Please be careful. Unplug first when working on anything electrical. 110V AC can kill you!
Step 1: Acquire parts
--Pair of amplified computer speakers. A medium-to-large case would make things easier. Also, a standard, integrated power cord (non-wall-wart transformer) can free up a lot of space inside. I used a used set of Gateway Edison 2.0 speakers I bought from a used computer store for $5.
--An outlet, which I bought at a local hardware store for $0.59. You can decide if a cover is required for your application.
--2 Rectifiers. I bought mine from RadioShack. I decided to buy the 4A, 400V-capable units, to be on the safe side. They cost about $2.50/apiece.
--2 Solid State Relays. I followed Alan Parekh's example and used 2 Crydom D2W203F relays. I bought these from Mouser for $8.80/apiece. These are by no means the only units that can be used, but they work great.
--2 Potentiometers, to match the original that will be replaced. Mine was a 10k. These cost me $3/apiece at RadioShack.
--2 knobs to fit on the potentiometers. These cost me $3/pair at RadioShack.
--If your speakers have a wall-wart transformer, you will need a power cord to get the 110V AC inside the speaker cases. You can buy these from RadioShack, hardware stores, or simply cut one off of an old appliance you are no longer using. Again, please use caution and unplug anything before working on it.
-Small-gauge wire (I used 22ga stranded)
-AC-rated wire (or just cut a bit off your power cord, like I did)
-Some others may be required. I also used scissors, a cordless drill, side- and end-cutting pliers.
Step 2: Take apart the speakers
Then, I carefully opened up the case on the computer speaker that contains the amplifier. I practiced on the other speaker first, in case I destroyed it accidentally. Unfortunately, the speakers that I was using were glued together, which made them difficult to pry them apart without breaking.
As I was taking the speakers apart, I forgot to take off the volume control knob and remove the nut underneath that held the potentiometer in place. This broke the potentiometer, which turned out to be a blessing, as I will show later.
Step 3: Position the outlet
Step 4: Wire up your rectifier/relay array
The purpose of the rectifiers is to take the alternating signal from the speaker outputs and convert it into a DC signal. This signal can be used to control 110V AC through the relay.
Solder the speaker output wires to the AC (or ~) leads of the rectifier. Then, using short wires, connect the + lead of the rectifier to the + lead of the relay. Connect the - lead of the rectifier to the - lead of the relay.
Cut the wire attaching the other speaker and strip the ends back. These will be your AC leads for the second rectifier. Repeat the process, referring to my pictures. Wrap all leads in electrical tape to prevent short-circuits.
We will leave the arrays as-is for now and focus on replacing the potentiometer next. This will make things easier, as connecting the AC severely limits working space.
Step 5: Replace the potentiometer
The next thing to do is solder short wires to the holes where the leads of the old potentiometer used to be. This should take 8 wires.
Next, you should make any necessary modifications to the faceplate. I drilled two large holes and two small locating holes for the potentiometers, as well as cut out a hole for the new power switch.
Test fit the potentiometers, and cut the shaft to length using a small saw.
Now, making sure to keep the same orientation, solder the other ends of the wires to the new potentiometers.
Then, solder the two wires left over to the switch. You may have to put the switch wires through the switch's hole before soldering. Wrap the leads in electrical tape to prevent shorts.
After this, I plugged in the wall-wart transformer and powered up the circuit to test. The LED power indicator lit, so I plugged in a pair of headphones and listened for changes in sound when I turned the potentiometers. This is how I figured out which Potentiometer controlled left and which controlled right sound channels.
Step 6: Wire up the AC and Relays
Solder the power cord and the additional wire stubs to the wall-wart transformer's plugs. You will need one short wire to go to the outlet, as seen in white, while the other side must go to both relays. I used a single wire and stripped some of the insulation from the middle and end of it to suit this purpose.
Now you are ready to solder your relays on. Connect the + to the Rectifier's positive and the - to the Rectifier's negative. Then solder the black wire from the power cord to one AC lead on each of the Relays. Then solder two short wires from the other AC lead on the Relays to each of the outlets, where the tab had been broken previously. Wrap all connections with electrical tape.
Here I will break down all the electrical connections required.
Right Speaker to AC legs of Rectifier R
Left Speaker to AC legs of Rectifier L
+ Rectifier R to + Relay R
- Rectifier R to - Relay R
+ Rectifier L to + Relay L
- Rectifier L to - Relay L
Power: 1 to Wall-wart, outlet side A
1 to Wall-wart, Relay L AC1, Relay R AC1
Relay L AC2 to Bottom outlet side B
Relay R AC2 to Top outlet side B
Step 7: Fit everything into the case
Also, don't forget to run the wall-wart transformer's DC power to the circuit board. In my case this meant wrapping up the excess cord and running a short bit of it out the bass port in the back of the speaker housing to the original location of the power plug.