This tool uses a modern light dimming switch to provide variable speed control of a motor (fan, pump, etc.) It turns out these dimmers use silicon controlled rectifiers (SCR's) which allow for very nice voltage control, and hence speed control in motors common to farm applications. More on SCR here.
Bill of materials
A good light dimmer switch from Lutron or similar. $20-60.
Other tools needed to build this tool
Basic wiring tools (stripper, screwdriver, etc.)
Step-by-step build instructions
Dimmer installation instructions from Lutron
Take care to check the current rating on the switch compared to the load.
