We found this micro amplifier in a technical documentation from National Semiconductor. The circuit can be used on an experimental phone, being powered even by alternative power sources.
The integrated circuit used is not very common today. Transistors can be replaced by more modern...
This circuit was found in old documentation. The frequency depends on R1 and C1. For the...
This circuit was found in a 1990 publication. The sensors are electrodes placed in the water tank...