I got some supercapacitors from Katie, and my first project was to replace the battery in my digital calipers with a capacitor. Digital calipers are great, exept for when the batteries are dead. Who has those tiny watch batteries laying around anyways? Now, my calipers can be charged up via USB, and I never have to worry about dead batteries.

The calipers fully charge in approximately 20 seconds, and they last for a few days of moderate use after a single charge. The supercapacitor itself is a Suntan 10 farad capacitor. The conversion was fairly straightforward. Basically, the capacitor is just soldered in to the battery contacts inside the calipers. I also added a switch to go from charge mode to discharge mode. This switch is probably not required, but I wanted to keep the high charging currents isolated from the caliper circuitry, so it's just a safety measure. I also added a simple female 0.1" header to act as the charging connector.

cover removed showing wiring
the charging port and switch
finished calipers with cover removed
finished calipers

The capacitor is rated to a voltage of 2.5 V, but USB supplies 5 V. If were to charge the capacitor to 5 V, I would probably break both the capacitor and the calipers (which run on 1.2 V). So, I made a simple charging circuit to regulate the 5 V from USB down to 1.8 V. The circuit is very simple and consists of a 1.8V, 500 mA, MCP1825S linear voltage regulator and a 2 ohm, 2 watt resistor to limit current. The whole circuit is built on to a USB cord to take up minimal space.

usb charger
detail of charging circuit