This circuit was published in an article in Radio Electronics magazine in October 1988. It uses components that are still common today, except for the sensor, which is a special type from that time.
This circuit makes use of a PLL 567 and the components must be calculated according to the desired...
The transmitter can be that of CB9574E. This receiver uses a PLL to directly trigger a sensitive relay....
This circuit is from a 1967 US publication. The configuration can serve as a basis for designs...