Telephone ring detector

[Phil Biehl]

I'm in the process of adding telephone capabilities (autopatch)
to the UHF ham repeater I am working on. I would like to be
able to detect the ringing current produced by an incoming
call. What I need is a CMOS compatible signal indicating that
ringing current is present on the drop. That is, a pulse for
every ring burst.

I would like to have as much isolation from the drop as possible
and have been considering optoisolators to connect the logic to
the line.

I would like to get a consensus on your favortie ring current
detector. Please mail or post responses. Thanks much.

Phil Biehl
(503) 685-2122
Tektronix Inc., Wilsonville OR

tektronix!orca!philb

[Scott Dorsey]

In article <20...@orca.UUCP> ph...@orca.UUCP (Phil Biehl) writes:
>
> I'm in the process of adding telephone capabilities (autopatch)
> to the UHF ham repeater I am working on. I would like to be
> able to detect the ringing current produced by an incoming
> call. What I need is a CMOS compatible signal indicating that
> ringing current is present on the drop. That is, a pulse for
> every ring burst.

My suggestion is one of the neon optoisolators on the market. Just
hook one end to the phone line and the other one to a 741 op amp. Or,
taping an NE-2 bulb to a CDS cell will probably work, although I wouldn't
bet on reliability. I know that HP makes some neon optoisolators, but
you can probably get them cheaper...

--
-------
Disclaimer: Everything I say is probably a trademark of someone. But
don't worry, I probably don't know what I'm talking about.

Scott Dorsey " If value corrupts
kaptain_kludge then absolute value corrupts absolutely"

ICS Programming Lab (Where old terminals go to die), Rich 110,
Georgia Institute of Technology, Box 36681, Atlanta, Georgia 30332
...!{akgua,allegra,amd,hplabs,ihnp4,seismo,ut-ngp}!gatech!gitpyr!kludge

[Larry Lippman]

In article <20...@orca.UUCP>, ph...@orca.UUCP (Phil Biehl) writes:
> I'm in the process of adding telephone capabilities (autopatch)
> to the UHF ham repeater I am working on. I would like to be
> able to detect the ringing current produced by an incoming
> call. What I need is a CMOS compatible signal indicating that
> ringing current is present on the drop. That is, a pulse for
> every ring burst.
> I would like to have as much isolation from the drop as possible
> and have been considering optoisolators to connect the logic to
> the line.

A simple ring detector can be made from a GE H11AA1 optoisolator,
which is a dual LED device which detects AC. Connect the LED input in series
with a 3.9 K resistor and a 0.68 uF capacitor, and then across tip and ring
of the telephone line. The optoisolator output is a transistor; overall
sensitivity can be varied by using a resistor between the base and emitter.
Some integration should also be done to minimize dialing transients from
being detected; this can be done with a capacitor across the base and emitter.
For CMOS use, the emitter can be at Vss to sink a logic input using the
collector.

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