This proximity sensor was found in a 90s Popular Electronics. The base is a PLL567 that operates at around 100 kHz. The circuit operates by capacitance sensing.
In the figure we show another interesting application for integrated circuit 4001. This circuit...
This audibility or loudness control for sound equipment is from 1992 documentation. The circuit uses...
This circuit was found in an application note of the LTC1164 from Linear Technology of the 1990s....