This circuit was found in an Electronics Now of the 80s applying to NiCad batteries of the time. The circuit is powered by a 12 VAC source, for example, the secondary of a transformer.
The basis of this circuit is a CA3130 operational amplifier. The transistor can be BC548, and the...
This circuit, found in a 73 magazine for radio amateurs, is used in radio localization. The...
This circuit is from an American magazine Popular Electronics of March 1991. The circuit produces both...