Analog (POTS) telephone off-hook detection

Question

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.

Answer 1

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.

^{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.

Answer 2

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.

Answer 3

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.

^{simulate this circuit – Schematic created using CircuitLab}