This sophisticated circuit with two LF411 operational amplifiers appeared in the November 1984 issue of Radio Electronics magazine. The circuit uses a Mallory buzzer as an indicator.
This circuit appears elsewhere on this site, but from different sources. Using the LM3900, we find this...
This interesting source or isolation transformer also allows to vary the output voltage by...
In this circuit the CA3140, an operational amplifier, is used as a comparator with the zener diode...