DTMF encode using CM8870

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sumittokle

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hi.............

we use CM8870 for DTMF encodig.......................
and we made circuit according to the datasheet but then also we don't get output

i attached CM8870 datasheet with this email.....................

1: according to the page no 6 we make our ckt ...................
2: connect mobile head headphone on pin no. 2 after capacitor.........................
3: give the 6 volt power supply (using 5 voltage regulator)................
4: but on output pin we don't get any output when we call ....................



where we get wrong...........................


please reply us.......................
 

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  • CM8870.pdf
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1. why 6v? The 5v regulator will need atleast 7v at the input to give 5v regulated output,and the CM8870 datasheet says the supply limit 4.75-5.25. Try to apply more volt at the regulator input
2. when u call, you won't get any output!! for getting output, you have to press keys on the transmitting cellphone once the call is received by the cellphone where the CM8870 is connected!!!

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Design with Microcontrollers
 
dtmf decoder cm8870

i press the key but i didnt get any output.................

and also i


does we directyly cut headphone wire and insert into our ckt.................
 
sumittokle said:
does we directyly cut headphone wire and insert into our ckt.................
Click to expand...

Yup!
cut one of the two cables going to the two ear-plugs. Remove the sleeve for uncovering the two wires inside the cable (one is signal and other common). connect the signal wire to DTMF ic and the common wire to ground.

Actually, the circuit which you are using from datasheet is already tried by many (including me), it should definitely work. For testing your connections or the DTMF ic, better you connect the circuit first with a telephone landline. There you can directly dial any digits from the same phone keypad and find the digital output from the DTMF decoder (keep the telephone in 'tone' dialling mode).

---------------
Design with Microcontrollers
 
You need to make sure you do not overdrive the input of the DTMF chip. The DTMF part has a min/max AC input spec in the data sheet. Your input to the DTMF chip should be in this range.
 
I think the DTMF input level is too low since the two 100k resistors make a gain of only 1. Try 10k for the 100k resistor in series with the 0.1uF capacitor at the input.

Pin 15 has nothing to do with ringing. A cell phone doesn't ring like a telephone anyway. The datasheet says:
"Presents a logic high when a received tone pair has been registered and the output​
latch is updated."
 
i have connected the PSTN line with cm8870. my project needs to hookON the phone automatically, how it is possible? ............
how the cm8870 detects the pstn ringing?
 
katteldanda said:
i have connected the PSTN line with cm8870. my project needs to hookON the phone automatically, how it is possible? ............
how the cm8870 detects the pstn ringing?
Click to expand...
The old CM8870 is a DTMF decoder. A separate circuit is needed to detect a phone line ringing and to answer the call. Then the separate circuit places the CM8870 in the audio path.
 
Hi i am using CM8870 dtmf decoder i.c. But not getting output
Can anyone suggest me the alternative for CM8870 I.C. Reply if anyone has the solution for it
 
I am also having the same problem well i think so replacing CM8870 I.C. with MT8870 of HT9170 I.C. is better option

The hands free connection has four wire when you will cut it blue colour is tip and red one is ring and rest two are seleve which are not connected
Connect tip to pin no as shown in CM8870data sheet and ring to ground.
 
Hey i am also having the same problem i think replacing the CM8870 I.C. with MT8870 of Ht91708 is an better option, go for it and if you
get the result successfully the do reply back
 
Sandeep_1905 said:
Hi i am using CM8870 dtmf decoder i.c. But not getting output
Can anyone suggest me the alternative for CM8870 I.C. Reply if anyone has the solution for it
Click to expand...

Post your circuit diagram, I forgot my miracle orb. I am not sure of any alternate device as I suspect it is done with embedded applications these days.
 
i too have the same trouble as said by "sumittokle".

I connected the circuit to mobile phone by cutting the wires going to one of the earphones. But there is no output.

so iam planning to connect to Land line directly. But is it possible to connect this Ic directly to land line? i mean do we need some other circuit in between the LL and dtmf decoder?
 
If you connect a telephone line to the obsolete DTMF decoder IC then the IC will blow up. It must have isolation and protection circuits.

The telephone line is 50VDC and has ringing that is 177V peak in Canada. It might be different in your country.
We played with DTMF controlling things 30 years ago.
 
connecting 8870 to mobile phones

well as some of the friends in this community, i too experienced some difficulty in interfacing 8870 with mobile phones.


the solution is very simple- the main problem is that we have to establish a good electrical contact with the circuit and signals coming out of the mobile phone. The circuit given in datasheet(single ended input) works very well. Take a wired headset of the cell phone(i used an old one of nokia1110), cut the wire going to earphone. Now you can see two shiny conductors. Most of them mistake that they can be directly connected to circuit(infact they have an insulation coating). Infact each of those conductors have a teflon coating. Just take a lighter and heat those conductors so that the coating burns awar. Now solder these to the circuit through a pair of wires.


it works!!!!
DING DONG
 
to audioguru--doubt

well... then how can we interface the 8870 with land line?

How to findout the line voltages in different countries? And can u pls post some interfacing circuits for the same and some resources related with telephones?
 
The 8870 was used 25 years ago in telephone answering machines. It never connected directly to a telephone line. It always had answering circuits and maybe a transformer with protection diodes.
A few countries still use DTMF instead of digital for caller ID.
 
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