Land line circuit suggestion / DTMF project

Question

I am hoping to make a system that I can call in and using DTMF chip can control few things. I don't want to use a cell phone so it has to be a land line, however, it has to be automated so when the call comes in, I can switch the circuit to my MCU, do my thing, and after I press a certain key or after a minute it disconnects. I am not looking on how to build this, I am mostly interested in dev board that gives me this capability. Does anyone know about 56K data modems? I rather keep it in the embedded level so I don't get into software programming for window, but if you think it's easier please let me know,

Answer 1

Multitech makes a modem that will work in voice mode, we use it all the time for IVR purposes, and for us it seems to work about the best of known voice modems. You will need one of the -v models. The internal modem of the MT9234ZBA-V is one of the socket modems.

http://www.multitech.com/en_US/PRODUCTS/Families/SocketModem/

http://www.multitech.com/en_US/PRODUCTS/Families/MultiModemZBA/ MT9234ZBA-V

This will take some effort, you'll have to open and close the modem, listen for dtmf, etc, but its doable from a micro with uart. Do all your learning from a terminal on windows, then transfer to micro.

Answer 2

An ordinary modem will not be sufficient if you want to pass DTMF. If you can't find one that is designed to detect tones, then what you need is the part of a modem called a Data Access Arrangement. It comprises the isolation from the telephone line, audio balancing hardware, hookswitch, and typically a ring indicator for incoming calls. To build your embedded controller, you would then add a DTMF to binary decoder such as the now-defunct Silicon Systems SSI202. Texas Instruments also used to make a few DTMF decoder chips. You can probably find enough parts online for a one off, otherwise, Microchip has published code to make a PIC decode DTMF.

The actual circuitry of a DAA is not difficult: a 1:1 ratio 50V transformer, optoisolator for ring detector, reed relay for a hookswitch and a capacitor to couple audio will make a usable one.

^{simulate this circuit – Schematic created using CircuitLab}

This is a basic DAA that I have built in the past. It shows the "phone" offhook. The LED and transistor comprise an optoisolator that will generate a pulse when an incoming call is received. To take the phone offhook, activate the HOOKSWITCH relay. Now you have an audio connection to the telephone on the other end through capacitor C1.

This is far from an ideal DAA, but it will work well enough to receive DTMF and control external circuitry. I drew it from memory, hope I didn't miss a minor detail :-)

Here is one that uses a triac instead of a mechanical relay. Xecom and Cermetek made many DAA modules. I don't know if they still do.