Phone-off-hook indicator

[Matty F]

Elderly people living on their own often have an emergency call system whic h is a button they can press that calls an ambulance service. For that to w ork, all phones in their house need to be on the hook. Naturally they have phones all around the house upstairs and downstairs, and it's easy to leave one off the hook. So what is needed is an off-the-hook indicator, which is a very simple circuit with a red LED that shows when a phone is off the ho ok. No other features are needed.

I have seen some such devices for sale but are out of stock, or they don't work in some countries or with broadband. So I could make one, but I am hav ing trouble finding the specified transistors, e.g. BS108. I am told that n obody bothers making their own circuits these day so nobody bothers stockin g transistors. My favourite shop says that when they have sold the transist ors they have in stock, they will get no more.

So what I want is a circuit that lights an LED at about 10 volts DC and tur ns it off at 30 to 40 volts, and with components that I can actually buy.

I'm not interested in discussions about how people should hang up their pho nes, or any other solutions.

This looks a suitable circuit (but I can't get the transistors):

[meow2222]

ich is a button they can press that calls an ambulance service. For that to work, all phones in their house need to be on the hook. Naturally they hav e phones all around the house upstairs and downstairs, and it's easy to lea ve one off the hook. So what is needed is an off-the-hook indicator, which is a very simple circuit with a red LED that shows when a phone is off the hook. No other features are needed.

t work in some countries or with broadband. So I could make one, but I am h aving trouble finding the specified transistors, e.g. BS108. I am told that nobody bothers making their own circuits these day so nobody bothers stock ing transistors. My favourite shop says that when they have sold the transi stors they have in stock, they will get no more.

urns it off at 30 to 40 volts, and with components that I can actually buy.

hones, or any other solutions.

Most scrap electronic appliances will give you transistors, no need to buy any. In that circuit nearly any old fet should work fine. Bipolar transisto rs are much more common in scrap though, why not use those.

NT

[Matty F]

which is a button they can press that calls an ambulance service. For that to work, all phones in their house need to be on the hook. Naturally they h ave phones all around the house upstairs and downstairs, and it's easy to l eave one off the hook. So what is needed is an off-the-hook indicator, whic h is a very simple circuit with a red LED that shows when a phone is off th e hook. No other features are needed.

n't work in some countries or with broadband. So I could make one, but I am having trouble finding the specified transistors, e.g. BS108. I am told th at nobody bothers making their own circuits these day so nobody bothers sto cking transistors. My favourite shop says that when they have sold the tran sistors they have in stock, they will get no more.

turns it off at 30 to 40 volts, and with components that I can actually bu y.

phones, or any other solutions.

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

Because I know nothing about transistors and I'm probably too old to learn, 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.

[Andy Burns]

2N3392 are easily available ...

[Adrian]

[Matty F]

Very expensive and they won't post to NZ. My usual supplier has thousands of transistors. Why won't one of them do?

[Matty F]

The circuit looks good although I don't need an LED for not in use. My usual supplier has no 2N3392, or any of these equivalents

2N2921 2N2922 2N3390 2N3391 2N3391A 2N3392 2N3393 2N3394 2N3395 2N3396 2N3397 2N3398

[jrwalliker]

That circuit could die when the phone rings if the trimmer is set incorrectly (so that the first FET doesn't switch on). It puts half the input voltage onto the second FET gate which has a maximum voltage rating of +/-20V.

During ringing the peak voltage can be considerably more than 100V. Although the gate current is limited, the gate oxide could still be broken down by high peak voltages, either impairing the performance or destroying the transistor.

John

[Terry Fields]

Having two LEDs is a good idea: not only is there an indication that the line is is use (which is what you want), there is also an indication that all is in order with the line (in that it has volts on it), and a negative indication (no lights) that the line has failed in some way. The latter condition means that the elderly person will have to resort to her Plan B: arouse the neighbours, use her elderly-person's emergency mobile phone, or whatever the backup plan is.

For the trivial extra cost involved, it's well worth it for a safety-of-life issue like this.

[Geoff Pearson]

Try it and see - the circuit can be made to work with either kind and not much harm can be done if it doesn't work.

[Brian Gaff]

One of the issues here seems to be that some phones do not have a very good rest switch. Some of the big button ones have this issue. The only way I've found to get around this is to add internal weight to the rear of the handset with plastacine. No don't laugh, it seems to work, though gaining access to some handsets can be a bit of a mensa test.

The problem is that rather than use one single switch it seems there are several, so even if it appears to be OK there might be one little switch not in the right position with variable results like line ok but no phone will ring etc.

Maybe a redesign of the innards so only one switch is there which makes a multi pole relay do the deed instead of the rest?

Brian

[Brian Gaff]

Yes, I'm sure I have bags of old transistors some place. I cannot imagine you cannot get them somewhere. mail order and hobby magazines have adverts still as far as I'm aware. Brian

[cl]

Loads of places still sell transistors (and everything else you need), the obvious ones are:-

CPC (part of Farnell) -

Farnell - (more aimed at the professional market) Rapid Recall - Radiospares - usually relatively expensive eBay - Loads of suppliers, often cheapest for small quantities

[Martin Brown]

You basically only need a generic NPN transistor with Vce>100v for that circuit to work. I'd also suggest you get the bits from someone like RapidOnline since they will sell you trivial quantities at lowest cost (actually the odd place that caters for radio hams may be slightly cheaper including postage and may even have arcane obsolete parts).

This one is a bit overkill but for 8p each not worth looking any harder.

Use modern LEDs and increase the resistors a bit despite what you say above you probably *do* want a reassuring green health LED as well as an "off hook" red LED. Not sure how much current BT like you taking. (so basically aim to draw as little current as possible)

Otherwise they will see it as a possible line fault.

[Martin Brown]

Any with Vce >100v (better 200v will be fine).

Most common transistors are in the 30-60v range and will quickly lose their magic smoke if attached to a phone line.

[Adrian]

We've already discovered that Matty's problem isn't really that nobody sells them, but nobody near him sells them cheap enough...

[Clive Page]

Can't the whole thing be done more simply by just putting a suitable LED and resistor in series with a zener diode with a suitable voltage spec? It's years since I've made any devices myself but that's what I'd have tried first. Or maybe you can't buy zener diodes any more?

[Matty F]

Thanks, everything there is so cheap I may as well buy all the components there.

[Dave Plowman (News)]

They're not a transistor.

Small signal high voltage N-channel mosfet. I'd guess there are lots of alternatives for this simple application.

[David.WE.Roberts]

There is another potential use for this - as a 'scam buster'.

There is a scam where fraudsters ring up and tell you that your credit cards are compromised (or something similar) then tell you to ring back your bank/credit card provider on their standard number to confirm they are genuine.

However, they keep the line open when you hand up, then play a dial tone so you think you are dialling the number when in fact you are still connected to the scammer.

So a device such as this should show that the line is still 'off hook' and you know that something isn't right.

Which might make this particular scam unworkable if most people had this device.

Any reason this wouldn't work?

I assume that Virgin Media phone lines behave in all ways electrical the same as BT phone lines.

Cheers

Dave R