Phone-off-hook indicator

Matty, ICBA reading the thread so this may already be covered, but IMHO something as important as a panic alarm dialler should, all by itself, be able to have exclusive access to the line if it is triggered.

This would mean it is placed "first" on the line and all other phones are plugged into it rather than directly into the line so a relay in the dialler can disconnect them.

I feel sure the better personal alarms will work in this way but if you can't find one that does, then your DIY efforts would be better used to achieve that, rather than off-hook LEDs etc.

The reason this wouldn't work is because the electrical conditions applied to your phone have nothing to do with the caller's phone. Your phone is always from 48V source via a resistance. The voltage at your phone goes down to a low value when you take it off hook.

Also, when the phone rings, 90V AC is applied, so the bridge rectifier in the proposed circuit would produce about 126V DC, requiring something other than junk-box FET's.

buy any. In that circuit nearly any old fet should work fine. Bipolar trans istors are much more common in scrap though, why not use those.

n, and I need a circuit with all the components specified. The last circuit I made was in about 1954, a valve radio, and I still have it here.

OK. You need npn transistors with at least 100v rating. You can get them ou t of a dead CFL lightbulb, each of which contains 2 transistors. A multimet er can tell you if theyre npn or pnp, npn are much more common.

The downside of high Vce transistors (ie 100v +) is low current gain, which would be an issue on a phoneline. You can solve this by replacing each tr in the circuit with a darlington pair, which is 2 transistors connected up to give the same result as one high gain tr.

NT

Yes - it is a condition in the phone network/local exchange not in the house wiring.

So is there any way to identify the above condition?

I assume you would need some kind of notification from the exchange that the call had cleared down.

Cheers

Dave R

In this case the personal alarm is just plugged into a socket in the bedroo m and is definitely not first in line. I don't think the alarm company has even bothered thinking about the problem. I will phone two of the companies involved and tell them what I think. Pressure from the media will force th em all to rewire all their installations. They advertise on TV every day.

However if my friend's phone is just off the hook I can't phone him so have to drive round there. So I'll still make the LED device. I may even start a company to manufacture them in the thousands. That's the free-market in a ction!

En el artículo , Matty F escribió:

That's because it's BC108, not BS108

En el artículo , Matty F escribió:

I doubt it.

The circuit shows a MOSFET. BS108 is a MOSFET which is rated at 200V. BC108 is silicon NPN and rated at 20V so it wouldn't last long in that circuit.

But not MOSFETS.

Just be warned ... you can't attach it to phone lines if not approved... So if you ever have a phone fault it would need to be removed before supplier visits.

Anything you attach to the line ultimately draws current (unless self powered) and as such will also affect the REN.

Not saying don't do it - that is for your own morale judgement ... just making you aware.

If you use fets or darlingtons, along with the high R values in use, and use an ultrabright LED on say 100uA, i is very low and won't significantly affect REN. Most households put working hv transistors in the dustbin, so most of us already have them.

NT

I hope not, BC108 is a quite different device!

At one time *the* small signal general purpose NPN transistor - if there was such a thing.

So Telecom NZ have the same rules and regs. as the UK?

They are probably similar.

Just thought that a "Privacy Adapter" might do what is required:

Maplin have 'em at £6.99 (!) not convient for Matty but maybe a lead?

Thanks. If I end up making one I don't suppose they will complain about a t eensy LED sucking all the power out of their network. However I won't go into large scale production!

Matty F :

I've come to this thread late. I use a different circuit and you can find further information here:

(There are two circuit diagrams, one that I found somewhere on the web and one showing the modifications that I made for my purposes. Sorry some of the photos are blurry.)

I've seen the discussion of the high voltages that have to be endured by the transistors and I suspect this circuit avoids that problem, so you can use a wider range of transistors. But ICBW about that and I'll leave others more qualified than me to comment on that. I just used the first transistors that came to hand.

The circuit is *battery operated* which is good in one way - it lights up if it's unplugged from the phone line or the phone line is dead - but obviously there's a problem if the battery runs down. FWIW the example on this desk has been running since 27/11/2003 on the same battery and still looks as bright as ever, and I fully expect it still to be going strong after another ten years.