[dcb]

just did some bench testing of device in post 25:

+ connected to 12v battery
- connected to battery ground
o connected to 12v via a 5k resistor.

output at o with no metal near tip, 12 volts

output at o with metal at tip, 0 volts

device current draw @ 12v output, 8.25 milliamps

device current draw @ 0v output, 11.6 milliamps

notes:
the steel had to be pretty close to the tip to set the output low, like less than 1/4 inch.
There were no intermediate voltages on the output.
the voltage output at o should work fine as a speed input to the guino.