Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!cmcl2!phri!cooper!gene
From: gene@cooper.UUCP (Gene from EK Enterprises)
Newsgroups: sci.electronics
Subject: Re: How do you build phone off-hook indicator
Message-ID: <1005@cooper.UUCP>
Date: Wed, 9-Sep-87 16:54:24 EDT
Article-I.D.: cooper.1005
Posted: Wed Sep  9 16:54:24 1987
Date-Received: Fri, 11-Sep-87 06:50:33 EDT
References: <12660@bu-cs.BU.EDU>
Organization: The Cooper Union (NY, NY)
Lines: 36

in article <12660@bu-cs.BU.EDU>, acm@bu-cs.BU.EDU (ACM) says:
> Now, I know there is a voltage differential between on-hook and
> off-hook (I believe it's 24 volts on-hook and 10 off-hook but this is
> purely from memory and I could be wrong).  How can I build a simple
> circuit to light an LED when the voltage drops?  I'd prefer the
> simplest curcuit possible that's pretty reliable.

Well, I've always had problems with emailing, so I'll post...

If memory serves me correctly, on-hook voltage is up to 48V, with off-hook
voltage being close to 4V or so. _This_varies_with_your_particular_site!_

In fact, it's possible (I've done it) to make a primitive listener-detector
by monitoring the voltage. Something like 6V with just one phone off-hook;
4V or so with en extension lifted.

Anyway, a nice IC to use is the 3453 or whatever low-voltage detector.
While it's not exactly micropower (or is it?), it can be either run off
the line voltage (*REGULATED*) or off a separate battery. just set it to
about a 10V cutoff. Over 10V is definitely on-hook; under is off-hook.

(The 34xx whatever is a Motorola IC.)

					Experiment and have fun!
					Gene

					...!ihnp4!philabs!phri!cooper!gene


	"If you think I'll sit around as the world goes by,
	 You're thinkin' like a fool 'cause it's case of do or die.
	 Out there is a fortune waitin' to be had.
	 You think I'll let it go? You're mad!
	 You got another thing comin'!"

			- Robert John Aurthur Halford