This is a follow-up question of "Multiple DTMF circuit interfaced to one microcontroller, will it work?
Multiple DTMF circuit interfaced to one microcontroller, will it work?
I used the differential mode of the CM8870 DTMF decoder IC to isolate the three phone lines from the ground. I have finished the board. When I connect the phone line to the RJ11 jack, I do get a dial tone. When I pressed a number, I also get the number pressed displayed in my console. But when I try it to get the caller's number, I do not get anything anymore. Here is the circuit.
Analysing and comparing the circuit from the previous one (note that the previous one worked!), I thought of increasing the signal gain might solve the problem. I increased it to have 3.3 signal gain and added a capacitor of 0.1uF in pin 17 and pin 18 to filter noises (since I realized that this is really needed.) Unfortunately, it still didn't work.
Comparing the previous circuit to the present one, it seems there are differences which are as follows:
(Present) versus (Previous)
1.) Differential mode versus single-ended mode.
2.) 330K ohm resistor versus 390k ohm resistor in pin 17 and pin 16.
3.) pin 5 and 6 are grounded, as opposed to floating.
4.) 100pF series capacitor versus 0.1uF series capacitor
Am I missing something?
Best Answer
Are you trying to decode caller-ID signals? If so, those signals are FSK, not DTMF.
If you are NOT trying to decode caller-ID signals, please explain better what you are trying to accomplish.