Notes
This simple and hardly odd ambit can acutely appearance the akin of the accumulation voltage (in a beyond device): as continued as the indicator has acceptable 12 volts at its input, LED1 gives steady, ceaseless (for the naked eye) chicken light. If the ascribe voltage avalanche beneath 11 V, LED1 will alpha to blink and the blinking will aloof get slower and slower if the voltage drops added - giving actual bright and automatic representation of the supplys status. The blinking will stop and LED1 will assuredly go out at a little beneath 9 volts.
On the added hand, if the ascribe voltage rises to 13 V, LED2 will alpha to glow, accepting at about abounding ability at 14 V.
The appropriate voltages can be adapted primarily by adjusting the ethics of R1 and R4.
The base-emitter diode of T2 basically aloof stands in for a zener diode. The emitter-collector aisle of T1 is inversely polarized and if the ascribe voltage is aerial abundant - T1 will account oscillations and the abundance will be proportional to the ascribe voltage. The alleviation oscillator ceases cycling back the ascribe voltage gets so low that it no best can account breakdown forth the emitter-collector path.
Not all baby NPN transistors appearance this affectionate of behavior back inversely polarized in a agnate manner, but abounding do. BC337-40 can alpha oscillations at a almost low voltage, added types about crave a volt or two more. If experimenting, be accurate not to bite a aperture through the accessory beneath test: they oscillate at 9-12 V or not at all.
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