This circuit can be used to display the variation of an audio signal in a group of 8 LEDs, behaving like a VU meter. We could use any specific integrated circuit used for VU meters, but we decided to use a readily available LM324 op amp. I.C.

Car battery monitor circuit using 4 LEDs

This simple Car battery monitor, allow us to know the voltage of the battery at all times. To achieve this 4 leds are placed somewhere on the dash of the car. Each LED has a label indicating the battery voltage at that time.