128a-3db@e260-3b.berkeley.edu (Jonathan Dubman) (04/11/89)
I realize this was asked recently, but I just started reading this newsgroup and I think I missed some of the responses. We need a sensor circuit to determine whether a telephone line is off-hook. I have lots of parts handy and I suspect it is a relatively simple voltage-range detector circuit, but I'm not sure of the voltages. Is it this simple? The reason we need this sensor is to prevent somebody from interrupting a download in progress. It would be more fun and slightly cheaper to build rather than buy if the circuit is indeed simple. Thank you very much! -Jonathan Dubman 128a-3db@web.berkeley.edu
irwin@m.cs.uiuc.edu (04/13/89)
/* Written 11:44 pm Apr 10, 1989 by 128a-3db@e260-3b.berkeley.edu in m.cs.uiuc.edu:sci.electronics */ /* ---------- "Sensing telephone off-hook" ---------- */ >I realize this was asked recently, but I just started reading this newsgroup >and I think I missed some of the responses. >We need a sensor circuit to determine whether a telephone line is off-hook. >I have lots of parts handy and I suspect it is a relatively simple >voltage-range detector circuit, but I'm not sure of the voltages. Is it >this simple? >The reason we need this sensor is to prevent somebody from interrupting >a download in progress. It would be more fun and slightly cheaper to build >rather than buy if the circuit is indeed simple. >Thank you very much! > -Jonathan Dubman > 128a-3db@web.berkeley.edu /* End of text from m.cs.uiuc.edu:sci.electronics */ At the church that I attend, there is a "privacy circuit" on the phone. There are several extensions in the church, but if the Pastor's phone is off the hook, all of the extensions are disabled, to prevent those who might tend to eaves drop from doing so. I was helping the pastor one day, making some changes in locations of extensions. We decided to look into how the circuit worked and found that there was a current sensitive relay in series with his phone. If his phone was off hook, the line current picked up the relay, which opened the contacts and the extensions were disabled. I put a meter in series and found that the line current was about 45 mills. I would think you could use this technique. Get a sensitive relay and put it in the circuit, wire the extensions through the relay contact, when you have the phone off hook for computer work, the other extensions will be dead and can not cause you problems.
ART100@PSUVM.BITNET (Andy Tefft) (04/17/89)
A while ago a friend of mine mad a simple circuit with a couple of transistors that would light a green led when the phone was on-hook and red when off-hook. There's a fairly LARGE voltage difference (around 30 volts or so i think) between off-hook and on-hook, which should be quite easy to detect. If you engineer the thing right you can run it off the phone company's power. I just made a voltage divider and stuck an led in there, set up so that the on-hook voltage would light the led ok, but the off-hook voltage was too small for any light.