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I have a school project which it requires me to design a logical circuit that tests a 12v battery if it is full or not or half-full and light the corresponding LED, but, I am having a trouble because I want a component that only allow a voltage to pass if it is above 12v in first branch and 6v in second branch and 0.5v in third branch, but, I don't know what to component to use to achieve my goal. I am thinking to use resistors but I do not know what are the values to assign to it to achieve my goal. NOTE: I am using Proutuse isis

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  • \$\begingroup\$ If a 12V battery has only 6V output under any reasonable load, it is practically completely discharged (empty, flat). Searching for "battery gauge" should give you some idea how this is usually done. \$\endgroup\$ – Brian Drummond Dec 19 '15 at 14:59
  • \$\begingroup\$ @BrianDrummond yes, but, in general would you please tell me what I can use to only allow 12v or above to pass so if the voltage is under 12v nothing pass? \$\endgroup\$ – Бассел Жаббор Dec 19 '15 at 15:33
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    \$\begingroup\$ Use a comparator or an op amp in comparator mode. \$\endgroup\$ – Passerby Dec 19 '15 at 16:07
  • \$\begingroup\$ Does your circuit have to be powered by the battery being tested? Or is there another "known good" supply available also? \$\endgroup\$ – The Photon Dec 19 '15 at 16:51
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    \$\begingroup\$ A "12V" lead-acid battery that reads 12V is already dead. So step one here might be to research 12V battery behavior for whatever chemistry you are reading (lead-acid is the default assumption at that voltage, but there are others.) \$\endgroup\$ – Ecnerwal Dec 19 '15 at 17:23
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This is a possible solution. As Passerby stated comparators (or op amps) could be used.

If the voltage (from the battery) is more than the voltage at the minus-pin on the comparator then there is positive output from the comparator (i.e the voltage "passes").

The voltage at the minus-pin of the comparators can be constructed with simple voltage dividers.

The XORs at the end makes sure that only one lights up at a time.

This solution needs an external voltage source of at least 12V to power the comparators, XORs and also create the voltage dividers connected to the minus-pins of the comparators.

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  • \$\begingroup\$ is it possible to use a method that does not need external power source? \$\endgroup\$ – Бассел Жаббор Dec 19 '15 at 17:56
  • \$\begingroup\$ You might be able to do it with zener diodes or just normal diodes. A diode has a "forward voltage" that is, it requires that there is some voltage across it before any current can pass through it. For example an 12V zener diode connected to the battery's plus and minus would only allow current to pass through it if the battery had 12V or more. A 6V zener would only allow current to pass if the battery has at least 6V etc. If you connect a LED in series after the zener diode (or diodes) then you would have to take the LEDs forward voltage into consideration as well. \$\endgroup\$ – fredrik.hjarner Dec 19 '15 at 19:17

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