This small phonographic amplifier was found in a 1967 General Electric documentation. The integrated circuit is from that time, providing 1.5 W of output power.
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...