Problem with DTMF decoder.

I constructed the circuit with cm8870 (DTMF decoder) as shown in it's data sheet**broken link removed**
It is working wen I connect it's input to my PC and generate DTMF.
but when I use to connect my cellphone headset output as input to the DTMF decoder, it is not working.

Hi, it could be due to the phone, or the phone service provider, or even due to which provider or calling card company the caller used. DTMF gets decoded and sent out of band through the network, and then gets converted back to tones again. And maybe many cellphones are not designed for outputting a high quality tone through the headset, since most people have no need to hear it. That last bit is just a guess. What does it sound like through the headset, clear?
Also, what is the voltage level coming out of the cellphone? It needs to be between about 50mV and 1V when connected to your circuit. If it is not in that range, adjust the volume.

sky_123 said:

Hi, it could be due to the phone, or the phone service provider, or even due to which provider or calling card company the caller used. DTMF gets decoded and sent out of band through the network, and then gets converted back to tones again. And maybe many cellphones are not designed for outputting a high quality tone through the headset, since most people have no need to hear it. That last bit is just a guess. What does it sound like through the headset, clear?
Also, what is the voltage level coming out of the cellphone? It needs to be between about 50mV and 1V when connected to your circuit. If it is not in that range, adjust the volume.

Click to expand...

when I connect the head sets i can hear the DTMF tones.
and I'm not sure about voltage livels. I have no scope to mesure. can i do it with multimeter?

the circuit cant decode even wen PC volume is low.
and I set cellphone volume to full. but no use.

It's very hard to say. Also, make sure you stick a ferrite through the wires between the cellphone and your circuit, in case any RF is getting into the decoder ic.
Also, maybe the cellphone is not outputting any audio if it detects that the headset is missing (again this is just a guess). You should keep the little speaker connected, in case it checks. Or find some other way to confirm this point.

can i use a audio amplifier to boost up the signal ?

sagar474 said:

I constructed the circuit with cm8870 (DTMF decoder) as shown in it's data sheet**broken link removed**
It is working wen I connect it's input to my PC and generate DTMF.
but when I use to connect my cellphone headset output as input to the DTMF decoder, it is not working.

Click to expand...

On Page 7 in 8870 datasheet PDF file on first circuit example in input section after 0,1uF capacitor reduce 100K resistor to 5,1K or 6,8K and try. Second resistor leave 100K, he connect pin3 and pin2.

This should do the job. :wink:

If possible, post a picture here of how you have connected the headphones to the decoder.

The service provider of the GSM has nothing to do with dtmf tones which are generate dby your handset. If you are connecting your cellphone headsets to the DTMF decoder with 8870, Note that some headsets doesnt use ground from the cellphone and your DTMF decoder needs a ground as reference. Cross check the headset connections and re- connect ground of the cellphone to ground terminal of decoder circuit and live wire of the cellphone earpiece to + terminal of the decoder (usually it is Pin2).
Never use a high power or boosted DTMF signal for the inputs of the 8870. They are highly sensitive and can detect even weakest signals. If you have an acess to an Oscilloscope verify that the DTMF are been received by the 8870 at its pin 2.

Cheers