Telephone in use indicator Help

The first thing I'd do is use a voltmeter to measure the voltage across the phone line. It should be above 45 volts on-hook, below 10 volts off-hook. If that's not the case, let us know.

The second thing would be to look for a wiring error in the input circuit (the diodes, the 22 M and 1 M resistors, and the first transistor). Maybe even a defective first transistor.

Yes. When disconnected, it is picking up stray electrical fields from its surroundings.

Frankly, that strikes me as a very complicated circuit to do a simple job. I'm sure there are simpler designs, but if this one works for other people, it should work for you.

Thanks for your comments.

I tested the voltage On Hook its 51 VDC. Off Hook its 51 VCD.

I have tested the four Diodes they seem to be ok. Closed one way .5 to .6 Ohms the other way. If I understand it right.

I have gone over the circuit one more time looking for human error. Cant seem to see anything wrong.

The one thing I did not mention was that I am using the 2n4401 instead of the MPSA-18, as the schematic suggests I can do.

I must say when I tested the resisters they do vary a lot when your at the 22M size. My little digital meter reads them anywhere from about 21 to about 25. My meter has a 20M scale and then a 200. So I have to set it on the 200M scale to test 22M. I figure its doing pretty good to get in anywhere near 22M.

Short of any other answers I could test the Transistors. But IM not always sure I know what to look for or how acurate it is on my little Digital MM. (which is all I have being a Novice.)

The reason I chose this circuit I suppose is because it looked relatively simple and used an external power source which should last for some time. Im always wary of all the warnings about causing problems with stealing power from the poor phone company. And possible other issues that can come up.

Any thoughts?

Regards

Correction..

SORRY I MIS WROTE

I mean On hook is 51vdc and off hook is 10.9vdc

sorry

Same voltage both ways? No...

Open one way, 0.6 volt (the diode test range on your meter reads in volts, not ohms), is correct.

It's the meter's problem, not the resistors... It's hard to measure resistances that high. I doubt there is anything wrong with the resistors.

Does it have an hFE (transistor gain) measurement? If so, try that.

It looks like a reliable design, although a bit on the complicated side.

Please check over and over again for wiring errors. Even the best of us make plenty of them.

Ah! Much better.

I'll check the transistors. Yes my meter does have hFE measurement. Perhaps as I test them It will give me another opportunity to check that every thing hooked up right.

I'll get back with hFE readings.

Is this thing supposed to flash just when the phone is off-hook? That's not the way it's done anywhere else, at least in the USA. When you're off-hook, it's supposed to be steady on, and it flashes when you put the line on hold. (to put a phone line on hole, I've been told, just put a

Frankly, overall, the circuit looks a little flaky to me, but I'm just a tech. ;-)

Good Luck! Rich

I learned that you need to be wary about such things, too. I believe that the total loading on the line when on-hook should be greater than about 5M ohm. That's not so easy to achieve.

I did take the trouble, though, to design a circuit that presents a constant load of 40Mohm when the home is on-hook and pulls a constant current of 400uA to drive an LED when off-hook (I use a very low current voltage trigger to set the threshold.) I have another that presents about 40Mohm at all times and blinks a tiny LED slowly at low voltage and faster at high voltage. That one is probably better, as it doesn't ever load the line much.

I suspect that a professional could do better still, too, and adhere well to the loading specs. The reason I targeted 40Mohm is that I wanted to use a couple of them on the line.

Jon

I tested the transisters

reading from left to right the first 2N4401 is hF 266 the second 2N4403 is hf 287 the third 2N4401 is hf 241.

Not sure, Im guessing these numbers are ok.

Yes, definitely. They are in fact toward the high end of the range, and it's conceivable the designer of the circuit had lower ones, and this might have affected the performance of the unit in some way. You might try swapping out the first transistor, just to see what happens.

Also look for bad solder joints.

my seven cents worth! a few years ago i did some house hold phone research for a project of mine!. this is what we have..

When ringing, 100 V AC! when not ringing and phone on the hook (no line connection). 50 V DC of a fixed polarity. when the phone is taken off the hook, it drops in the area of 6..12 volts of the same polarity if memory serves. when dialing with the old rotary, this turns into a 0 to 6..9 volt pulse., the Tones simply modulate the line like does the voice.. and the last and most important is this, when the party on the other end picks up!, the polarity changes on the line! i guess this is how answering machines know when the party is still there because when they hang up the polarity switches back. don't know if it is still that way in the USA but i would guess it is.

and don't forget, the polarity reverses depending on the state of the connection to your party!

Remove the 2N4401 nearest the battery. Does the LED still glow? If so you are seeing the unavoidable (in this circuit) current from the battery + through the LED and the 1 meg and 2.2 meg series resistor to the - of the battery. That current is very low -

Anyway, I expect you will not see it glow with the transistor removed. If it does not glow, put the transistor back in, and add a 1 meg resistor from its base to ground. If it does glow, check the resistance (with the thing disconnected and the battery removed) from the junction of the two

Ed

Thanks ed. I'll do that and report back.

Results

Removed 3erd tr. Nearest Battery. (4401) Very subtle glow when (circuit not connect to telephone) Connect same circuit to phone line On Hook seems to glow Off hook dims (of course in the above test there is no flashing)

Added resister and put back tr. When I hook up to phone line. there is a glow ON HOOK.

Checking the resistence from the juncktion of the two 1 Meg Reisisters.

Im not sure what you mean by with the thing disconnected. Im asuming you mean not connected to the phone line.

With your 1 Meg R. in place it settles down to about 3.2M W/out the addition of your 1M R. It grows from about 1.4 to 2.7 then back down again to 1.4 backup to 2.7.(roughly) I guess this has to do with the capacitor. charging and releasing??

Those are my results.

A POTS line in the US does not change polarity when the "far end" answers.

Don

Ok. Get a different circuit. You've got roughly 3 uA current which is unavoidable with this circuit, and it makes your LED glow. You either have to live with that or change to a different circuit. I can't see any glow with only 3 uA through an LED, but it is your eyes you need to satisfy, not mine.

Ed

The polarity doesn't change for the duration of the call, but it does after the far end hangs up. Its called reverse answer supervision. When you call and an answering machine answers - and then you hangup, the Central Office momentarily reverses the polarity back towards the called party and the answering machine knows to hang up.

Thanks I guess that waht I will have to do. Yes the glow is faint. I turn off the light int he room Im in and there it is glowing. I have tried various resistors in various places of various sizes. I can reduce it but not get rid of it completely.

Rats. Just when things are working they dont.

Regards