Happily there is a solution in the form of capacitive sensor probes ... The explanation of the probe’s operation is an interesting part of the write-up, unexpectedly it uses a 555 configured ...

[James Eckert] got his hands on a Freescale MPR121 capacitive touch sensor. The chip in question ... It’s a sweet little part. So [James] had to make a twelve-key capacitive keyboard on the ...