Low battery voltage (below than ~6.9V) will show "RED" light up.
Else will show "GREEN" LED light up.
The basic, if there is a small amount of current go through the zener (reverse bias),
the first transistor (left) will be turned on making the "GREEN" LED turn on.
Other than that the second transistor (right) will be turned on and "RED" LED will light up.
Modification - Add in buzzer
You add in buzzer to the circuit to make it give an audible alarm.
The best is to alert us whenever the cable under test is not "GOOD".
So, adding a resistor, transistor and a buzzer/beeper to the last NAND (near "GOOD" LED) will do the job.
Use your own creativity for this. :-).
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