Telephone in use indicator Help

Pretty much anywhere in U.S. phone companies, been around for as long as I can remember. I know it worked on the old 1A2 key systems. Not a feature that you order, per se, but its just the way the system works by default.

I mean disconnect supervision.

Your information is incorrect.

With loop-start and ground-start lines (POTS and most small business lines) there is no polarity reversal to denote the calling party has hung-up. Answering sets determine when to hang-up by detecting the absence of voiceband signals.

Don

Nope, THAT is absolutely incorrect. The absence of any voice band signal is rarely used as the primary means of determining the call has ended, but is used as a fallback.

The telco will send a Calling Party Disconnect (CPD) signal. This will be a momentary battery reversal or a momentary drop of loop current.

Battery reversal is on its way out as no FCC registered equipment is required to recognize it, but drop of loop current is very much here.

I once had an answering machine that would turn itself off when I picked up the extension.

Cheers! Rich

That's because it sensed the voltage drop when another phone went off-hook.

maybe it could be an RF source?

try a capacitor (0.1uF ceramic) in addition to a resistor in parallel with the LED.

Bye. Jasen

Hey thanks. I will give it a try. Hate to toss out something Ive already put together.

By the way Im not exactly sure how this works. If anyone is able to give a quick overview of how its suppose to work.Im a little confused (novice) on what the first transister does connected to the phone line. thanks.

When the phone is on hook, the base of the first transistor is a little over +1 volt, so the transistor is biased on. That places ground on the emitter of the second transistor, which is a PNP, so it is biased off. With the second transistor off, the cap can't charge and the 3rd transistor is not turned on. When the phone is lifted, the first transistor is off, which removes the ground from the 2nd transistor emitter. Current from the battery through the led and the 1meg resistor charges the cap - when it rises to about .6V higher than the base of transistor 2, the transistor is biased on, which in turn biases the last transistor on. That provides a path for ~ 40 mA current to light the LED. The cap discharges into the base of the 3rd transistor, biasing the second transistor off and the cycle repeats.

There are two high resistance paths that drain the battery when the phone is on hook. The combination of the 1 meg and

2.2 meg resistors connect to the base of transistor 2 is one path. The second is through the other 1 meg resistor and the first transistor, which is biased on by the phone being on hook. You need a different circuit if you want to avoid draining the battery. This one will draw around 10 uA with the phone on hook - and earlier experiments showed you can see the glow at only about 3 uA. (I'm still scratching my head over that. You must have good eyes!)

Do you have a DMM that has a uA scale? It would be a good idea to measure the draw from the battery with the phone on hook. If it is only ~ 10 uA, it's really not worth going to a different circuit. A typical 9V battery should last a long time if you don't use the phone. Where this circuit really uses current is when the phone is off hook. In rough numbers, a 1 minute phone call burns up as much battery life as 33 *hours* of the phone being on hook. You ought to be able to see the LED easily at ~2 mA instead of 40 mA. That would reduce the off hook battery drain by a factor of 20 - which is well worth doing. To do that, change the 180 ohm resistor to a 3.3K and see if it glows brightly enough for you with the phone off hook.

Ed

I'd use a better and simpler circuit. But looking that this one, you said there is no glow when the 2N4401 is out of the circuit - that means you're not getting leakage through it. Looks like it might be leaking through the 1M and 2.2M resistors between the 2N4403 and MPSA-18 transistors.

So, on to a better circuit.

GND + V + V | | | | X 47Kohm X 47Kohm | X X | | |----- > Goes | | | low | | |/ when | 0---------| off-hook | | |\\ emitter | | | o----------||----------| Bypass diode

Thanks. I'll try your suggestions. My meter does 200 and 20 M. ( I asume is Ma) I tryed to test it but it the glow does not register. When it flashed its about .22 -.18 M. Tested in series of course. You may be right a larger R. may reduce the loss a bit and still last some time.

Regards