This circuit was obtained in a QST magazine from the 90s. The circuit makes use of components that are uncommon today. The circuit includes a gain selection switch.
In the figure we have the way to use a low-cost digital clock chip to trigger an alarm. The...
This circuit is the traditional 555 oscillator that produces a hum in the piezoelectric...
This circuit was found in a computer publication from the 1970s. The transistors can be the BD135...