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LED DC Voltage Indicator

LED DC Voltage Indicator

This LED DC voltage indicator circuit is a voltmeter, rather than simply a battery tester. As such it may measure voltages as low as 3V.
It utilizes venerable LM741 operational amplifiers applied as comparators that drive LED indicators. Voltage thresholds are 3, 6, 9 and 12V. Above each incremental threshold an additional LED turns on.

DC Voltage Indicator Circuit Schematic

LED DC Voltage Indicator

Vcc = 18V

Many voltage indicators rely upon the measurement source voltage for power, but in this voltage indicator case using the LM741, 3V is simply too low for satisfactory operation—the 741 is not specified for low voltage applications. Vcc is higher than what we usually see because it is necessary to have sufficient headroom to bias shunt zener regulator D6. Vcc could have been reduced to 9V by dividing all the threshold reference voltages by a factor of 2, but that would preclude using the LM741 because its input common mode range is specified no lower than 3V from the negative rail—It would definitely not work at 1.5V.

To conserve battery life a “Push to Test” pushbutton switch is used. Another way of doing it would be to use ±9V, and then it could sense voltages down to zero volts if desired. However, this would require the same number of batteries and complicates the pushbutton switch.

Not built or tested

Generally what I submit has been built and tested – this one has not. I have built stuff like this before and used the LM741 as well as the dual version (LM1458), so I have a high degree of confidence that it will work as expected. However, Murphy is always lurking somewhere, just waiting for such an opportunity. The project builder will have the experience of troubleshooting in case it does not function as expected.

Input voltage protection

It is not a good practice to run op amp inputs to the outside world where they would be subject to ESD. R1 and D5 provide the required protection.

D7 protects against accidental reverse battery connection.

Input impedance

Because this dc voltage indicator with LEDs circuit does not derive power from the measurement source, the input impedance is very high. Input impedance is 1M (high but not infinite). In parallel with this is the op amp input bias current that is roughly 4 * 80nA or 320nA. Due to the high impedance, it may be possible to turn on all the LEDs by simply touching the input with your finger. If input noise is an issue, a 0.1uF capacitor across D5 should be helpful.

LED brightness

LED brightness may be varied simply by adjusting the 3.3K series ballast resistor. Most LEDs specify max current to be 20 or 30mA, but efficient green and white versions are blinding at this current so they need much higher value ballast resistors.

Native or source file link

This may be read and /or edited via RFFlow software—great, friendly and inexpensive flow chart and drawing software: www.rff.com A trial version may be downloaded.

Glossary of undocumented words (for our ESL friends)

Murphy – reference to Murphy’s Law (anything that can go wrong will go wrong –Edsel Murphy)
http://www.frontiernet.net/~wmooney1/Fixed_files/murphy.pdf

12 Comments

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  • Dbillgate

    Urgent HELP!!!Why i put the circuit in Multisim 12 and breadboard,It totally can’t run???

    • Jim Keith

      This is the voltage source input from the batteries under test.

    • Dbillgate

      I have solve the problem, we need change the 3.3k resistor connected to LED to 1k resistor then the circiut can run.

    • Dbillgate

      What are the uses input+ and input-?

    • Jim Keith

      Try simplifying –use only one comparator to debug simulation –plot Ein vs output voltage

  • asreejithmenon

    Respected Sir

    I have seen many circuits using a voltage divider network to give the reference voltage to a series of comparator and the input voltage is the voltage to be detected is also given to the comparator.Based on the input voltage the led glows showing the level of input voltage.Like a battery voltage indicator.I wish to replace the led by using a 7segment display.Also since say for a maxm of 12v which should show display of 1v.So i would require 12 comparators.

    So it looks like a flash adc.Now i wish to reduce the cost,hence instead of that wish to implement a succesive approximation adc or some other kind of adc.How do i implement this.Please help me out

  • JKPieGuy

    Wow, I’m so glad that I found this schematic that would have been perfect for the application that I’m working on. But I’ve already have soldered most of the circuit already onto “perfboard” so it would be ashamed not to finish. Anyways, I’ll still give this circuit a try on the breadboard, just for fun.

  • Chibuzor

    Sir,
    I want to inquire, constructing a automatic street light controller system, can it be possible using LM741 op-amp as comparator, LDR and PIR motion detector to build a street lighting system, if it is, how is the circuit is going to be design? This is my mail: ichibuzor@ymail.com and it my final project work. Please!!! help.

  • vasudha

    sir,
    thanks a lot!
    works perfectly fine . just made a few changes according to my project.

    • Koushik

      Hi Vasudha,
      Can u please tell me the changes which you had made in the circuit so that we can implement in our circuit.

  • Jim Keith

    woops, see a problem right away –D5 should be 15V (1N4744A), rather than 12V for obvious reasons.

    • Koushik

      Hello Sir,
      Can u please tell me the changes which you had made in the circuit so that we can implement in our circuit.

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