MT8870 DTMF Decoder

[dramrattan]

In the datasheet for the MT8870 there are two configurations in which it can be connected to the TIP and RING lines of a telephone. I want to use the differential input configuration but how do i know what values of capacitors and resistors to use? I know the input capacitors is used to block DC and the resistors form a voltage gain for the amplifier. But again how do i know what values to use? The telephone line is 48V when on-hook, about 90V when ringing and about 10V when dialing or during conversation so do i have to use these values to somehow calculate the capacitor and resistor values to be used?

 

[mneary]

Do not attach the MT8870 directly to the telephone line. The MT8870 is intended to be installed behind a network interface device (not just a few resistors and capacitors).

You have considerable research to do.

 

[Mikebits]

Do not attach the MT8870 directly to the telephone line. The MT8870 is intended to be installed behind a network interface device (not just a few resistors and capacitors).

You have considerable research to do.

I am not so sure about that. See app note.
https://www.electro-tech-online.com/custompdfs/2009/03/mt8870dtmf20decoder20application20notes.pdf

Standard POTS interface should work.

Telephone interfacing circuits

 

[dramrattan]

Mt8870

Yea it is said by most that the MT8870 should not be connected directly to the telephone and should be connected through some 600:600ohm centre tapped transformer but there are products on the market that is working perfectly fine and properly packaged using the direct connection. Thus the application note uses it directly. I guess i would just go ahead and use the values suggested in the MT8870 application note of figure 10 and connect some LEDs at the output and see if i getting the correct readingss when a digit on the telephone keypad is pressed.

 

[Sceadwian]

I haven't brushed up on my FCC law recently but isn't directly attaching a device to a telephone line in the US without isolation circuitry illegal?

 

[dramrattan]

Illegal

Yes attaching equipment is illegal but it's my final year project i currently working on so i'mnot doing anything legal. I'm following instructions given to me by my lecturer in charge All blame is on him if anything happens

 

[mneary]

In a classroom situation Figure 10 is fine. In the real world, the epanorama document is a very good start for developing the interface that I suggested.

Mikebits, thanks for that reference - I didn't know it was so nicely in one place like that!

 

[sumeshm.pnr]

hey can anyone suggest me how can i make a caller id...
it should have an lcd display...... and it should tell some thing to the in comming call like welcome.....