The 12v battery level indicator unit in the photo has no reverse polarity diode and R2 is the calibration potentiometer. LM339 datasheet. Tags Electroschematics. Jim Keith. About Jim Keith. I grew up ...

The BC557 transistors’ base is biased by the 10V zener diode. As long as the battery voltage stays above 11.6V, the zener keeps the base of the BC557 transistor high. When the battery becomes ...