Found in a 1980s documentation, this circuit uses transistors from that time. Modern equivalents can be used. The alarm is a piezoelectric buzzer with na included oscillator.
This circuit generates noise pulses for audio equipment probes. The circuit is from a 90’s 303...
This modulated light transmitter was found in a 90s Popular Electronics. In place of the lamp with...
This circuit, found in a 1958 magazine, is for reference only, since the components are not easy...