DTMF problem

[josi]

hi! i'm building a dtmf decoder. I builded the circuit exactly like it's on the picture (attachment) the only difference is that i used a nokia 3310, when i press the keys on the 3310, the dtmf chip (MT8870DE) doesn't decode the tones as expected! can anybody help? thanx

 

[aljamri]

hi! i'm building a dtmf decoder. I builded the circuit exactly like it's on the picture (attachment) the only difference is that i used a nokia 3310, when i press the keys on the 3310, the dtmf chip (MT8870DE) doesn't decode the tones as expected! can anybody help? thanx

the 3310 signal is much lower than the land line telephone. By the way how do you connect it into the mobile?!

 

[Mikebits]

From first glance C1,C2 are wrong value. You show 100pf pretty small for audio, should be more like .1uf.
Also the Q outputs of the mt8870 don't have enough current to drive leds at least, not to well.

 

[josi]

the 3310 signal is much lower than the land line telephone. By the way how do you connect it into the mobile?![/QUOTE
i connected the 3310 speaker on the 2 caps, and i mesured the frequency of each key! they r standard value!

 

[Mikebits]

To verify what I say about your input caps. Just use the formula

XC=1/(2pi(FC)) Where F is DTMF frequencies and C is 100*10^-12.

 

[josi]

To verify what I say about your input caps. Just use the formula

XC=1/(2pi(FC)) Where F is DTMF frequencies and C is 100*10^-12.

Mmm! so basically what u saying is if key 1 frequency is1.2 khz i shoul do this:

1/(2pi(1.2K*10^-12)) wich is 1326291.19 isn't that too big???

 

[Mikebits]

Mmm! so basically what u saying is if key 1 frequency is1.2 khz i shoul do this:

1/(2pi(1.2K*10^-12)) wich is 1326291.19 isn't that too big???

Exactly! If you note from the formula: With F constant, Xc (Kinda like AC resistance) decreases as capacitance increases. Ergo, you need to increase your C1,C2 values.

Take a read here which talks about Xc.
**broken link removed**

 

[aljamri]

the 3310 signal is much lower than the land line telephone. By the way how do you connect it into the mobile?![/QUOTE
i connected the 3310 speaker on the 2 caps, and i mesured the frequency of each key! they r standard value!

That's the frequency, what about the amplitude ?

 

[josi]

Exactly! If you note from the formula: With F constant, Xc (Kinda like AC resistance) decreases as capacitance increases. Ergo, you need to increase your C1,C2 values.

Take a read here which talks about Xc.
**broken link removed**

higher value caps are giving me better response than lower values in terms of signal stength, but i still hav the same problem where i can decode some tones and not other. in other words keys 2,5,8,0 have the same frequency when pressed (1.3Khz) but only 5,8 and 0 giv me an output (wich is not even the binary equivalent as it was suppose to be) and 2 hiv me nothing! (same for 1,3,4,7,*) i don't understand!

 

[Mikebits]

Are you saying it is working better with the new caps, but not quite? You may need more gain on your input. You could try increasing R3 a bit. One other thing is that your circuit seems to be missing a .1uF cap from pin 17 to +V, try this first. See data sheet.
https://www.alldatasheet.com/datasheet-pdf/pdf/77085/MITEL/MT8870.html

Hope that helps