Two telephony projects
1/5/97
Below are a couple of computers that I made for
Ed this weekend.
The first one just needed to be able to control a modem and have some
onboard storage to hold some numbers so I decided to make it very small
and build it right into the serial connector. I used the modem's DSR
pin to power it so we wouldn't need an external 'wall wart'. This
technique is similar to the way that mice power themselves off an RS-232
port. In fact the processor that I used
(PIC16C58)
I've actually found in a few mice I've opened.
The second was slightly more complicated. It needed to be placed inline
between another telephone device and the wall and be able to tell when the
receiver is lifted. When that's detected it dials out to a predetermined
number. The fun part about this one is the method used to dial the phone.
In order to eliminate the need for a dialer chip the tones are calculated
in software by adding two sinewaves and sent out a digital output pin as a
pulse width modulated signal - so the whole dialing circuit is a 5 cent
capacitor!
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