Digital Calipers – connector circuit

The caliper runs from a 1.4v battery and cannot be driven by the 5v typical for microcontrollers. Like many other devices the ATMEGA also has a 3V3 output. Some people report successfully running a caliper from 3.3v but with the current one this did not work.
So for the final connection between caliper and microcontroller there are two things to be done. The supply voltage has to be brought down to drive the caliper and the output signals (Clock, Data) have to be level shifted up to ~5v again.
For the power supply a shunt regulator with a 470Ω resistor and two diodes works fine. Shifting the signals back to 5v is done using two transistors (BC548C) each in an emitter circuit. For the flexible connection between caliper and circuit board a usb cable (A/B) is used.

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