DIY telephone handset-headset switch box

I wanted to do something a  bit different one day at work (I had been working like a crazy man and had to give my brain something completely different to do to keep my sanity), so I decided that I wanted to be able to use a computer headset instead of the handset at work. The best part of this switch is that you don’t need any active or passive components to change impedance or signals!!

4P4C (sometimes called RJ9, RJ9, RJ10 or RJ22) plug information

To begin with, you must understand how telephone handsets are wired to the 4P4C plug. Fortunately, the connections are dead simple to understand. The 4P4C connector has 4 contacts. Contact 1 and 4 (the two outermost) are the microphone contacts (polarity doesn’t matter here) and contact 2 and 3 (the ones in the middle) are the speaker contacts. See diagram 1 for more details.

4P4C contacts

Diagram 1 - 4P4C (RJ9 RJ10 RJ22) contact diagram - ©DnetSvg CC-BY-2.5.

Schematic Diagram

Telephone headset switch schematic diagram

Telephone headset switch schematic diagram

As you can see, the schematic diagram is very simple. The most complex part in this schematic is connecting the ground signal together ;-) .

Part list

2x 3.5mm stereo jack connectors, preferrably the screwed type.

2x 4P4C (RJ9, RJ10, RJ22) connectors

1x dual state switch

1x hobby box (plastic box to keep everything tidy)

Some spare wire (I used a bit of Cat5 cable)




Here you can see the internal wiring of the switch box. To hold the 4P4C connector, I used superglue.

Handset switch wiring

Handset switch wiring


Below you can see the box completed. The only thing left to do is to label it and put it to use.

Completed switch box

Completed switch box

The switchbox on my desk. It is really easy to use and the sound quality doesn’t suffer a bit (the sound quality in phone systems are horrible to start with .. ;) )

Switchbox in use on my desk.

Switchbox in use on my desk.


If you build this box, please let me know in the comments below!! :)


  1. Salvador says

    Hello Sigurður,
    Great and clear post.
    I’ve just made my own two days ago. I wonder if you get the problem of having audio only on one side of the headset, as I have.
    If so, do you think it could be fixed by adding some resistors in the mono to stereo wiring before the 3.5mm jack of the stereo?
    I’m trying (unsuccessfully) to find a schematic of that wiring (as this should be kind of common case), and I don’t posses enough electronics skills to figure it out by myself.

    • siggi says

      Hi Salvador and thanks for the comment! :-)

      Do you have a stereo jack for the mic and the headphones?
      I used stereo jacks and just soldered the left and right channels together. I get perfect audio to both of the speakers.
      I didn’t worry much about matching the impedance of the speakers or the microphone since I get a decent power transfer. If you want to match the impedance of the speakers, connect the left and right channels together and add a ~10 ohm resistor in series with the speakers. You shouldn’t have to worry about the microphone (if your headset doesn’t have dual mics).

      If you want, I can create a new drawing for you, but as a first attempt, solder the left and right channels together (shown on the drawing), and see if doesn’t work for you :-)

      Kindest regards,

  2. Kevin says

    Hi, great post, very similar to what I am trying to do on a linksys SPA942.

    However, I am try to use a smartphone headset (an iPhone compatible one) which of course has a common ground between the audio and the microphone. Pin outs I am using are here:

    In yours you have used RJ9 3 and 4 (green and yellow) as ground. So when I soldered mine, I soldered both 3 and 4 to pin 3 (ground).

    When the phone is on mute, it works fine, but when I unmute it the audio immediately breaks up and then stops. The microphone also does not work. If I leave RJ9 pin 4 (yellow) unsoldered, the microphone does not work (which makes sense)

    Can you tell me what I need to do about having two separate grounds?


    • siggi says

      Hi Kevin.

      I don’t think you need to worry about two separate grounds, the phone is so new that the ground for the mic and the audio are probably the same.

      Do you have a 4 pin jack, or only 3 pin jack? (You need the four pin jack)


  3. Matt says

    I have a question, do you still have to pick up the receiver while on a call and then switch to the headset to use it?

    • siggi says

      Unfortunately, yes. I did not design any automatic receiver lifter with this (although, I thought about it…).
      It’s not a big deal for me though.

  4. Matt says

    Ah, I work for a small contracting company and have been looking into a way to use a standard 3.5mm headset instead of paying for plantronics headsets. Do you think it would be hard to incorporate a diy handset lifter?

    • siggi says

      I don’t think it’s hard to incorporate a diy handset lifter.

      I would get a servo motor and create a two state PWM circuit, one for picking up and one for hanging up.
      You would also need to create some mounting bracket to position the servo correctly for the handset.

      It should be easy enough to design a PWM servo controller using a 555 timer circuit and a switch.


