This circuit is from 1994 documentation by National Semiconductor. What it does is convert the square root of a voltage to a proportional frequency.
If the input voltage falls below 12 V the LED glows up. Other voltages can be monitored by...
This Low Speed circuit was found in an American documentation from the 1980s. The integrated...
This variable high pass filter covers the range from 23 Hz to 700 Hz with the components used. The...