From: alung@megatest.com (Aaron Lung)
Subject: Re: Telephone on hook/off hok ok circuit 
Organization: Megatest Corporation
Lines: 59

In article <1993Apr15.144826.4607@bnr.ca> moffatt@bnr.ca (John Thomson) writes:
>: >> >Just a thought of mine here:
>: >> >Since an on-hook line is aprox 48-50V, and off-hook it usually drops below 1
>: >> >How about an LED in series with a zener say around 30V.
>: >> >On-hook = LED on
>: >> >Off-hook = LED off.
>: >> >Would this work? If anyone tries/tried it, please let me know.
>: >>
>: >> Aye, there's the rub -- if you draw enough current to light an LED, the
>: >> equipment at the phone company will think you've gone off hook.
>: >> In the on-hook state you're not supposed to draw current.
>: >
>: >Which means you should just use your Digital VoltMeter. You can use an
>: >old VOM but the phone company equipment can detect that and might think
>: >there's something wrong with the cable.
>: >
>: 
>: Look Guys, what's the problem here?  If you want a light that goes on when 
>: the 'phone is *Off* hook, all you need it to run it in *series* with the 
>: line, as I mentioned in my previous post.  If you want a light that goes on 
>: when the 'phone is *on* hook, all you need is a voltage threshold detector.
>
>If you're going to do the series Diode thing (which is the easiest), just
>make sure that the LED can take the current (I can't recall it off-hand, but
>it's something like 100mA or more?)

Careful now folks... Also consider the 90VAC+ @20Hz that is forced on ring
and tip when the phone's supposed to ring!  Even with a simple zener
and LED setup, you might end up with some carbon real quick.  Whatever
scheme you use, make sure you've got at least 200V-rated components on 
the frontend.

Also remember that, if I'm not mistaken, the phone line is a 600ohm
equivalent circuit.  Any current you draw from the 48V or so gets
dropped across that 600ohms.  That's fine until you're down to roughly
12V, when Ma Bell considers it to be off-hook.  But dropping it that
far down is probably a big no-no.

The easiest implementation to accomplish the above??

    tip  ------->|-----\/\/\/\-----+----------+
              rectifier  resistor  |          |
              diode                |          \ 
                                   V          /
                          zener  /---/        \  resistor
                                   |          /
                                   |          |
                                   |          V   LED
                                   |         ---
                                   |          |
    ring --------------------------+----------+

This is only a suggestion...go ahead and blow holes in it, but you
get the idea.  Choose a high-efficiency LED so you don't need much
current to get it to light up.  Choose values for the other components
as required.

aaron

