Dummy batteries let you use an AC adapter


We find it frustrating when battery operated consumer electronics don’t include a way to connect an external power supply. We try not to purchase disposable alkaline cells if we can avoid it, and this dummy battery AC adapter hack will aid in our mission.

The battery compartment shown above is for a motorized baby swing. It accepts C sized batteries (who has those just lying around?) and lacks a barrel jack to connect a wall wart adapter. [Jason Smith] mentions you can get around this by connecting your positive and ground wires directly to the conductor springs. But using a dummy battery makes it a bit easier to remove the adapter if you do want to use battery power.

Each of the orange dummy is a wooden dowel with a screw at each end. The screws are connected with a piece of jumper wire, shorting the two terminals. This completes the circuit in the battery compartment and allows him to power everything from the adapter cell at the bottom. The adapter uses an LM317 adjustable voltage linear regulator. He used fixed resistor values to dial in his target voltage. The equipment should be rather forgiving as battery voltage starts higher than the printed value and drops as the cells are used up.

This technique has been around for a long time. One of our favorites was a hack that converted an Apple Magic Trackpad to USB power.

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Non-invasive AC power adapter for exercise equipment

We often look at battery-operated hardware and shake our heads at the wastefulness of throwing away disposable batteries. There are some devices that minimize the waste, like those TV remotes that seem to never need new cells. But the C cells that [Quinn Dunki] kept replacing in her elliptical trainer were only lasting about three months at a time. The manufacturer hadn’t cared enough to build a power jack into the machine, so she built her own AC adapter without modifying the stock hardware.

The first thing she did was to patch in a couple of wires between two of the batteries. This let her measure the current consumption, which topped out at around 200mA. This is good news because that’s easily sourced with a cheap linear regulator. Out of the junk box came a 12V/1A wall wart transformer, which just leaves the need for a fuse and some capacitors to finish out a voltage regulator circuit.

Since [Quinn] didn’t want to permanently alter the exerciser, she came up with a way that it could take the same physical space as the batteries. Two long stand-offs are used as prongs to interface the spring terminals in the battery compartment. They attach to a piece of protoboard which hosts the rest of the circuitry. Now she just needs to remember to unplug this from the wall after each session and she’ll be in business.