Put together the circuit you attached, my original circuit just had the
output (primary) of a 120/12v 300ma transformer with a 4.7uf non
polarized cap across the output going to the phone; this gave me 37.8
volts ac and rang the phone. Your circuit gives me 20 AC with only 3.2
D.C at the phone connections using your instructions. I built the
circuit according to your schematics; I can only assume I have done
something wrong, perhaps the phone is connected to the wrong place? The
only component I did not have from your schematic was the 2.2uf cap I
used a 4.7uf 50v is this critical? At the off-hook connection if I place
a meter here the voltage seems to fluctuate between 1.9v and 0dc with
the phone on-hook if that helps. I tried a 4.7uf Non Polarized and a
10Uf Non Polarized in your circuit and it seems to give a higher output
with the 4.7uf Non Polarized but still won't ring the phone.

Regards,
Steve

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of David Duffy
Sent: Monday, February 04, 2002 11:11 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]:Adding a DC bias to and AC signal

At 10:20 PM 04/02/2002 -0500, you wrote:
>Hello All,
>
>A prelude to my last post about Ac signals and a problem with detecting
>an off hook if I can't get this condition detected without the dc
>component of the signal. How can I add the dc component, even if it's
>only 12vdc instead of the regular 48vdc the Telco uses so that I can
use
>it to detect an off hook condition. I'm detecting this condition in a
>closed loop off a relay that is not connected to the telephone company
>it is a ringing signal that I have created on my circuit that does not
>have the dc component, unfortunately this does not give me a way to
>detect an off hook. A bridge rectification of my ringing signal gives
me
>41dc during ringing and 14vd off-hook but the ring cadence would be a
>problem as it would provide a false reset signal during ringing silence
>between ringing pulses and the dc voltages are too high. The bridge
>rectifier also seems to load the ringer circuit down a little too much.
>So my next thought is to add enough dc to the loop to detect the off
>hook and get a good logic high or low that is independent of the
>ringing, I am not sure how to add dc to the ac without blowing the
thing
>up.

Are you using a transformer to step up the ring voltage? If so, connect
the secondary winding "on top" of the 12V rail so that you have 12Vdc
with the ring voltage riding on top of it. The phone hangs between the
free end of the tx sec and a 1K resistor that goes back to 0V. The
junction of the phone/1K goes via a 10K to a 10u bipolar capacitor
which feeds the base of an NPN transistor via a signal diode...
Oh crud, this is too hard to explain... Here's a quick drawing !  :-)
Regards...

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu