Bouncy Seat Hack: Power from a Phone Charger

The Equipment:

Battery-powered automatic bouncy seat and music player / noisemaker – $35 from a Once Upon a Child store in Sioux Falls.

The Problem:

I love this bouncy seat. My son loved it almost as soon as we first put him in it, and it became his bed every night. The subtle bouncing and soft music (something more original than the standard “Twinkle Twinkle Little Star” in an endless loop) were a real boon during the first couple months at home with our boy. But the thing eats batteries for lunch. I’ve been changing out 3 x C batteries (in series – remember this) almost every week since we started using the bouncy seat. At about $1.25 per battery, that’s $15-ish per month. So cancel my paid subscription to this machinated baby-sitting service: we’re going hacking!

The Plan:

3 x 1.5V batteries in series yields 4.5V. A typical USB phone charger produces 800mA of 5V DC. First, power the bouncy seat from a wall outlet using a USB phone charger alone without batteries. Next, since the phone charger and batteries connect to the same points to power the bouncy seat, add a switch to isolate power sources so I can leave the batteries in and not end up charging very-not-rechargeable batteries.

The Parts:

DPDT 2-position .5A 50V slide switch $4.51 for 10 pcs.
I probably could have used just a single-pole double-throw switch for only the positive leads, but it seemed smarter to isolate negatives between power supplies as well. If you can educate me about why that is a good or a bad idea, please leave a comment!
3′ USB printer cable kinda like this that I had lying around. Free, I think.
About a foot each of 20-something gauge red (still too big, but it’s what I had) and black wire. Wire is cheap.

The Process:

This was such a fun, simple little job! To start, I made sure a phone charger could power everything by turning on the bounce and the music and measuring the current draw from the positive to the negative terminal in the battery bay. It measured something like 1.54mA, if I recall correctly. It was really tiny anyway, easily powered 500 times over by an 800mA charger.

Sidetrack: the first time I opened up the case for the bounce mechanism, I expected to see weights oscillated by a cam and motor or something like that. What I found was nothing so idiotic. The wizard who designed this rightly determined that a motor and cam was far too prone to wear out. The bounce motion is created by a weighted arm with a magnetic coil in the center that is energized in pulses to overcome the spring on the bottom in its attempt to wrap its magnetic arms around the steel pin in the center, and then release and bounce back up. Nothing touches! The only wear parts are the hinges! Genius!

Anyway, I snipped the B end of the USB printer cable and stripped off the insulation, shield, and data wires. A USB cable has four wires, two for power (red and black), and two for data (green and white). Batteries were off the table during this stage, so I soldered the wires directly to the battery terminals for simplicity.

Once I got the switch, I traced its outline onto the case with Sharpie and drilled 1/16″ holes at the corners and then in a series along the edges until I could snip the rest out with a flush cutter. A little touch-up with my set of needle files yielded a fairly square hole. Luckily I had two of the tiniest screws possible from an old laptop case to secure the switch to the case. I desoldered the cord from the battery terminals and attached them to one side of the switch. The switch isolates the two sets of three terminals so it doesn’t short. I used new wire to connect the other side of the switch back to the battery terminals, and then came the slightly trickier soldering.

The negative lead that used to connect the PCB to the batteries was a very tiny stranded wire. It took a couple tries to get it stripped with enough strands left and then bend it to shake hands with the wire from the switch. The positive lead went straight to a capacitor, so I stripped a longer end and sidled it up to the cap and bridged them with solder. I covered both neg and pos joints with appropriately-colored heat-shrink tubing. I don’t have a heat gun, so I just held the tip of my 25W soldering iron near the tubing and let radiation do the work. Surgery complete. Close and confirm patient viability. Success.

Review:

I think one of the best things I could have done to make this project better would’ve been to use wire a couple gauges smaller. Soldering wires of unlike gauge and construction (solid vs. stranded) seldom ends well for me. I also would have liked to use install a female USB micro jack to avoid a dragging a power cord tail everywhere with an otherwise rather portable bouncy seat, but actually finding and procuring the right configuration of such a jack is not as easy as a trip down the stairs. Maybe someday.

The choice of switch worked out perfectly. There’s even enough overlap in the slide that if you move it fast enough, the bouncy seat doesn’t lose power, making transition between power sources seamless.

Ciao for now! Ideas, criticism, and education welcome in the comments!

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