DTMF dialer and phone interfacing circuit

Other Hardware General Electronics
1

Hi all,

An alarm GSM - sms communicator accepts messages from the main alarm panel in the form of DTMF coded pager messages. Connecting a standard DTMF phone set in the place of the alarm panel and following the same DTMF coding enables a user to also send SMS messages from the telephone set to the GSM communicator. In other words, the GSM communicator simulates the behavior of the telephone exchange and appears to the telephone set as exactly that. It has a tip and ring terminals as well.
I am trying to use the above feature to send SMS messages using an arduino in place of the standard telephone set and a dtmf encoder chip like HT9200B. In addition to the IC and the associated components listed in the datasheet, I have used an 600:600 ohm isolation transformer to interface with the phone line. The primary winding is connected to the phone line (gsm communicator) via a relay and a 470ohm resistor in parallel to enable the arduino to seize the line once the relay is energized. A 0.33 uF capacitor is also connected in series to block the DC signal from loading the transformer.
I have tested this with a standard telephone line and it works fine. The line is seized when the relay closes and the arduino is able to dial a number with dtmf.
Next I replaced the phone line with the alarm panel connection. The Arduino is again able to seize the line, but the DTMF codes are not recognized by the alarm panel. (the communicator’s dial tone doesn’t go off)
Trying the same with a standard telephone set works fine.
This is abviously an issue of the interfacing circuit I described above, but I cant figure out why it works with a telephone set but not with the alarm panel which essentially simulates a telephone exchange.

If anyone has experience with phone line interfacing circuits and could offer assistance would be greatly appreciated!

Thanks

2

Hi, The transformer winding should not go directly across the phone line. Not sure from your description. The resistor alone should be connected directly across the line. In my experience, the best actual value is 1000 or 1200 ohms. The capacitor should block DC from the transformer..

Try different resistances across the alarm panel telephone line to see what it expects. Too low and it may not work. The PlainOldTelephoneSystem is very forgiving of electrical sins...

3

Hi, The transformer winding should not go directly across the phone line. Not sure from your description. The resistor alone should be connected directly across the line.

The resistor is connected across the phone line once the relay closes. The winding is also connected to the phone line with a capacitor in series as mentioned earlier.

Using higher resistors, the line is not being siezed. Thats why I ended up with 470 Ohm. I might be able to increase it a bit though.