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 😉 .

(The diagram has been updated according to comments)

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!

      • DennyMo says:

        Did you post the updated schematic yet? It looks like it’s still wrong, wanted to double check before I start soldering anything.

        • siggi says:

          Hey Denny,

          Thanks for reminding me to update the schematic..!! :)

          I didn’t find the original file, so I just recreated it in Cadsoft Eagle. I’ve updated the post.

          • Jack says:

            Hey Siggi,

            I’m sorry but, one more time on the diagram please.

            Lines 1 and 2 are still crossed in the diagram.

            I’m sorry to ask, but would you kindly post another corrected diagram, shownig the straight through wiring you described in the text.


          • siggi says:

            Hey Jack,

            Haha .. guess I was in a bit of a hurry when I redrew the schematic.

            Thanks, I’ve fixed it now .. hopefully I didn’t make another “hasty” mistake 😉


  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. 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???

  11. Terrence says:

    Heyja Siggi.

    Great project. I am trying to make something along the same lines.

    a little box which you plug your PC, cell phone and landline telephone into… Then you plug your smartphone’s earphones into it as well and select which one you want to listen to with a rotary switch.

    Works great for the PC and Cell, but the landline just kills the connection.
    It’s as if the Landline Mic ground and Loud speaker ground can’t be connected. But I can’t think of any other way to do it.

    I see that Kevin had the same problem, but never actually came to a conclusion.

    So my question… Do you know of any way around this?

    • siggi says:

      Hey Terrence,

      OK, so here are my thoughts on this:

      1. It’s possible that the original headset is able to disconnect the call by joining the grounds.
      You can test that by disconnecting either the speaker or mic ground and use the rotary switch. If it disconnects after the mod, then you know that this is not the case.
      If that’s the case, then you can use a small capacitor between the landline mic and speaker grounds.

      2. It’s also possible that when you use the rotary switch emits a pulse when you switch.
      You can test that by keeping the rotary switch on the landline, then make a phone call. If it doesn’t disconnect when you pick up the phone, then you know that this is the case.
      If this is the case, then I suggest measuring the pulse using an oscilloscope and create an RLC trap on the ground line or switch before you pick up the phone.

      Let me know if either of these solutions were the case, or if it STILL doesn’t work! :)

      Best of luck!


      • Terrence says:

        Wow, Thanks for the reply.
        I’ve not even managed to get to the switch yet. I am just trying to use my smartphone headset with my land line. Straight through connection.

        I did come right though. Using a line transformer to create a second ground for the Mic, one separate from the speaker.
        Works great now.

        Thanks for the help!

  12. hassan says:
  13. Grounded says:

    I build this, but I read the pinout for the RJ9/4P4C connector backwards. I also didn’t source the phone parts from the “Radio Shack” near me. I sourced them at the nearest “Value Village” thrift shop by buying a phone for $3, instead of buying each connector for $4. I also bought a 2 pack of 3.5mm stereo audio jacks for $4 and a project box for $4. If I had time, buying a pre-built box off of eBay would have been cheaper.

    Since the phone I bought only used 2 pins – pins 2 and 3 – which is the minimum for connecting to the public telephone network – I used cat 5 cable to make the pins without having to solder. I replaced all pins so that the spring across all pins was equal.

    I removed 2-3cm of coating from the cat 5 twisted pairs. I used the colour code Blue, Orange, White/Orange, White/Blue. The white’s being my grounds and the solid colours being the T1 and R1 pins.

    I put the wires through the holes the connector pins came through and then bent them back. The RJ9 connectors I had, had slits in the back for the pins to go in. I put the wires through these slits. I used needle nose pliers to pull the wires all the way up and in, so that the plastic sheathing was as far ahead as it would go. I then cut off any excess.

    I put some electrical tape around the 4 wires so that they wouldn’t move very much. The CAT5 wires aren’t as springy as the brass pins that were in there, but they’ve got plenty of spring memory to them for this purpose.

  14. Ayub says:

    RJ9(Male) to 3.5mm Smartphone headset connector (Female)

    Please check the above link and help me

    How do i make DIY Headset Buddy 3.5mm Smartphone Headset To RJ9, When i try
    As you aware TRRS female connection has only ground pin, and i’m unable to connect to RJ9 mail connector. when i connect getting a beep sound.

    need your help on this. Please provide me the circuit diagram for this and guide me how to increase volume in headset. please reply as earliest

    T- Left
    R -Right
    R- Ground or MIC
    S- Mic or Ground

  15. Ayubkhan says:

    RJ9(Male) to 3.5mm Smartphone headset connector (Female)

    Please check the above link which i did.

    I tried the DYi connection but it is not working, need your help. when the common ground (Green & Black) is connected there is a beep sound , if i remove (Green or Black) then either MIC or Speaker is not working not sure how to fix this issue , Please help me ([email protected])

    • siggi says:

      Hey Ayubkhan,

      There are two types of these phone headsets, the only difference is the MIC and GROUND pins.

      What I suspect is that you need to swap the ground and mic connections.

      This is your current configuration:


      Try changing it to


      Let me know how that works for you! :)

      • Ayubkhan says:

        Thanks for your reply,

        I tried changing it as mentioned bleow.


        Great there is an improvement , MIC is working but there NO sound in speakers.

        then i tried combined both wires (RIGHT & LEFT) and common connection (GND) getting a beep sound.
        do we need to add any resistor or capacitor ?

        if i tried below getting beep sound

        there is any other thing we can try ?

        • siggi says:

          Hey Ayubkhan,

          Try connecting only one speaker (left or right) and disconnect the other.

          Don’t connect the speaker with the ground.

