This circuit from the 80's makes use of two transconductance operationals of the time. Modern equivalents can be used. The circuit does not need to be powered by a symmetrical source.
The resistors R determine the frequencies of the oscillator together with C. The circuit is from a...
This configuration can be implemented with equivalent operations. The circuit is from the 1990...
This proximity sensor was found in a 90s Popular Electronics. The base is a PLL567 that operates...