He describes his project as:
Auto Bed Leveling using piezo electric discs for very high sensitivity.
Mounted between the bed springs and the z-stage, any force down on the bed can be registered and used as a probe signal. Hence one can measure when the hot-end touches the bed surface. No modification to the hot-end or carriage is therefore needed, and the probe is insensitive to temperature.
This is still a work in progress, so although it is working for me in it's current form, it needs constant tuning as the circuit is not yet perfected, and the mechanical mounts have some known issues. A new circuit and new mounts have been designed but not yet tested.
So far, I've tried to only use parts available from sparkfun. I'm guessing a total cost for all parts of less than $20 USD.
Video of the setup in action:
You can find full description and how to guide at: