The LM39X4 acts as a LED-driving vol-tometer that has its basic maximum and minimum readings determined by the values of R2 and RV2. When correctly adjusted, the unit actually covers the 2 volt to 3 volt range, but it is made to read a supply voltage span of 10-10 volts to 15 volts by interposing potential divider R1-RV1 between the supply line and the pin-5 input terminal of the IC. The IC is configured to give a `dot` display, in which only one of the ten LEDs is illuminated at any given time
If the supply voltage is below 10,5 volts none ofthe LEDs illuminate. If the supply equals or exceeds 15 volts, LED 10 illuminates.