3 LED battery monitor circuit

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Description. This is the circuit diagram of a 3 LED bar graph type battery monitor circuit that is ideal for monitoring the voltage level of an automobile battery.When battery voltage is 11.5V or less transistor Q1 will be On and the LED D1 will be glowing.When battery voltage is between 11.5 and 13.5V, the transistor Q2 will be On and the LED D2 will be glowing.When battery voltage is above 13.5V the transistor Q3 will be On and the LED D7 will be glowing.

Circuit diagram with Parts list. 3-led-battery-monitor-circuit Notes.

  • Assemble the circuit on a general purpose PCB.
  • The battery to be monitored can be connected between the terminals namely A and B.
  • It is always better to use LEDs of different colour.
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49 thoughts on “3 LED battery monitor circuit

  1. Gulshan bhatia

    Dear sir
    please provide me 3LED battery monitor circuit job or project numberr
    I want to by the project kits.
    Gulshan Bhatia

  2. Aura

    Hi Seetharaman,
    On Sept12.2011 in response to a query from another reader with regard to components values for highr voltages,you have mentioned that you’ve comments about this circuit in the 26th Nov.section. Where/how do I find it? Thanks.

    • Seetharaman

      26th nov 2010 i have given the operation of the circuit. If the voltage tolerance of the zener diodes are more you get for different voltage. Now you select the zener for the voltage in which the circuit should operate and adopt the same circuit with different zeners. in fact for say 6 volt battery you can use D6 as 5.1 volt and D10 as 6 volt zeners and so on.

  3. Mae

    When D7 light is on, D2 light should be off right? But how come the D2 light doesn’t turn off at 13.5 V (but D7 is on). We used 1N4740A as a replacement for BZX9C10 and 1N4742A as a replacement for BZX9C12.

  4. my article submitter

    Neat blog! Is your theme custom made or did you download it from somewhere?
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    Please let me know where you got your design. Many thanks

  5. johnny_lee

    good day sir. can you modify/add circuit so that it can indicate also if the battery is being overcharge? An additional LED circuit perhaps

  6. Vuyolwethu

    thanks i did recognize that now could you please help out here because in my project report they require a formula based on selection of components..for example how did u come up with a certain resistor value,transistor type and also diodes on this particular circuit

  7. Vuyolwethu

    i am student i want know the selection of components whether how did you came up with formulas regarding the componet selection and also function of each component

  8. Reez

    does this circuit really work? i’m working on my final semester project and i got my hands full on 4 different circuits already. so i need to be sure that this works well as time is very limited for my project.

  9. patrick vaz

    Hi Seetharaman,


  10. patrick vaz

    hi setharaman,

    1234.. would be fine, if it is not a problem, have a mixture of red, white & blue led. qty of each, what ever u decide, should be a maximum of 300 to 400 leds, would really appreciate,

    best regards

  11. Seetharaman

    Hi Fahad this circuit is based on the breakdown voltage of zener diodes. by sensing the supply voltage rail the circuit illuminates different LEDs in different band of voltages. from minimum (3volts)up to 11.5 volts the red LED will light up and other two will be off. when voltage lies between 11.5 to 13.5 yellow LED lights up and other two will be off. when the battery voltage is greater than 13.5 volt green LED lights up and other LEDs will be off. (the supply rail to the circuit itself is sensed for the voltage levels no other supply is required for this circuit).(please note the voltage indicated for LED change over depends on the tolerance of the zener diodes)

  12. Seetharaman

    Hi Fahad you can provide a fuse of say 500mA (even 100mA will do if readily available) in the positive line from the battery close to the battery.

  13. Fahad Naveed

    some one was saying that we should add a fuse to it. what was that? where should the fuse should be added and of how much Amperes?

  14. Seetharaman

    In Place of battery let us connect a 0 to 15volt dc supply. slowly increase the voltage from 0. around 3 volts D2 will conduct through R3 R2 and BE junction of Q1 – Q1 will get biased and D1 will light up. further incresing the voltage at around 12volt D2 D5 D6 conduct through BE junction of Q2. Q2 conducts – D2 lights up and D1 goes off. further incresing the voltage D8,9,10 conduct through Q3 BE junction biasing Q3 – D7 glows. Q2 collector current flows through D3 and Q3 CE hence D2 goes off. only D7 glows. by decreasing the voltage the process will get reversed.

  15. tom foxe

    Hi Manu, I have made the monitor using any old silicon rectifiers and low power transistors and it works just fine, thank you very much, Tom

  16. seetharaman

    Hi Ralph The zener may be of a higher tollerance one. even then around 14 volts it should lightup. normally 12 + 2 forward diodes + one be junction. that is 13.8 volt is minimum requirement.

  17. seetharaman

    Hi any collector base junction. not base emitter, as they breakdown at low reverse voltage itself

  18. tom foxe

    Hi again – do the diodes have to be IN4148 ? it’s just that I have a lot of 1A silicon diodes – can I use them ? Cheers Tom

  19. tom foxe

    Hi Manu, speaking of battery current I have a few concerns:
    1. Cars have devices to stop overcharging, and a warning to tell you if your battery isn’t charging, so you’d only need this circuit if say, your alternator warning light wasn’t working & a repair was expensive.
    2. A car battery can deliver 100s of amps so it’s quite important to put a low value fuse in this circuit for protection, car fires are sometimes caused when a high and low power circuit short together – the low power cable gets so hot. The circuit will take less than 0.05 amp so a 1 amp fuse would be suitable.
    3. Maybe the circuit designer could add a fuse ?

  20. manu

    nice one, I’ll try this on my car soon, but can you please tell is the BZX79C12 a zener diode, can you post is it a 12v zener diode or something else ?

    will this circuit handle the car batter amps ?

  21. tom foxe

    Hi, this is a really useful circuit for me as I’m planning to do a workshop on bike-powered generators. I had planned to include all parts in the workshop price, but I now see that this is too expensive, a battery charge regulator alone can cost over £100 ! I’ve decided to dispense with a regulator and just make this circuit which will cost a few quid. Just one typo – In the circuit description you refer to to the overvoltage LED as D3 when it’s actually D7.


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