This unusual circuit from a Popular Electronics of the 90s uses LED indicators, operating as a bridge. The components used are common. It is based on the tuning of the signal by a PLL.
This circuit was found in an old publication about transistors. The circuit can be used to drive...
Using a symmetrical source this circuit reverses the rotation of a small motor through light...
This circuit was found in a 2005 Linear Technology documentation. The circuit is based on LTC4213...