My aim

I'd like to use a 'normal' telephone as an audio and switch interface for a project. I want to design a circuit that has the telephone A and B wires on one side (ie. an RJ-11 socket) and has four connections for my other work on the other:

  1. Receive a high/low signal as to whether the phone is on or off the hook
  2. Receive the audio being spoken into the phone's microphone (only needed when the phone is off the hook)
  3. Send an audio signal which will be delivered through the phone's speaker (only needed when the phone is off the hook)
  4. Send a high/low signal for whether the phone should ring (only needed when the phone is on the hook)

I've been reading some old usenet posts but my electronics engineering days are a long way in my past, I can't quite decipher what I should be building. Can anyone help?

I've put a representative schematic below.


simulate this circuit – Schematic created using CircuitLab

What I think I know

The speaker/mic

I think I can split out the speaker/mic signals like this


simulate this circuit

The ring

I think I'm also going to need an inverter in this circuit, to generate the 75vRMS 25Hz AC signal to power the ringer (UK ring is 0.4s on, 0.2s off, 0.4s on, 2 sec off).

I still need to digest this inverter circuit, but I can probably figure out the components I'd need for the above (though I'd probably have difficulty isolating it properly)

The off-hook detection

I don't know if I need to generate the 50V base voltage that a telephone network usually provides — I imagine this requirement will come from being able to detect if the phone is off the hook or not.

Where I need your help:

  • I don't know how I'd stop the ringer signal from impacting the mic/speaker setup
  • I don't know if anything above is very wrong

Does anyone have any advice?

  • \$\begingroup\$ Do you want this to operate with any telephone that can connect to the PSTN, or is this a unit modified for your purpose? \$\endgroup\$
    – Jim Mack
    Commented Mar 28, 2020 at 21:56
  • \$\begingroup\$ that istructable is mostly wrong. do a search on "SLIC schematic" for some better circuits. \$\endgroup\$ Commented Mar 29, 2020 at 0:09
  • \$\begingroup\$ @JimMack ideally it'd work for any telephone I connect, but if there are inescapable differences I'd have to make for different phones then I'd consider starting with the one I have (which appears to have a modern, but physical bell ringer) \$\endgroup\$
    – JP.
    Commented Mar 29, 2020 at 8:00
  • \$\begingroup\$ @Jasen Subscriber Line Interface Circuits look to be exactly what I'm looking for! I'm assuming most work like this, so I'll be looking for one I can buy and understand during the lockdown. Thank you :) \$\endgroup\$
    – JP.
    Commented Mar 29, 2020 at 8:03
  • 1
    \$\begingroup\$ many Some IC manufactureres offer samples and test boards, SLIC circuis are also found in devices that provide FXOs - like IP telephony gateways and PABXs, I am not surte which suppliers are staill open \$\endgroup\$ Commented Mar 29, 2020 at 11:39

2 Answers 2


It looks like you will need to hold the speaker on your ear because its power amplifier is missing. Then the hybrid matching to the phone line will not be perfect so you will have a noise into the mic will come out the speaker and the mic will pick it up and the sound will go around and around as acoustical feedback howling. Very old speakerphones used a push-to-talk pushbutton that disconnected the speaker when you talk and disconnected the mic when you listen. The old and modern mics need to be DC powered.


OK; the hybrid circuit looks cheezy, but if Instructables says it'll work, what could go wrong?* If you somehow think that Instructables may not be the best and most reliable source of information on the internet, search on "POTS hybrid circuit" or some such, and engage your BS filter.

You need to power the phone -- when you detect off-hook by detecting current, you're detecting the current used to power the phone. You can get by with 12V through a 600 ohm resistor, 24 is better. 48V is excessive -- the central office supply is 48V, but it's designed to work through miles of wire. It was quite common back in the day for small PBXs to use 24V to the phones.

I would disconnect the transformer from the line when you're pumping your 75VAC into it. I might, if I knew the thing were going to get used more than once, experiment with a protection circuit for the audio bits -- but a simple relay circuit takes very little design time, and Just Works (TM).

Note that a lot of cheap electronic phones will ring if you just chop the 12V supply to them at 25Hz -- old mechanical ringers need lots of power, but new phones don't. If you're running a Western Electric model 500 phone, then yes -- give it a 75VAC sinusoid (or at least 75V RMS at 25Hz). While you're at it, if it's an old mechanical ringer make sure you give it enough current -- those things want lots of power (comparatively).

* Yes, my tone of voice was sarcastic there.

  • \$\begingroup\$ I corroborated the instructables circuit using an ePanorama article — take a look at the section titled "Modified circuit" in the 'Audio interfaces to telephones' section. That seemed pretty well researched and accurate to me — I'm basing a lot of what I'm learning on that article, do you think it's ill informed? \$\endgroup\$
    – JP.
    Commented Mar 29, 2020 at 7:51
  • 1
    \$\begingroup\$ That coupler will serve to put "microphone" output onto the line, and put line energy onto the speaker -- but it'll also put microphone output into the speaker. It works for modems, because modems are either half-duplex, or they separate the input and output by frequency. This Wikipedia article shows a proper telephone hybrid -- which still doesn't entirely kill the cross-talk, but does a much better job than what you're showing. \$\endgroup\$
    – TimWescott
    Commented Mar 29, 2020 at 15:56
  • \$\begingroup\$ What is your goal with this? Are you just trying to achieve half-duplex, or are you trying to achieve full duplex with minimal crosstalk? \$\endgroup\$
    – TimWescott
    Commented Mar 29, 2020 at 15:57

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