Interfacing HT9200A to the phone line

[Fandango]

Hello!

Anybody could please suggest a simplest circuit to interface the DTMF output of the HT9200A serial DTMF generator to the phone line, using an isolator transformer (1:1)?

Thank you in advance.

 

[Mikebits]

https://www.rane.com/pdf/ranenotes/Interfacing%20Audio%20and%20Pots.pdf

 

[Fandango]

Thank you for your post Mikebits. However it didn't explain how to interface the DTMF output pin of the HT9200A to the line transformer. Did you have some idea or example?

 

[Externet]

To feed or extract audio to/from a telephone line, from any chip / source / input / output with a transformer, connect any audio output-type transformer secondary (low or 8Ω impedance) in series to one telco wire.
Connect the primary leads (or high impedance) to the chip you mention output pin and ground.

Telco-------------------------------------Telephone
Telco---------------uuu-------------------Telephone
..........................=====
DTMF out pin------nnnnn------gnd

(ignore the dots)

To use a 1:1 ratio 'wet' transformer, it better be a ~600:600 impedance and can be connected in parallel to the telco wires.

Telco----------------./ .----------------W600Ω 600Ω---< audio pin
Telco-----------------------------------W600Ω 600Ω---< gnd

W= 'wet' winding end

Edited: added---> the DTMF level injected into the telco line should be around 0.3V rms. (~1V p-p)

Miguel

 

[Fandango]

Thank you Miguel !

I guess I did understand. So I think that could be better to use some kind of amplifier between the DTMF pin of HT9200A and the wet transformer, may be an operational like the LM358 with DC decoupler.

 

[Externet]

Hi.
If that chip itself outputs a decent level you need no amplification (highly probable). Just measure the DTMF amplitude in a low AC multimeter scale and as you plan to use a 1:1 ratio transformer, that same level will be the injected level into the telco line, you may end needing to attenuate with a resistor instead of amplifying.

0.2 to 0.3 V rms will be enough.

Edited: added ---> Checked that chip data sheet; shows output being 0.15V rms into 5KΩ. It may work on a lower level. Try it.

DC decoupler ? No need. The transformer does it.

Miguel

 

[Fandango]

Thank you again Miguel.
I will try to build a prototype to check the levels and also write some firmware to control the chip for test it. After this I post the results.
Best regards!

 

[Raz00]

Hey im doing the same project but with pic16f84a and TP5088 and i have a problem interfacing them to the telephone line but i cant find the audio transformer can i use anything else capacitors and resistors ?????

 

[audioguru]

An old telephone answering machine or dial-up modem used a transformer that you can use.

 

[colin55]

You can capacitively couple it to the phone line or use an optocoupler, or even use a transistor and load resistor.
It all depends if you want 5kV isolation or not.

 

[dr pepper]

Heres how holtek suggest you can do it, they make the device.

The 2 trannys and 2 resistors in the centre of the schematic are for on/off hook, you could leave these out and connect the output from the diodes straight to the transistor with the dtmf applied to its base, allthough then it will hog the line.

**broken link removed**