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On a call indicator light Answered

I need to figure out a cost effective way to let people know I'm on a phone call. This solution needs to light an LED automatically when I am actively using the telephone. Is there a way for a simple circuit to sense when a phone line is in use and not cause any line interference? The circuit doesn't need to be powered through the phone line, it just needs to be able to sense that the line is in use.

I know how to solder and understand basic electronics concepts but am also very teachable. If someone can point me to a schematic I can build it.

There are ready made, but expensive, solutions for sale but the costs are restrictive. Any help is greatly appreciated.


Jack A Lopez

10 months ago

I am guessing that you are talking about the kind of phone that plugs into a jack in the wall, rather than the newer kind of phone that has a radio transceiver it it, the so called, "cell phone" or "mobile phone" or "smart phone"

The old kind of phone is sometimes called POTS, an acronym for, "plain old telephone service"

Regarding the question of how to find a circuit diagram for an in-use, indicator light, my first guess was to try an image search for this.

As an example, here is an image search using DuckDuckGo, to look for, "circuit for phone line in use indicator light"


I dunno. This looks like a nice one,


Although, the line connection does not have the four-diode, bridge rectifier, so I guess you have to get the polarity right.

This one looks interesting too,


Moreover this one looks like it does not need batteries.

Although you kind of have to wonder about that. The 3.3 K resistor in series with its LED was probably chosen so that this LED current is so low it will not keep the phone line in an off-hook state. The trade off is that LED is not going to be very bright. I mean it would be easy to see at night, but maybe not during a sunny day.

Anyway, I think basic lesson here is to use an image search when looking for circuit diagram ideas.