In this phase shift circuit, the transistor can be any general-purpose transistor. The circuit is from an Electronics Now issue of June 1993.
Other CMOS gates can be used in this sensor, which we found in the July 1995 Electronic Now...
This Texas Instruments integrated circuit, available from Mouser Electronics, has a 91 dB common...
This circuit was obtained from an English magazine from 1979, but it can be easily assembled today...