Sensing circuit How-to with Digital Inputs

What kind of circuit would someone build to sense very low voltage transmission without altering the voltage or resisting current too much that the original circuitry failed to operate?

Situation:

  • A keyboard.
  • The "spacebar is pressed"
  • An electrical connection is made when the spacebar contact connects the open contacts on the PCB for the spacebar
  • The circuit is closed until the spacebar is released.

Question:

  • How could a passive "sensing" circuit be placed in-line and be fed to the inputs of an ardunio to be read?

  • If my Arduino were reading the value of an input pin every 100ms and capturing the result (perhaps printing to serial, irrelavant) how could I capture this circuit going "live" without interrupting the monitored circuit's normal functionality?

Whoops, just noticed this is the software forum. I'll post this in hardware interfacing.