Applications for this circuit include hearing aid, stethoscope, etc. Transistors are general purpose and powered by a single 1.5 V battery.
This circuit was found in a 80's Popular Electronics. The sensor can be any phototransistor. The circuit is...
This circuit produces rectangular signals from a sinusoidal input. The circuit uses a unijunction...
This circuit can source 3 to 12 V with currents up to 500 mA depending only on the transformer....