For measurement, rulers and tape measures can get you in the right ballpark, but to measure down to the thousandths of an inch — or hundredths of a millimeter — you need a pair of calipers. If you use the digital version, your device will likely include an interface port that allows you to connect it to a computer via JohnMackey’s excellent hack.
The interface includes a custom PCB, along with a 3D-printed connector, enabling you to form a solid connection without modifying your device. It runs on an ATtiny84A MCU, and uses a TLC3702 comparator chip to shift the 1.5V caliper signals to the 5V needed to work with this MCU.
The board can be configured to output only when measurement values change, and can communicate via TTL serial and/or an I2C interface. It’s been tested with a pair of calipers and a tire gauge, and should be able to work with many cheap Chinese calipers on the market.
As the board provides power to the calipers, it’s able to reset/control power for the unit using a BSS138 N-channel MOSFET. The board is not capable of changing from mm to inches, but one could conceivably hack the caliper to simulate a press of a button, or perhaps even switch to your preferred unit system in software.
How you use this device is left to your imagination, but it certainly presents a great option for a wide variety of automation and measurement applications!