Found in a Popular Electronics from the 90s, this circuit uses components that are still current. The circuit triggers the vehicle's horn. The sensors can be increased in number.
This switched circuit was found in an old documentation and no information about the transformer...
This circuit divides the frequency of a 2 kHz signal by 2. The circuit was found in Philips...
This small FM transmitter has a sensitive modulating stage and is therefore suitable for listening...