DTMF/InstallDTMFencoder.exe )
Anyway, I'm pretty sure you'll be easily able to build a hardware-based encoder easily. ( Like this one maybe.. http://electrosofts.com/dtmf/dtmfencoder.jpg ) Regards..

 

techadiya can you please send me the working circuit for dtmf decoding with all the issues resolved.