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.