Often a backyard electronic system could become imperfect as a result of a perishing battery. If the battery voltage is no more than 9V for a 12V operated system, the output should not be anticipated to be of high quality because of disparities in the circuit efficiency.
For instance, several car users consider their battery for granted seldom offering it a servicing. As the winter season advances the requirements made on this vital power source increase. Along with the unavoidable process of aging and lowering capability to store a charge for an extended time, this makes the necessity of a straightforward support to monitor battery voltage constantly apparent.
The proposed indicator had been for that reason designed to prevent any incipient failure by giving 'at a glance' info on battery condition with three colored LEDs - green indicating battery voltage satisfactory for normal use, yellow-colored that the voltage was reasonably low and red that the cell had been lifeless.
Once the battery is in th best condition, its output voltage is going to be about 13V and, obviously, even higher if a short while ago charged.
This potential is utilized via D8 via R4, D7 and R5 to the base of T2. This switches T2 ON, triggering D6 to light up via R3.
During this period T2 successfully allows a short circuit over the remaining circuit via D6, preventing D2 and D3 from giving out light.
As the battery voltage gets lower, T2 starts to switch off since the threshold of D7 is arrived at. This enables D2 to conduct as T1 provides all the time to be switched on via R1, D1 and R2. (D5 improves the voltage necessary for switching On D2 when T2 is ON.)
The current hence utilized via R3 precludes D3 from lighting up as the potential across it and D4 is not over the zener level.
Sooner or later, at still lower battery voltage, T1 will probably switch off in the same manner as T2, enabling potential at the junction of D2, D3 and D6 to elevate more than 5V zener level of D4 which will start to move current illuminating D3.
Zener diode could be determined for some other switching points and/or battery voltages.