This circuit uses an unusual sensor nowadays, but the configuration can be adapted to more modern types, as well as the operational amplifier. The circuit is from an 80's documentation.
This circuit was found in a Radio Electronics of July 1984. It consists of the traditional 1 A...
This circuit is from a 90's Radio Electronics. The circuit operates accurately up to about 15 kHz...
I found this circuit in an old British magazine from 1976. It is possible to test operational...