🚀 go-pugleaf

DI3QC@CUNYVM.BITNET writes:

>     I would like some help with a project I am working on. I am trying to buil
>d a circuit using a 68hc11e2 which will sense a phone ringing, pick up on n rin
>gs and then wait for a n digit code. Once correctly entered the user will get a
>new dial tone via the switching to another line. Are there any circuits all rea
>dy available to do such? Any ideas to this project I could use some help.

To detect rings, I'd put the phone line through a bridge rectifier, so the
polarity will not matter, then a voltage divider, to make it a reasonable level,
and then to the input of a 555 timer set up as a one shot with a delay longer
than the ring.  That way each ring puts the 555 through exactly one cycle.
Remember to protect the input of the 555 with reverse-biased diodes to the
power supply rails.

Then of course you feed the ring one-shot signal to any counter you like.
Many counters will give you an output for 1,2,4 or 8 rings, or else
you can detect any state of any counter and count any number of rings.

To make your circuit pick up the phone, have it switch a 600 ohm (or so)
resistor across the line with a relay.  To suck audio off the line once
it has been answered you could put a transformer in series with the
resistor, and feed its secondary to an audio amp.

There's a nifty chip to detect touch tones.  If you need a number I can
look it up for you.  This would detect any touch tone and strobe its value
out on 4 data lines.  You can feed these to shift registers, flip-flops,
or what-have-you to detect special combinations.

To finally connect in the other line, I've actually gotten away with
connecting them directly in parallel, observing polarity of course!
This is not recommended but it can work.  The real way to do it is to
use some sort of inductive network.  I'm not your source on that one.

					good luck -- cool project!
					little david
-- 
I'm a poet and I'm unaware of it.