This circuit was found in a 1968 documentation. It makes use of both NO and NC reed-switches and small magnets attached to a card in the form that results in the drive combination.
I found this circuit in a 1986 English magazine. It can still be easily assembled, replacing the...
This field strength meter circuit is for testing small transmitters. The circuit is from a Radio...
This circuit for the 60 kHz range was found in 73 magazine and published in 1995. The circuit uses...