Goals:

Support stock Ender 3 (or similar machines)
Use the nozzle touching the bed as the probing mechanism
Cost less than $10 per probe

How it works:

This replaces the stock X carriage of the Ended 3 with a custom PCB. This PCB has a strain gauge built in that can sense the force on the nozzle. When the nozzle pushes against the bed, it outputs a trigger signal, which goes to the mainboard.