DTMF 8870 not working properly

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toshalimitra

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I am using dtmf 8870 decoder for my project of cell phone based voting machine.The decoder is giving correct output whenever any input is being given by the phone(say, M1) connected to the circuit..but whenever i am giving any input from another mobile (say M2) through M1 it is not responding to the inputs other than 3,6,9,#..

what is the reason behind it? and what is the solution?:-?:?:
 

check the capacitors and crystal oscillators if possible change them and than check...
 

i checked them twice..and if the problem would have been in the capacitors and crystal oscillators, then how come the mobile connected to the ckt is performing perfectly? cannot understand..confused :|
 

This may sound silly but can you actually hear the tones from the second mobile through the speaker on the phone connected to your circuit?

When keys are pressed their number is sent over a data stream but not always the audible tone itself.

Brian.
 

yes, i can hear through the speaker connected to the first phone but can't get the output through the dtmf. only 3,6,9 and # are perfectly giving a output but not the others
 

If some tones are working but not others it points to a volume level problem. The way the decoder works makes it unlikely that a frequency error would still detects some numbers correctly. Can you explain how you have the decoder connected to the phone please. For example, are you connecting across the loudspeaker or using the earphone output socket?

Brian.
 

i am using earphone output socket..is there any problem with the earphone and hence the error????
 

i was working on dtmf decoder(MT 8870) lately and during that time i found that it was giving proper o/p on galaxy fit but not able to decode and give o/p on galaxy Y.Just try to plug in the device which gives you undesirable output and make a call from which you are getting correct output.

P.S when you jack in aux wire make sure your phones media volume as well as ringer volume are at Full otherwise you wont get output properly.
 

I see this problem occurs frequently. The 8870 is very sensitive so the volume level should not be critical but on many mobile devices the earphone socket is used for different purposes depending on what is plugged into it. It is possible the phone does not see that an earphone is plugged in so it decides the socket is being used for a different purpose and doesn't send audio to it at all. Try simulating the presence of an earphone by connecting a resistor of say 100 Ohms across the earphone connections. Also check the wiring to the socket, look at real earphones and see if the plug on them matches the one you are using. Some phones use 3 pole jacks, some use 4 pole so a standard plug may not be connecting properly.

Brian.
 


yes..actually i was getting an error because of not setting the volumes high..but still i'm getting the same error..actually i'm using nokia 2730c for both the cases..but not getting the result...getting an o/p for only 3,6,9 and # :-(
 

what is the resistor value of R2(Resistor for gain)try to increase that value and check.That is the only thing which can cause an error of inappropriate output.

P.S Make PWDN pin and INH pin to ground.
 

Pleased post the schematic you are using, including how it connects to the phone and how it is powered (if from AC, please show how the PSU is wired). The gain of the 8870 is usually quite high and certainly enough to work from even a quiet earphone signal.

Brian.
 

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