          Can you take a photo of your current setup? Maybe there is something there that needs fixing.. :)


  16. Ruan says:

    Hi there, this looks interesting.

    Perhaps you could clarify/help. I have a plantronics CS60 headset that connects to a landline telephone. We use this in a operating theatre to communicate to a surgeon during surgery. On the other end of this line, we connect a modified landline phone to a speaker and microphone (IP cameras also active) – for teaching purposes. I am, however, unhappy with the audio quality (no doubt the telephone lines).

    I would like to modify the plantronics headset (or make a connector box) to plug it into a pc instead and use Skype (or something like it) to have clearer quality audio. Note, by-pass the telephone completely.

    Would the above help set me on the right path?

    Thank for any help you could give.

    P.S. I have no budget for this, otherwise I would just have gotten another headset…
    Anything I’ll have to buy will come out of my own pocket.

    • siggi says:

      Hey Ruan,

      Yes, this can help you connect the headset to your computer.
      To do that, you have to connect pins 1 and 4 to a microphone jack and pins 2 and 3 to the loudspeaker jack.
      Then you can connect the headset to your computer.


  17. Hagi says:

    Thank you siggy for providing this great information about the headset wiring. It helped me a lot.
    I tried to built a simple 4p4c to 3.5mm adapter (Without the switch) for a client of mine that didn’t had a built in headset support on his Flexset 280D office phone (’ve attached a sennheiser PC21 single-side headset, But the volume of the headset is too low (I thought that bridging the stereo will amplify the output, but then remembered that the original output is mono…..).
    I ordered Plantronics M12 phone amplifier that will hopefully enhance the headset volume and will head some switching functionality since I didn’t installed a switch (
    Will let you know how it worked out once I’ll install it.
    P.S. Connected the adapter I’ve built to an Uniden AS7412 phone (, and the mic seems to be the speaker (reverse headset connections)

    • Siggy says:

      Hey Hagi,

      You’re most welcome. I love building stuff and sharing it with others :)

      Let me know how the M12 works :)

      As for the Uniden phone, it doesn’t surprise me. This is how vendors “lock in” their customers to only buy add-ons from them. But it’s an easy fix, either swap the mic with the speaker plug or swap leads to pin 3 and 4. :)


  18. Fred says:

    Works great if I connect a headset but then I wanted to go further… put the phone speaker to line-in of my pc and mic into line-out… I was hoping to hear voice from phone on my computer speakers will sound play on pc going to phone… Unfortunately when connected to pc it don’t work at all… can it be a question of impedance or ground separation in pc?

    Any idea to fix it will be very appreciate…

    • Siggy says:

      Hey Fred,

      I suspect this is because of audio settings in your computer.

      You can test this by using your headset and connecting the mic to line in and the headphones to line out.

      Then you need to talk into the mic and see if it comes out of your speakers. If nothing comes out of the speakers, you need to configure your computer so the line in goes directly to your speakers.
      After that you need to play something on your computer and see if you can hear it in the headphones. If you hear nothing, you need to configure your computer so the line out plays sounds from your computer.

      This is a very complicated configuration for a computer and I suspect it will not work like you want it to once you have everything configured. But please do try it out and let us know how it goes :)

      • Fred says:


        Thanks for the feedback.

        I made some extra test and I suspect it’s linked to the phone. If I place a splitter on the speaker output of the box in order to be able to connect a real headset and same time send speaker to line-in it work (I can hear voice from phone on both pc speaker and in the headset) but if I remove the plug from headset it also cut on laptop… I need more try…

        Finally my goal is to use a Bluetooth headset connected with the pc and send all back to the phone… you are right will be quite tricky to make it work…

        I will let you know if I succeed…


  19. Chris says:


    I’m hoping you can set me straight on wiring. I have a AT&T deskphone that has a headset out 2.5mm (TRS) that I plug my Plantronics headset into directly with a quick disconnect cable. The clarity on both the earpiece and mic are excellent. I’m trying to build a switch box like yours to switch between my PC and phone and only using the single headset.

    I’m having trouble determining the pin out combination for connecting a 4P4C modular to a TRS 2.5mm. For the sleeve ground, are green (pin 3) and yellow (pin 4) the common ground? Leaving black (pin 1) as the mic and red (pin 3) as the earpiece.


    • Siggy says:

      Hey Chris,

      TRS is only 3 pin/ring
      What you probably have is TRRS which is 4 pin/ring.

      There are two standards for TRRS connectors:
      1. (OMTP – old standard) Left – Right – Mic – Ground
      2. (CTIA – new standard) Left – Right – Ground – Mic

      The earpeace is either left or right audio wire, OR left and right connected together.
      If you’re only using one audio channel, I would just wire the left and right together.

      To figure out if you have the old or new standard TRRS, you can measure resistance between the tip and pin 3/4.
      If the tip and pin 3 show resistance, then you have the old standard.
      If the tip and pin 4 show resistance, then you have the new standard.

      Best of luck! :)

  20. Ram says:

    Hey Siggi,

    Great work !..

    I am trying to do a similar project. When i tried measuring the signals coming from RJ-9 connector, it had some dc content in it ( DC offset ).. Does that cause any problems to headphones?


    P.S. : The headphone that i used for testing is not working anymore ! 😛

    • Siggy says:

      Hey Ram,

      Thanks :)

      Well, if there’s constant DC voltage then that might have a negative effect on the headphones. You might want to cut the DC by using a coupling film capacitor for the headphones and microphone.
      You’ll get better audio and less chance of destroying your equipment 😉

      If you do end up adding a coupling cap, remember to add a ~10K (high ohm) resistor behind it to the ground ..


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