Audible voltage indicator (CB21332E)

Suggested by a Popular Electronics from the 90s, this circuit uses a buzzer to indicate the presence of voltages in the range of 4 to 220V AC and DC.

Power CMOS Shield (CB19763E)
Bipolar transistors are types of general use like BC548, while the power MOSFET can be of any type...
Noise Generator (CB17203E)
This circuit generates a signal rich in harmonics that extends up to close to 1 GHz, serving for...
Salen Key Low Pass Filter (CB4865E)
This second-order low-pass Salen-Key filter was obtained from an American 1980s documentation. Formulas...