This circuit was found in a 90s Radio Electronics. It produces an ultrasonic signal in a special transducer at the frequency of 2.25 MHz. No information was given about the transformers.
This small earphone amplifier with germanium transistors was obtained in a Japanese publication from...
The sensor for this circuit is a phototransistor. We found it in the April 1971 issue of Radio...
Found in a 90s Popular Electronics, this circuit allows to select the operating ranges between...