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.
As you can see, the schematic diagram is very simple. The most complex part in this schematic is connecting the ground signal together .
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.
Below you can see the box completed. The only thing left to do is to label it and put it to use.
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 .. )
If you build this box, please let me know in the comments below!!