We found this circuit in a Radio Electronics from February 1966. The transistor is from the time and so are the transformers. The circuit is battery powered. Observe the impedances.
This simple 555 biofeedback circuit was found in the December 1983 issue of Radio Electronics...
This circuit has a programmable alarm that can be triggered with different water levels. The...
Found in a 1991 Hobbyst Handbook, this circuit monitors both battery and alternator voltage. The set...