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.
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.
