DTMF with AT2051

DTMF generation generally requires a lot of microcontroller resources, so it really depends what else you require the micro to do whilst DTMF is being generated. I usually prefer to use a seperate DTMF chip such as the Holtek HT9201A. Yes, you need to simulate keypresses, but it really isn't that difficult.

I have modified PCB's from a phone to do the dialing with a 8051 micro. Wire an opto accross a keypad, resistance is low enough when the opto is on. Drive the opto's with the micro. normally the keypad is arranged in a 4x4 matrix. A AT89C4051 has more flash memory.

Thanks for your feedback. I am constructing a simple 5-zone home security system under $50. The alarm detection will use interrupt. Once the alarm is detected, it will run into a dtmf loop calling the home owner handphone 10 times with 5 minutes break. There will not be a dial tone detection and the dtmf tones will be sent after with 3 seconds delay after the phone loop. The Panic button would be done using a SCR and a N/C button switch instead of a port pin.

I'd take a look at the datasheet of HT9201A as well as its availablity. Have anyone used a PCD3311/3312 by Philips before?

Allen

I don't quite get it here. Are you wiring 'one' opto-coupler to the keypad and send out decadic pulses OR are you wiring "seven" optos on the column and row lines and on each digit, triggering two optos to send out the dtmf tones.

I was thinking of using the line interfacing part of a modem for experimenting with my DTMF sending.

Allen

I am building a 5-zone home alarm with dtmf sending if the alarm is triggered. The zone detection is using interrupt and once triggered the uC would call my handphone 10 times with 5 minutes-breaks. Dial tone detection is not included and instead I've put a 3 seconds-delay after off-hook.

The panic button is implememted in hardware using a SCR and a N/C switch. I have been looking at the datasheet of PCD3311 by Philips and can't find a dealer locally. Has anyone used this chip before? I have also visited the Holtek website as you mentioned and they had announced the discontiunation of the HT9201A chip. Now Holtek is making some RC decoder and encoder chips using infra red and RF and other more advanced microcontroller chips.

Allen

Have a look at:

for DTMF "transceivers" which can interface to a micro with 4 data lines and a few control lines.

Alan

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If you are connected to the phone lines, why use DTMF at all? Rotary dialing (pulse) still works. You may need some configuration data for international use. Interface - one relay, already needed for on/off hook.

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Chuck F (cbfalconer@yahoo.com) (cbfalconer@worldnet.att.net)
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     USE worldnet address!

yes, 1 opto on each keypad. If you get a phone with a keypad to grab the line (VISTA 350) wire an opto across that as well and you have a remote off-hock function.

Good points, I choose DTMF firstly I want to see how hard it is to interface to DTMF stuffs. And if the project is successful, we might market it locally. So DTMF is an important selling point and also its ability to talk to PC via its serial port.

Nowadays it's hard to find a rotary type telephone ( I am still keeping an NEC 8B phone for antique). Well, if I fail to do in DTMF, decadic is a good alternative then.

Allen.

You don't need a rotary telephone - just pulse the line. The only penalty is the aforementioned internationalization, and the time to dial. You can't signal after connect, either.

Experiment - substitute atdp for atdt in your modem dialling script. It should work, but may not be able to dial '#' and '*'. Modem line control relays today are not large enough to handle the inductive line surges, and may eventually weld themselves. I had a Hayes that did that in about 6 months of use. You, being prewarned, will not use the smallest possible relay.

--
Chuck F (cbfalconer@yahoo.com) (cbfalconer@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
     USE worldnet address!