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
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.
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 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)