It's annoying to have the time to exercise, but no entertainment because the tablet is out of batteries. So I decided to add a USB charger to our Schwinn 420 elliptical. The process is probably very similar for any other exercise machine powered by DC from a wall-wart (I wouldn't want to deal with mains power, and I don't know what it's like to deal with the fancy machines that have a generator).
Parts and tools needed:
- 9V/12V/24V to 5V DC-DC step-down buck module USB-output ($0.70 on Aliexpress)
- wire (24 gauge; I used scrap pulled out of a CAT6 cable)
- soldering iron and solder
- file, hacksaw and/or rotary tool
- hot glue gun, or 3D printer (for this model) and two smallish screws
This is the second in a series of three upgrades I'm making to the elliptical. The first were snap holders for a Kindle Fire. The third will be tapping into the wheel encoder to make the elliptical into a USB controller for PC games (I am waiting on some JST connectors from aliexpress so I can get one).
Step 1: Identify DC Power Input at Convenient Location
Our Schwinn 420 has a console with four screws in the back. Unscrewing the screws revealed a ribbon cable and a 3.5mm mono connection. I disconnected the ribbon cable and with a multimeter systematically tried pairs of pins until I found a pair that had about 13V between them. These were pins 1 and 3 when looking from the back, but I'm not going to show you a photo, because you should check this yourself as even if you have the same model, you might have some other variation on the connections. I then checked to make sure that there was close to zero ohms between these pins on the ribbon cable and the corresponding connections on the DC power jack.
I also checked the maximum current draw while the elliptical was operating (fan on max, elliptical shifting resistance) to make sure that the wall-wart's 1.5A rating could handle the charging. I also checked how much current was being drawn by the buck converter while it was charging two of our tablets, and made sure the buck converter wasn't getting very hot in use.
Oddly, the wall-wart is labeled 9V but actually puts out over 13V. Fortunately, both are well within the buck converter's range.
Step 2: Make Hole
I took apart the console and using a hacksaw, a file and a rotary tool made as neat a rectangular hole for the USB female socket on the side.
Step 3: Solder
I looked for a good place to solder wires to the charger from the DC power connections identified earlier. To my joy and delight, there were unpopulated through-hole solder pads on the elliptical's motherboard that were connected (with about one exception?!) to the ribbon cable. So it was really easy to unscrew the motherboard and solder wires in place. Then I soldered ground to the negative connection on the buck converter and plus to the positive connection.
Step 4: Mounting
I was originally planning to just mount the buck converter by the hole with hot glue. And I guess that would work--my previous experiments showed the buck converter didn't get hot when operating, and there would be a lot of air space around it. But I wanted to do a neater job, so I spent some twenty minutes and designed a little plastic holder with holes for screws in OpenSCAD. The holder just holds the end to maximize airflow around the converter. I screwed this in place (alas, the screws went through the front panel. And it works.
Then I put it all back together. And it works.
It is slightly annoying that there is now a blue glow constantly from that area of the console due to the buck converter's LED. It's tempting to snip that LED. I am not decided on that.
Final hint: Make sure you find ways to drape the charging cable so it doesn't get in the way of movement.