off hook signal detector for land line phones.Does this exist?

 My grandfather leaves his cordless phone off the hook all the time. Since it is a multi phone cordless system it is frustrating to find it or go there and take care of it?  I would like to build a unit that will detect the sound/signal from the phone company to alert you that a phone is off the hook, and have it cycle the power on the base to reset the connection to the handsets.  Any ideas?

sort by: active | newest | oldest
In North America, the "off hook" warning signal is comprised of 1400 Hz, 2060 Hz, 2450 Hz, and 2600 Hz, on for 100ms and off for 100ms.

I have an instructable on how to use a PIC processor to detect specific audio tones. It produces an output which goes high while the desired tone is detected. Heres a link:


It can only detect one tone at a time,and it has an upper detectable frequency limit of 2148 Hz (so It wouldn't be able to detect the 2450 or 2600), but it still could be used effictively in your application. You wouldn't necessarily need to confirm the presence of all four tones, maybe just one or two. You could decode the lower two tones and AND them together to help eliminate false hangups from occuring.

The output of the frequency detector could then trigger a relay or whatever you want to use to cycle power the the phone base unit to get it to "hang up" again.

I don't know if you are comfortable with using a PIC, but if you are this one idea. There are ICs out there that are designed to detect these and other phone tones, and you may find them under the description of "call progress" or something like that. I had tried to find sources of them in the past without luck, as most were obsolete. Higher power DSPs are used nowadays in applications that need to detect these kinds of tones.
justmefjs2 (author)  LargeMouthBass5 years ago
Question while I gather my supplies. Do you think I will get away with just dropping this in parallel on one of the phone wires to avoid having it take the line off the hook while monitoring? Yes...in line with the phone base.
The monitoring circuit you are building should be placed in parallel on the phone line, just as if it were a second phone or an answering machine. It could be connected to any phone jack in the house, even in another room.

When designing your circuit, you will have to take into consideration the interface to the phone line itself. There are fairly large DC, AC , and transient voltages present on landlines, and any equipment you design must withstand them. The normal on hook voltage is about 50volts DC, and the ringing voltage for an incoming call is on the order of 90 volts!

Since your circuit doesn't need to take the phone line "off hook", it should be designed to not load the phone line. This usually means capacitors to isolate the large DC voltage, plus transformer isolation to pass the audio to your detector circuit.

Phone interface circuits and all the details is too much to go into great detail here, but do some web searching if you aren't familiar with it. One site with a good guide is :


justmefjs2 (author)  LargeMouthBass5 years ago
This looks great. I have been looking for a reason to get an pic programmer, so this may be it. Thank you!
iceng5 years ago
good reasoning, what are your skill set and allowance.

justmefjs2 (author)  iceng5 years ago
My skill set is pretty flexible, and I have seen the line indicators. I was hoping to find something that would sense the klaxon sound they use to try and get your attention. I wanted to avoid the timer since he is pretty coherent, and makes long and short calls.
If we assume you know what you are doing, then look for "off-hook telephone line indicator"

The next trick is to put a relay on the line to drop it all out, and restart it. You'll have to decide on a suitable time-out for the circuit - If grandpa's phone calls are long and rambling, you need a long time out !!! Then you'll need a timer.

Alternatively, you COULD be really clever, and use a cell phone to tell you that the line is open (after a period) , it sends you a text to tell you, and you text back to reset the line