I bought an old knife blade switch off of eBay to use as a decoration. As soon as I got it, I knew I had to make it actually work. Running full 120V power through exposed metal on the wall wouldn't be very safe, so I ordered a DC-AC relay. I used an old phone charger to provide 5V of power to run the relay. It's been in use for over 4 months now without any problems.
Step 1: Materials
- You can get these switches on eBay for anywhere from $10-50 dollars. Just search "vintage knife blade switch." I paid $40 for mine.
- The relay I used was a SSR-40DA Solid State Relay. It cost $4 dollars shipped.
- Any low voltage DC power supply with work. I had an old phone charger on hand.
- Long machine screws that fit electrical box holes.
Step 2: Make It All Fit
I wanted all of the components to fit and mount inside a regular double gang electrical box. I didn't want to modify or repair the wall. First I drew the outline of the box.
Step 3: Phone Charger Too Big
My phone charger was too big, so I took it apart. The circuit board was the perfect size.
Step 4: Find 5V Positive/Negative Leads
I used my multimeter to find the positive and negative terminals of the USB jack. This is where my 5V would be coming from. Then I soldered my wires to the board to connect to the relay.
Step 5: Wiring Diagram
This is how I wired everything up.
Step 6: Cut Down Phone Charger Case
I cut the phone charger case down to the size of the circuit board. This worked perfectly and fit right inside the box.
Step 7: Fixed a Mistake
Luckily I realized that if I centered all of the components, it would overlap the trim of the door next to the switch. So I moved everything over a few inches and glued it in place.
Step 8: Drill Mounting Holes
Using a spare box as a template, I drilled new mounting holes in the switch.
Step 9: Mount on the Wall
Mount on the wall using 4 long machine screws. I used a flash in the picture so it doesn't look very flattering. You can't really see the mounting screw heads very well in normal light. I may use some fake screw heads to cover the holes on the 4 corners.
Step 10: Final Test
I metered the current and it was only drawing 20mA at 5V. It's been in use for several months now without any problems. I was concerned a solid state relay would provide too much heat, but it seems either the switch acts as a heat sink or the relay just isn't getting hot.