In this interesting circuit from Popular Electronics, the output voltage is the square root of the input voltage. Equivalent operational amplifiers can be tried out.
When the input voltage exceeds the value set in R4, the LED lights up, indicating this. The...
In this circuit of the 80's components of the time are used. The circuit controls a direct current...
Found in a 90s Elektor, this circuit is based on a Maxim integrated circuit of the time. The...