3
\$\begingroup\$

With a single POTS telephone line and two or more handsets, when a call comes in, each handset can join in with the call. This can sometimes be an issue when one user doesn't know the line is in use, and picks up the phone and starts dialing, or listens in to the original call.

Doing some research, I've found that there is a nominal 50v on the line in idle, the ringing is a 25hz wave superimposed on the 50v, and normal "off-hook" voltage is ~10v. Telephone Technical Information

What I'd like to do is detect the 10v and light an LED to give an indication of this "off-hook" call-in-progress state.

I suspect a simple circuit powered by a AA battery should be easy enough to do, but not sure how to go about it, nor have I found anything that does something similar that I can hack together.

\$\endgroup\$
3
  • 1
    \$\begingroup\$ Also, be aware of the voltages present for ringing, as far as I remember that's another 100-ish VAC at 25 Hz (not MHz or mHz) or so. Your circuit has to survive that as well. \$\endgroup\$ Apr 30, 2018 at 22:27
  • \$\begingroup\$ Just typing into Google "phone line in use indicator" comes up with a number of circuits. You use to be able to buy these, but apparently not anymore. They're built into phones now. \$\endgroup\$
    – user71659
    Apr 30, 2018 at 22:50
  • \$\begingroup\$ The specifications for POTS (in the US) requires that the maximum load on the line, on-hook, is \$5\:\text{M}\Omega\$. Off-hook, you may be able to pull power from the line for the LED without a battery. However, you'd have to work out a way to release that load immediately when things go back to on-hook. I also wanted occasional blinking -- every 15 seconds -- to indicate functionality. I could just barely meet this with a high efficiency LED and circuit. No batteries, though. It will be much easier with a AA available: 3V circuit. \$\endgroup\$
    – jonk
    Apr 30, 2018 at 23:54

3 Answers 3

3
\$\begingroup\$

You could also use a small moving coil meter (eg VU meter). They draw 50uA for full deflection, which IIRC is less that the 500uA you can draw when on-hook.

Put a flouro orange sticker on the needle as a flag, and cover half the meter face. When the meter falls, the flag is visible.

schematic

simulate this circuit – Schematic created using CircuitLab

Telepermit specification states (Can't see a US one, but I don't imagine it's much different)

6.7
On-hook line current
(1) The total current drawn by all equipment connected to a PSTN 
line within a customer's premises in the idle condition shall not 
exceed 1 mA.

(2) In the on-hook state the direct current drawn from a nominal 
50 V supply by any single item of equipment shall be not greater 
than 120 µA.

* The on-hook current drain should be as low as possible to allow 
customers the freedom to install several telephones without undue 
risk of exceeding the maximum value for the line.
\$\endgroup\$
6
  • \$\begingroup\$ In the US, the spec is that the on-hook load cannot be less than \$5\:\text{M}\Omega\$, as the total for ALL devices attached to the line. I know. One of the long-time standards committee members lived nearby and came to my house to hand me the official specifications and talk with me about the standards, for a bit. Nice man. \$\endgroup\$
    – jonk
    Apr 30, 2018 at 23:55
  • \$\begingroup\$ I am pretty sure that 5M spec was for old phones with mechanical switches. As soon a phones got electronics the specs were much looser to allow for electronics to be kept up. \$\endgroup\$
    – Henry Crun
    May 1, 2018 at 1:00
  • \$\begingroup\$ The specification, to my knowledge, isn't changed often. The standard I received is a recent one, too. It was updated just 10 yrs ago, the copy I have. And I don't think there have been material changes to it, since. These things do NOT change fast. Lots of meetings, debates, etc. It's hard enough just agreeing. I'm pretty sure the standard remains. However, that doesn't mean central offices don't change their equipment. But it does mean you cannot count on it. You'd be using the phone system "out of spec" and "at your own risk." (They need to detect leakage into Earth from broken lines.) \$\endgroup\$
    – jonk
    May 1, 2018 at 1:58
  • \$\begingroup\$ The spec above is much what I remember from circa 1990. The phones were for export, and I don't recall that they could not be used in the US, but europe,uk and jp were the big market IIRC. Maybe they needed a power adaptor for the US, but it seems odd that the US spec should be so out of step. Well, slightly odd anyway. \$\endgroup\$
    – Henry Crun
    May 1, 2018 at 3:56
  • \$\begingroup\$ Looking at the OP's handle, he is probably subject to something like the above, rather than US specs... \$\endgroup\$
    – Henry Crun
    May 1, 2018 at 3:59
2
\$\begingroup\$

The DC polarity isn't always guaranteed so use a bridge rectifier firtly and make sure the diodes are reverse rated for 200 volts minimum and that they don't have a sloppy reverse recovery time. Something less that 1 us will be OK but that rules out some suppliers of 1N400x devices.

I'd use a low voltage comparator powered from the AA battery (consider using 2 batteries in series to open your comparator choices up). I'd make a stable voltage reference for one comparator pin and use a potential divder from the rectified line on the other. If the line voltage is high then you want the comparator output to be high. If the line voltage is low (off hook) then this triggers the comparator and turns on the LED via a suitable resistor.

\$\endgroup\$
1
\$\begingroup\$

What about this? The LED will only be illuminated by the current drawn the off hook phone. The diode is required to complete at the AC ringing circuit and 1uF capacitor ensures that the off hook audio isn't distorted by the diodes.

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.