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Led voltage indicator

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PMK:
Hi, Chris_B.
Apologies for reviving an old'ish thread (new member here), but your circuit caught my eye. I've just breadboard'ed your design - works a treat. I made a similar gizmo a couple years ago. The main difference is that the unit is self-powered by the battery that it's monitoring (no separate supply needed). See http://www.pm.keirle.com/2_test/esv/esv.htm
Also, instead of the 10-digit display, a couple circuits shown below using just four LEDs. The 1st ckt uses 3x fixed-value 1% res's; the 2nd ckt uses 2x trimpots.
Designed primarily for the model plane/boat fraternity, can work on any number of cells up to 25 volts max.

Hope these sketches make sense.......

Chris_b:
Hi PMK

Nice collection of circuits you've got there, this thread could end up as the definitive reference for these!

The original challenge (which I failed to meet entirely) was to produce a self-powered circuit that would indicate the output from a dc generator driven by one of the amazing engineering creations you see on this forum. Folk had tried some basic led circuits and suffered from vapourised leds or a system where the leds didn't come on at consistent increments of input voltage. With a self-powered LM3914 you'll not be able to measured voltages much below about 4 volts, so I cheated and used a battery.

Designing with the LM3914 is not entirely straightforward as the function that set the led current/brightness is mixed up with the function that sets the voltage corresponding to FSD "full scale deflection" or all leds alight. As you'll see from my notes, my circuit did not produce the current I expected so I added a list of resistor values to produce useful led currents.

My circuit also included a potential divider on the input, so the range could be extended above the battery voltage, and on reflection (hindsight is always 20/20) I should have simplified the circuit by fixing the FSD at the absolute minimum which is 1.25 volts by connecting pins 6/7 to ground with a single resistor (that would set the led current) and grounding pin 8.  Then RX and RY could heve been chosen to set the FSD. That way the way you set FSD is independent of the way you set the led currents. It would also have meant the circuit, if self-powered, would have worked down to the lowest possible voltage; mind you, at 4 volts or so it still might not have been low enough for some folks. With a separate battery, and the 1.25 volt FSD above, the first LED would com on at 0.125 volts.

If anyone is interested in the revised circuit, shout here and I'll update the design above and repost.

Chris

PMK:
"With a separate battery, and the 1.25 volt FSD above, the first LED would com on at 0.125 volts."

Absolutely. That's the reason why your battery-powered version is ideal for measuring my single cells.
In my case, the unit had to be self-powered because, down at the flying field, they wanted it that way. They don't like parting with their ££ for more batteries and it had to be made reasonable small (read weight). They don't like current-hungry gadgets, neither. It's the same deal with the model boating fraternity. So having a 4-led version pulling just a few measly mA is music to their ears.
The '3914 is a great little chip - ideal for these sort of gizmos. I initially started out with using quad op-amps but soon found that the '3914 takes much less space on any given Vero layout. And pulls less current, to boot. Incidentally, if you want to manually vary the brightness of the LEDs, and if you can put up with a slightly larger board size, you could swap out the fixed res' on pins 6 and 7 with a 5k trimpot (another tweaker to play with).

Nice to meet you, Chris.
I'm sure I'm not the only one interested if you want to post your revised version.

<edit>
Forgot to say...

Your issues regarding R1/R2/current draw: I too noticed the discrepancy on the data sheet. Again, in this case, superbright LEDs are probably the better choice because you can skim as much mA as you dare in return for brightness. In fact, the photo above, it lies. The actual LED intensity is more than the camera implies. You could probably hike R4 up to as much 5.6k, maybe more, all depending on how good your eyesight is/whether the sun shine directly on the LEDs, etc. The higher the res' means the more mA you save.
I see that it's now possible to buy 10-digit displays in the superbright flavour. This is good because your R1 and R2 could be recalculated, and likely end up squeezing a bit more milage from the PP3.

Bernd:
PMK,

I know were I can use that circuit. Going to have to build one for my two six volt battery powered "Rescue Boat".

Thanks for that design.

Bernd

PMK:
Hi, BernD.

Is there a chance you might have a schematic for your light level indicator? I'm trying to eyeball the tracks in the above photo but it's a bit of a hit-and-miss affair.

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