Suggested by Sprague in the 90's documentation, this circuit applies to AC motors. The documentation does not indicate the maximum engine power that can be controlled.
The circuit presented is based on a mechanical tuning fork. The circuit is suggested by National...
This source, from a 1982 documentation, uses a triac to control the power and voltage applied to...
This circuit was found in a 1990 Electronics World magazine. The circuit can be mounted today by...