Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP 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 Lines: 42 Approved: telecom@xx.lcs.mit.edu From: tjput 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