For experiments with a disk rotating at various speeds, the April 1998 Electronics Now magazine suggests this PWM motor control. The current with the LM317 is 1.5A maximum.
This circuit was indicated in an old American publication to serve as a Laser alarm system as a...
This circuit, which uses only transistors as active components, was found in a 1987 documentation. The...
This sound meter or sonometer was found in a Popular Electronics from the 80's. The circuit can be...