Add an AC Adapter to a Battery-powered Device

With a new baby, we are acquiring an astonishing number of battery-powered devices -- bouncy seats, swings, activity jumpers, mobiles, ... -- and burning through an even more astonishing number of batteries. Now I know why Costco sells those immense packages of AAs.

My wife asked me whether I could attach an AC adapter to our baby's mobile. I'd seen an article in MAKE about modifying noisy toys, so I knew it was possible. It turns out to be surprisingly easy, provided you (okay, I) don't make some dumb choices along the way.

NOTE The project I describe will void the warranty for whatever toy you modify. The manufacturer (rightly) will not support you or provide you with assistance in doing this. If it works, be happy: If You Can't Open It, You Don't Own It. If it doesn't work, take it as a lesson not to meddle in the affairs of Corporations, for they are Subtle and Quick to Anger.

You'll notice that two of the Steps don't have useful images. I started writing this up after completing the project on one of our daughter's mobiles, and before starting the second. I definitely don't want to take the first one apart again just for some photos, and we've since decided not to do the AC adapter on the second mobile (the cord wouldn't be adequately secured).

Nevertheless, the descriptions in the Steps are accurate and complete, based on lessons learned the first time. I think this can still be useful to folks.

First, choose the toy you want to power from the wall. I'm going to show you a very nice infant's mobile from Tiny Love. The steps will be similar for whatever device you choose, although the internal details (and maybe the screwdrivers) will be different.

The parts you need to add an AC adapter are fairly simple, but harder to find than I expected. I ended up going to Radio Shack -- they carry specific-voltage adapters, individual tips, and panel-mount sockets to match the tips. At most places you have to by a "universal" adapter, and can't get the sockets at all.

For the TinyLove mobile, which uses 3 AA batteries, I bought

4.5V adapter (700 mA)	273-1765
Size "M" (2.1mm ID, 5.5mm OD) plug	273-1716
Size "M" panel-mount socket with switch	274-1582

For your application, choose an adapter with an output voltage that matches the type and number of battery your device uses.

You will also need lengths of thin (24 gauge or smaller) wire, one with black insulation and one with red. I cannibalized mine from some left-over four-conductor signal cable. Start with 6" lengths and trim them back when doing the soldering (Step 5). A wire stripper will be quite handy, but someone with some skill (not me) could use some small wire-cutters or scissors.

You'll need some tools: The TinyLove mobile is held together with weird triangular-bit screws. I found the bits at McMaster-Carr Industrial Supply (items 5941A11 through 5941A14).

A 21/64" bit will make the exact hole needed for the panel-mount socket. You could also use a 1/8" Dremel bit, followed by the Dremel ball grinder to enlarge the hole.

A soldering iron, solder and flux are needed to connect the wires. A solder-sucker might be helpful, too, when disconnecting the leads from the battery terminals.

Remove the batteries first; soldering the terminals with the batteries in place isn't the best idea. Open the mobile using the triangular bit on the four back-panel screws. Be careful when opening up the mobile, as the wires inside do not have a lot of slack.

Identify the red (+) and black (-) wires at the terminals of the battery compartment. Choose a spot on the side of either the back or front halves to mount the AC adapter socket, close to the battery terminals, so the red and black wires will reach it. You'll need enough space inside to fit the whole socket so it can be pushed through its mounting hole. Mark the outside of the panel with a Sharpie where you're going to put the socket.

Drill out the 21/64" (5/16" plus a little) hole at your mark for the AC adapter socket. The threaded end of the socket needs to stick out far enough to tighten the nut. You may need to clear some additional plastic from the inside in order for the socket to fit snugly up against the curved surface.

Mount the socket from the inside, with the washer and nut on the outside. Make sure that all three contacts are accessible (the non-contact second ought to be downward). Tighten the nut as much as you can with pliers, so it won't twist around or come loose (you may want to use thread-lock).

Disconnect the red and black leads from the battery terminals using the soldering iron, and the solder-sucker if necessary. Make sure you keep track of which terminal is which!

Solder the red lead onto the center pin connector (at the 9 o'clock position) of the AC socket. This allows the AC adapter to feed current into the circuit.

Solder the black lead onto the shell connector (at the 12 o'clock position) of the AC socket. This is the return path for current from the AC adapter back to ground.

At this point, the project is "complete," in that the AC adapter will provide power. However, you've disconnected the battery system entirely, so the mobile is not portable (nor will it work if the power goes out).

WARNING Don't do what I did the first time. If you cut the existing leads and try to attach the cut ends to the AC socket connectors, the halves will probably be too short to reach sensibly when the device is opened, and you'll have to splice in lengths of wire anyway to make up the difference. Just move the whole leads as above, and use a couple of whole new wires for the new connections (Step 6).

This AC socket includes a normally-closed switch on the shell (ground), so that you can power your device as built with the batteries, but when you plug in the adapter the batteries are disconnected (and not shorted to ground or reverse biased).

Strip the ends of the black wire (and cut it to a nicer length if desired) to expose about 1/8". Solder one end to the negative terminal of the battery compartment, and solder the other end to the NC switch connector (3 o'clock position) on the AC socket. When the AC adapter is not plugged in this provides the return path from the circuit board to the batteries' negative terminal.

Strip the ends of the red wire the same way. Solder one end to the positive terminal of the battery compartment, and solder the other end to the center pin connector (9 o'clock position) on the AC socket. You already have the built-in red lead on that center pin, so you may not need additional solder for this connection.

Now, the batteries are connected into the circuit almost as before. With the AC adapter unplugged, the batteries provide current through the two red leads, and the return path goes up the black lead to the AC socket, through the closed switch, and back to the batteries.

When you plug in the AC adapter, the batteries are disconnected from the circuit and current comes from and returns to the adapter through the original red and black leads.

You're now ready to verify that you haven't fried the device. First, set up the AC adapter by plugging the Size "M" Adaptaplug into the end of the adapter cable. Make sure that the "(+)" mark on the plug is aligned with the "TIP" mark on the cable. This specifies that the center pin is positive, and the outer shell is grounded.

Plug the AC adapter into your device and into the wall. Turn it on. For the mobile (since the armature is disconnected) you should hear the annoying digital music start to play. If not, unplug the adapter and check all of your solder connections.

Disconnect the adapter and put the batteries back into the compartment, without closing it up. Turn the mobile back on, and it should work as before. If not, see above.

Finally, with the batteries in place, reconnect the AC adapter and turn the device on again. It should work as before. You should also confirm (by touch) that the batteries are not getting warm, nor that there's any unexpected smells or smoke. The NC switch on the AC socket ensures that the batteries are cut out of the circuit entires when the adapter is in use.

With everything working, you can close up the unit. Carefully fold all of the leads into the space between the case halves (use bits of painters tape to hold them down). Insert and tighten the case screws. Put the battery compartment case back on.

Now you can put the unit back into action, powered either from the wall or from batteries as you wish!