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Path: utzoo!decvax!ucbvax!KESTREL.ARPA!king
From: king@KESTREL.ARPA.UUCP
Newsgroups: mod.telecom
Subject: Re: Hold Circuit
Message-ID: <8612240403.AA04246@ucbvax.Berkeley.EDU>
Date: Mon, 22-Dec-86 12:21:31 EST
Article-I.D.: ucbvax.8612240403.AA04246
Posted: Mon Dec 22 12:21:31 1986
Date-Received: Wed, 24-Dec-86 01:00:57 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
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Approved: telecom@xx.lcs.mit.edu


   From: tj 

   put a resistor, and LED, a Zener, and an SCR in series. The Gate of the scr
   goes through a resistor and a pushbutton to + phone line. Lets draw a 
   schematic online... (Hope this works)

   -----------phone line + -------+-------------+
				  |             |
				  R             |
				  |             |
				  \___PB_______SCR
						|
					      \_|_
						Z  \
					 +------|
				       \_|_     R
					 Z  \   |
					 |     LED
					 |      |
   --------------- phone line - ---------+------+

   The SCR is off until pushbutton PB is pressed making contact thus
   turning it on. Upper Zener is some voltage higher than the off hook
   voltage of the line. (say 10 volts) Until the line goes higher than
   10 volts it is off, even though the SCR is on. Hold the push button
   as you hang up. This turns on the SCR and as the line voltage rises
   the zener conducts and the current through the SCR will hold it on
   (get a low hold current scr, say 3 ma) The resistor in series with
   the push button supplies the small turn on current.  5 k should do.
   The lower zener limits the voltage across the LED. Make say 5
   volts. This would put the r in series with the LEd at about 150
   ohms.

   Like no guarantees...

I doubt this circuit would work, and it could be dangerous.  There is
no impedience between a 28 volt source and ground; that voltage will
be pulled down to 15 volts.  What would limit the current through the
SCR and two zener diodes?

-dick