From telecom@delta.eecs.nwu.edu Wed Jan 27 06:10:48 1993 Received: from delta.eecs.nwu.edu by gaak.LCS.MIT.EDU via TCP with SMTP id AA29057; Wed, 27 Jan 93 06:10:45 EST Received: by delta.eecs.nwu.edu id AA10632 (5.65c/IDA-1.4.4 for ptownson@gaak.lcs.mit.edu); Wed, 27 Jan 1993 05:10:32 -0600 Date: Wed, 27 Jan 1993 05:10:32 -0600 From: TELECOM Moderator Message-Id: <199301271110.AA10632@delta.eecs.nwu.edu> To: ptownson@gaak.LCS.MIT.EDU Subject: answer machine exclusion key Status: R >From telecom Mon Jan 25 06:07:25 1993 Received: by delta.eecs.nwu.edu id AA14620 (5.65c/IDA-1.4.4 for \telecom); Mon, 25 Jan 1993 06:07:22 -0600 Received: from world.std.com by delta.eecs.nwu.edu with SMTP id AA00657 (5.65c/IDA-1.4.4 for ); Mon, 25 Jan 1993 06:06:58 -0600 Received: by world.std.com (5.65c/Spike-2.0) id AA06197; Mon, 25 Jan 1993 07:06:51 -0500 Date: Mon, 25 Jan 1993 07:06:51 -0500 From: jon_sree@world.std.com (Jon Sreekanth) Message-Id: <199301251206.AA06197@world.std.com> To: telecom@eecs.nwu.edu Subject: Answering machine exclusion : schematic Status: RO Hi Pat : This is either for publication, or FAQ or what-have-you. I typed it up in response to someone's question, so maybe others will find it useful. ---- This is the schematic I traced for an answering machine stopper gadget : it cuts off the answering machine in case any line is picked up. Some answering machines have this feature built-in; the older or inexpensive ones don't. The gadget is called Message Stopper (R), by Design Tech International Inc., Springfield, VA. Several other mfrs make this kind of gadget, so this is just one I happened to buy. Cost, roughly $10. The gadget is in the same form factor as a wall-plug-in Y-splitter. That is, it plugs into the wall RJ11 outlet, and has two RJ11 outlets on it, one marked TEL, the other marked ANS. There are two LED's, the green one on top of the ANS outlet, the red one on top of the TEL outlet. The two outer wires of the three RJ11's are wired in parallel. The schematic for the center two wires is : .-------------- phone .----------------- | | C1 ----| C2 -| | | | | | | '----------------- answering machine | +--|<--+--|<--->|---- | | two zeners +-->|--+ two leds Top led is green, bottom is red. It turns out the leds are basically idiot lights. The outside design makes it look like the green should light up when the ANS is offhook, and the red when TEL is offhook, but actually, depending on the line polarity, only one will glow, and only when the ANS outlet is offhook. If a load is connected on the ANS outlet, both leds will glow when ring voltage comes through. I could not read the zener voltage off the diodes, but I measured 11V across one of them when operating. The theory of operation is straightforward. All telephone extensions in the house are in parallel with each other, and with any device connected on the TEL outlet. If any of these devices is offhook, the phone line voltage is expected to drop below 12V or so, at which point the zeners isolate the ANS outlet from the line. It's a fairly simplistic design, and will not work in all situations, but it mostly works. ---- Regards, / Jon Sreekanth Assabet Valley Microsystems, Inc. | Fax and PC products 5 Walden St #3, Cambridge, MA 02140 | (617) 876-8019 jon_sree@world.std.com |