We found this circuit in an old Mullard (Philips) documentation. The sensor consists of a reed-switch that is coupled to a magnet on one of the wheels of the vehicle.
This simple electroscope with bipolar transistors is not the most sensitive, but can detect static...
This circuit is suggested by Texas Instruments in his Optoelectronics Sourcebook. Any photo...
The frequency generated by this circuit depends on the crystal used. Frequencies up to a few tens...