This circuit allows switching on a LED when the battery voltage (12V) drops below a certain threshold. The operating system is based on a comparison of tension, which comes in a zener diode. If the supply voltage is 12V and the zener diode is a 10V model (this is the case in the diagram), we find the nodal point R1 / R2, a voltage of 2 V (12 V 10 V).This voltage is sufficient to produce in R2 and the base of transistor Q1, allowing current to saturate the transistor Q1, which then has its collector voltage close to 0 V. The transistor Q2 is blocked, the LED D2 is turned off.
On this link a few explanatory lines on the choice of component values, starting from the right of the diagram (LED D2) and up to the left (Zener diode D1). More photos on Facebook Fan Page.
Download : Battery Level Indicator
Autor: Mr. Sonelec