Somewhere back in the '80s someone decided intercoms were "cool" and "beneficial", but if you're like me you see right through these clever marketing ploys. Some of you may have been lucky enough to have a sweet Tape Deck, but I only had AM/FM radio. A lot of people now days are tearing them out in place of a whole house speaker system or just removing them entirely. Rather than go that route, I decided to make mine useful at a very low cost by adding a Bluetooth module to play music from my mobile device instead.
Step 1: Order Your Parts
You'll need some basic parts for this
1) The Bluetooth Amplifier - I bought mine on ebay from China for $14
2) A Power Supply - Anything that can supply half an amp @ 12 volts should be plenty. I actually ransacked my bucket o' electronic parts and found 12 VDC 600 mA transformer, which I was able to re purpose at no cost.
4) RCA Cable
Chances are, if you're a nerd you can re purpose this stuff from you junk pile, the only thing you'll likely need to buy is the Bluetooth amplifier. Mine took about a month to get here from China... so you could look for a local source if you want something faster.
Step 2: Disassemble the Main Intercom Unit and Test the Setup
Before you get started, keep in mind, there is likely AC power feeding your intercom unit somewhere, so don't get all hot fingers back there without fully understanding the wiring. In my case, there's a transformer that brings 120V to 40V AC that's supplying the main intercom - Still enough you might not enjoy touching it, so be safe and unplug it.
Undo the screws holding it to the wall, inspect the ball of wire. In my case it had a "AUX" plug with an RCA header installed, if you're not so lucky, you may have to get more creative in how to plug in your Bluetooth audio feed. I cut apart my old RCA cable, inside the cable you'll find a shield (ground) and an inner wire that will be your signal. Strip off the sheathing enough so you can connect your shield to the - speaker terminal and the inner wire to the + speaker terminal. Connect the other side to the Aux port. Connect power to your Bluetooth board, and plug your intercom back in.
It's a good thing to test that everything works before you get crazy with mounting it nice. For me, everything worked right away no problems.
Step 3: Permanently Install Power to Bluetooth Board
Now, you'll want to figure out how to get power to your Bluetooth board more permanently. I first attempted to steal power off the main board of the intercom. It had a couple AC/DC circuits, so I leeched off one and found it didn't have the power needed for my board. So I had to do it the hard way. Maybe you can get lucky and pull power off the intercom somewhere... but I had no luck. As always, while probing around with your multi-meter, be careful not to short anything or touch anything that may have high voltage, then remove power before doing any soldering on the board.
I went digging in my pile o' electronics crap, found a 12V AC/DC converter and started pulling it apart. Obviously, we're going to be working with 120V AC power, so don't do this if you're not entirely comfortable doing this sort of work. Also make sure you disconnect the circuit before starting work.
My transformer had the standard windings, then a little PCB with the diode rectifier and a capacitor. The polarity on the capacitor will show you where your V+ and V- outputs are. Solder in some new wire to connect up your 2.1 mm DC barrel jack.
My intercom already had AC power and a transformer, so I just connected my new transformer in parallel to the other one, and routed out my new 12V line (which I soldered onto the transformer) out of the electrical box and into the main intercom housing. I was able to keep both transformers in the electrical box, so no high voltage wires were accessible without removing two plates.
Step 4: Cram It All Back Together
Now that everything is hooked up, test it out once more. If everything is good to go, cram that ball of wire back into the intercom enclosure and enjoy your sweet new Bluetooth interface on your vintage intercom system.