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Hi all,

I am a highschool student and for electronics class I had to come up with an idea and build it. I want to make something, that picks up like an answering machine after so many rings, then you can input 4 digit number and it will then allow you to enter additional numbers to do things etc. The four digit number is kinda like a pin number. I know that I need to do DTMF for it, and that each number is composed of a high frq and low frq. I have been trying to study chips but i'm new to this stuff. How do I know what chip I want to use, and once I deciede how to i connect that to everything else?

I am currently looking at:

Mitel MT8888C, MT8870D
Silicon Systems SSI203 DIP-18, SSI204 DIP-14
Motorola, MC145436,
and UMC 9203 DIP-18, UM9204 DIP -14, 92870A/B/C

I'm think all of these are pretty good chips for me to be using in this project, but as i said i'm new and am not really sure. I think i might be leaning towards the Mitel dedicated DTMF MT8840. Has anyone ever done this?? or has anyone also researched this and help me find a good place to start. One of my main things, is if I choose the MT8840, is how to go about puting that in my own circuit, I know i'll need power to it, ground, among other things, but I'm not sure how to take the data from the phone and put it into the chip, and how to get my output.

Thank you,

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I have posted a snippet of a design with the 8870.
Once a valid tone has been detected (after 40mS) the
STD output (pin 15) goes high and the 4-bit digit data
(representing 0-9 and A-F) is presented on the output
pins (pins 11-14). The STD output will remain high
while the tone is still present and will drop low 40mS
after the tone stops.

Hope it helps.



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