While I'm not to sure what your hoping to achieve but you can buy a priority (can be called by another name) switch which basically is a 2 outlet telephone adaptor plug for less than a fiver that isolates the other connected line when in use. This is achieved from a array of transistors inside the unit which consumes it's power from the line. This may be what your looking for.
As Sue suggests go for a voltage sensor solution rather than a current one. On-hook a phone line has about 48v across the wires, Off hook it falls to about 9v so anything below (say) 20v can be assumed to be off-hook.
You might want to seek out a line interface that has the necessary approvals, especially if you are working on somebody else's line.
Well I do wonder what the OP is up to since they don't say..
I have a phone line, but I doubt the other house occupants would be pleased if I rigged up some kind of device to start recording in my absence. Of course, we have no idea what the OP wants to do, but I can certainly imagine uses that are unsavoury (I'm not suggesting that is the case).
I wouldn't help anyone along with such a device without knowing the end-use, but clearly that's just me. It also seems sensible to mention the actual usage since there may be a ready made/alternative solution besides jury rigging the phone line.
A few years ago Elektor magazine published a constructional project for a reed relay based self energising latch for dial up internet intended to defeat internet rogue diallers that break the connection you dialled yourself and redial a premium rate number.
The circuit consisted of a reed unit with an energising coil round it, wired in series with the reed, a push button is in parallel with the reed so the connection is made while dialling, once the hookswitch is closed and current flows through the coil the reed pulls in and the button can be released. For a rogue dialler to dial it must first drop the existing connection - which also drops out the reed making the dial out impossible.
Since this demonstrates that it is possible to wind a coil round a reed element that can be operated by the telephone line current, it might help if you can obtain a copy of the article for the coil winding details and modify the circuit for your use.
On Sun, 17 Jun 2007 11:43:04 GMT, "On Web" put finger to keyboard and composed:
My immediate thought was that a line in-use indicator could tell you if someone was talking on the phone before you tried to send a fax or dialled in to your ISP. Conversely, it could also be used to indicate that a dialup Internet session was in progress.
- Franc Zabkar
Please remove one 'i' from my address when replying by email.
"On Web" wrote in news:j3fdi.5003$ firstname.lastname@example.org:
I only see one post from him, the first one:
Jim wrote in news:Xns99526A09F520B5D4AM2@127.0.0.1:
There is no claim of any attempt to capture callers' numbers there.
Regarding issues of recording, deriving switch signals, this is legal in the UK, you just have to watch how the law applies in specific instances.
You talk of 'jury rigging' a phone line, which is as loaded a phrase as I ever heard, it implies some kind of illicit activity. Any real jury knows that a person is guilty until proven innocent.
If you want to be cautious, just point the OP to a page that discusses the regulations governing user connections to the user side of the master socket wiring, and the laws governing lawful making and use of recordings. The rest you must leave to their discretion, you can't police their morality.
The easiest course is to look at what is commercially available in a shop. If it's on open sale, it is safe to assume that discussing its function is not only legal, but wise, and it might be cheaper to make than to buy. Doubtful though, unless you buy from Maplin or other shop that charges around twice what many shops accept as reasonable.
Franc Zabkar wrote in news: email@example.com:
Indeed. It can actually help with privacy. Without it, what's to stop someone picking up the line and finding themselves a willing or surprised eavesdropper, even against their better judgement. Such a device is as innocuous as an engaged sign on a toilet door. Many people would encourage its use, that way at least it takes deliberation, with less excuse for 'accident' in shared households if a person picks up a handset to listen.