We found this circuit in Radio Electronics from the 90s. The components are current. The sensor is a common general-purpose transistor. The connection to the sensor is made by twisted pair.
We found this circuit in a publication from the 1970s. It produces intermittent clicks on a...
We found this circuit in a Philips documentation from 1969. It makes use of transistors of the...
This circuit was found in a 1999 English magazine, but can be easily assembled with BC548...