One-transistor oscillator (CB22064E)

Capacitors C2, C3 and C4 determine the frequency of this simple oscillator. Transistor can be general purpose equivalent.

555 Biofeedback Monitor (CB24775E)
This simple 555 biofeedback circuit was found in the December 1983 issue of Radio Electronics...
Water Tank Sensor with Alarm (CB16458E)
This circuit has a programmable alarm that can be triggered with different water levels. The...
Battery na alternator monitor (CB7136E)
Found in a 1991 Hobbyst Handbook, this circuit monitors both battery and alternator voltage. The set...