Input adapter for filament sensor

This small input adapter board allows you to connect the filament width sensor to a temperature input. Some 3D printer controller boards do not have general purpose analog (A to D) inputs, so this board conditions the signal so that an available thermistor input can be used.

An example board where this is useful is the Rambo Mini.

Some work is still required to update Marlin firmware to allow this option.

This board uses a small op amp in a voltage follower circuit. It decreases the output impedance of the signal so that it can provide the current required by the thermistor input. Without this, the filament sensor output is loaded by the thermistor input and the output signal is reduced, leading to inaccurate output.

This is not needed for a general purpose analog input because the current draw is very low for these inputs.