  5. Sean says

    Inspired by your idea, I have built one switch box for my phone between handset and headset. It worked very well. I have taken few shots of my box. But I don’t know how to upload them.

    • siggi says

      Hi Sean.
      You can’t upload on my website, but you can upload the images on a public network like, picasa or flickr, then post a link to it here. :-)


    • timelake says

      Read all the above and wondered if you can suggest.

      I have a transceiver with a, 8-pin mic connector.

      How complicated would it be to build a switch box to accommodate, say, six boom headsets and mics?

      I intend to have these plugged in all the time and just switch between them depending on envionmental factors.

      I’ll appreciate your help with this project.

      • siggi says

        Hey timelake.

        Thanks for the comment.
        Well, for your transceiver, it really depends on how the signal is interpreted from the 8 pin connector, but let’s assume that it only uses two pins for the speaker and two pins for the microphone.

        You can base your design on this design, but instead of using a two state switch, you can use a 6 state switch like this one:
        You connect all of the grounds together.

        If you want, I can draw up a schematic for you, but to be safe, can you please let me know what type of transceiver you have?

        Cheers! :)

  6. ColdBeer says

    Hi Siggi! Nice work!

    I’m planning to build your switch box, but I wonder why do you change the wiring between input rj9 connector (from the phone) and output rj9 connector as:
    1 – 2
    2 -1
    3 – 4
    4 -3

    Could you answer this question please? Thanks in advance.

    • siggi says

      Hey ColdBeer!

      Thanks :)

      Huh .. didn’t notice this in the diagram – I must have been in a hurry sketching it up.. ;)
      It should be like that as reversing 1&2 and 3&4 turns the microphone into a speaker and the speaker into a microphone! :p

      It should be direct like this:
      1 – 1
      2 – 2
      3 – 3
      4 – 4

      I’ll update the drawing. :)

      Good luck with the box!

  7. icefixer says

    Hi Siggy,
    I am trying to use Microsoft LX-1000 but i can’t make microphone work with your diagram. Do you have any idea?

  8. Onur says

    Hi siggi, Nice work and thank you for your post.
    I did the same box and it works. but my voice not enough volume to the other side. the person whom i speak cannot hear well.
    What should i do ? can i use any capacitance ?

  9. says

    Hello Siggi.
    I use a Jabra 9350e wireless headset, but not for phone. I use it with my Ham Radio. 400′ range 1.8GHz. I make a lot of cables for other hams. They must have VOX to work the Jabra. Works very well, now 3 years on mine. Now Ham radio is simplex. The audio/mic path’s must be separate, due to ground loops, RF, VOX tripping. Since the Jabra uses 1:1 in/output xformers. The Jabra grounds are not common. (by my fluke) It seems with some radios, there are still issues. HUM and VOX tripping. Kenwwod’s and Yaesu’s. Not Icom. I am trying to find a definite on polarity on the 4p4c hand set cable. I want the grounds going into the Jabra the way a phone would have. I checked with a fluke on an old phone. This is what I
    Found the 4 pins 1234 seem to be -Mic Gnd- Spk Gnd- Spk Hot- Mic hot. Input on this? Some phones have no polarity issues, but I do with some Ham Radios, and my cables. I made 30-40? or so, Audio on 2&3, Mic on 1&4, had about 3 issues. Older radios. I found some stuff on the 4p4c that is contradicting. In radio, the Mic gnd has circuitry in some rigs. The audio is straight forward, hot and chassis ground. Now Some refer to colors that have changed over the years. Others are talking about tip (+) and ring (-) The common hand set colors, red, black, green, yellow. Any Ideas? Positions, color, and mostly the polarity of the 4p4c? Pics would be great!
    Thank you, Eric

    • siggi says

      Hey Eric,

      Unfortunately 4P4C was never standardized so the polarity can differ between handsets.

      When you have Ham radios, I would venture a guess that the polarity depends on the manufacturer (that’s why they like selling their OWN adapters – because they only work for THEIR product).

      If you are creating cables in bulk for other Ham Radio owners, I would either create four variations with polarity crossing or add a DPDT switch on the end of the cable (to reverse polarity for either mic, speaker or both).

      If my assumption is correct, I would simply add two surface mount (or other small size factor) DPDT switch inline to reverse the polarity.
      Here is an example of a surface mount DPDT switch:

      Let me know how it goes! :)


  10. Vishal says

    Hi Siggy, your DYI is awesome. If I would have had the same requirements, I would have been so very happy :)

    I want to use my smartphone’s headset and mic with my workplace’s cisco 6921 ip phone. It’s got an RJ9 i/o. Materials I think will be needed are in this pic I have uploaded

    Before I cut the wires and make a mess out of it, I just needed to know if this would work. is it just connecting the wires to the right places or would I need something else like a resistor???

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