As the microphone base, I used a Retekess intercom system, which you can find on Amazon. This caught my fancy because it had a gooseneck microphone, a weighted base, a speaker amp that we can use for the audio receive, a power supply and a mute button, which I thought I could reuse for the push-to-talk function. Plus it looked decent enough in our setup, the top is real brushed aluminum. And this is quite well built, you get a lot for your 50 bucks.

The work started with the reverse engineering of the Retekess intercom. All the connections I ended up needing were on the small daughter board behind the control panel, with the PCB trace side drawn below. I was able to identify the supply (12V), and best of all, unused contacts on the mute button. The point marked "tone input" is where you hook up the output of the tone generator. 

You have to make two mods to the switch itself. You have to remove the click spring from inside it, so it becomes a temporary push-button instead of a two position on/off. Then I simply disabled the muting function by cutting one of the connections - can't remember which, see picture in the video.

I also figured out the wiring for the square connector at the back. The speaker connection is going to be our output. I'll use the mic connection to feed the voice down when we get to it, so it will power the internal speaker.

On the next page, I started to write down ideas on how I could make the pulse shaping circuit. I originally thought of using a 74LS121 single monostable multivibrator.

On this first rev. I also have a Schmitt trigger, which I ended up deleting because the multivibrator circuit absorbs the bounces anyhow. 

I thought I could use the single multivibrator to trigger both ways, which should be possible from the datasheet. But it does not work in practice, it favors one direction.

So the circuit above did not work, and I decided to use a 74LS123  instead, with two separate multivibrators: one triggered on the positive edge, and one on the negative edge. I then combined the signals with a 7400 AND gate. That circuit worked fine. 

However, in the final rev, I used a CB4096 instead of the 74LS123. The CB4096 is functionally equivalent, although the pinout is different, but it is CMOS. That allowed me to power everything with the 12V from the Retekess supply.

Note the difference in the pin layout and RC values of the CB4096B, which replaced the 74LS123 in the final rev. 

Next step was to make an audio gate switch. You could use an analog switch IC, but I did not have one on hand. So, instead, I used a MOSFET as the gate. The difficulty here is to prevent the tones from leaking when it's off. The first serial switch circuit did not have enough extinction ration - the BS170 must have had too much capacitance in the off state, and some tone leaked through. I thought I would need two stages, with one shunt and one serial switch, but then the first single shunt stage was good enough. So I ended up with the schematic at the bottom. And while I was at it, I also deleted the AND gate and made a diode OR gate to combine the signals, which further reduced the size of the circuit.

Here is the final schematic. The tone generator itself was lifted from another page of my lab book, and uses an XR2206 chip. The two tones have to be tuned to 2525 Hz (intro tone) and 2475 Hz (outro tone). I used my trusty HP frequency meter for that, but a frequency reading on a modern digital scope will do fine. Or do it by ear if you have a musical one!

 As I said above, I eventually replaced the 74123 by the CB4096, and had to change a few resistors - the values for the CB4096 are the ones circled in the diagram below.

Here is how I wired it up on an Adafruit "perma-proto" board, which is a prototype PCB that is wired like a breadboard. So you can make permanent prototypes, as the name implies. This sketch is missing the adjustable volume 50k pot, which I added later. That's why there is a note that says "replace 47k with 50k pot".

This is how it looked when finished. This early version is missing the adjustable volume pot which I added later as I indicated above.