Maybe the easiest would be to buy them as is then buy one around 500 ohms and another around 250 ohms. If it turns on to soon you can add the 500 in series with the 50k, to low add the 250 in series with the 20k. You can make a little tee - pee with the resistors.
We can start a pool on the switching voltage.

Now that idea I like. Simple, easy, and I get to use SMD resistors like dominoes

Thanks guys, will put together an order for some pieces and let you know how the build goes.

 

Ron, you've modelled D2 as a white LED (NSPW500BS). That results in D2 current being practically zero below the trip point, as shown in your sim plot. However, if you model it as a different LED you'll find the D2 current in fact ramps up significantly below the trip point. This means that both LEDs are lit below the trip point (albeit with a ~3:1 current difference and hence quite a brightness difference). Nevertheless the circuit should still provide a good indication of cell voltage.

 

Well shoot. I thought I checked that. It's worse at the low currents.
Here is a little dual diode that will shut the green one off.
Or we can feature it.... You might like the effect of the green one glowing if the voltage is close to 4.1 volts. You can then use 1 or 2 diodes in series with the green one. I lowerd the resistance a bit to get the current up at the same time.

 

The extra diode does the trick. Nice.

 

I'm thinking an op amp version can be a little easier than the open collector comparator version.

Use a 1k pot in this design.
You will need a 2.5 v regulator or voltage reference or Zener (shown as Zener)

Buy a low-voltage op amp that works with 3 to 5 volt single supply

To calibrate - Connect a battery when exactly at 4.1 volts and adjust pot until midway between two LEDs (one going off and other coming on).

 

Don't forget he has 1 to 7 batteries in series. So you can't share a quad op amp - each one needs it's own circuit.

 

Hey everybody,

I just had a reply from one of the guys on the Helifreak website who's a fellow enthusiast with a lot more knowledge than me on this kind of thing. He's recommending an off the shelf IC with a colour changing LED.

I'm still trying to get my head round exactly how this works, but it seems like a really simple idea:
http://www.helifreak.com/showpost.php?p=6240061&postcount=4

 

and the spec sheet is...

http://elcodis.com/parts/1351559/BD45291G-TR.html

 

Sorry, should have linked that. I like mouser here in the UK, they give you the full PDF spec sheets:
http://www.mouser.com/ds/2/348/bd46xxxg_series-e-209345.pdf

 

That looks like it should work. Maybe ebay would have them in onesies. Mouser want you to buy 1900.

 

No, you can still buy individual ones of these from Mouser, or at least I can here in the UK. I just need to work out how these work, and how to use colour changing LEDs.

 

I think you mean bi-color LED. They are just 2 LEDs in one package.
Mouser has them.

 

Hi folks,

I've found what I believe to be an IC that can switch on a LED when the voltage drops below a certain threshold:
http://uk.mouser.com/ProductDetail/...=sGAEpiMZZMunEhqKs81nFIsibbHJOKDwMDHU2omsPTQ=

And in the spec sheet, they have a diagram of a really simple battery charge indicator circuit that's pretty close to what I'm looking for:
http://www.mouser.com/ds/2/308/NCP302-D-86450.pdf

To my untrained eye, this looks fairly similar to the originally proposed circuit, but replacing the ZR431L and resistors with an IC that has a pre-set threshold voltage. While I like the flexibility of the ZR431L, for the size I want the finished system to be, and the number of circuits in close proximity, I think this may be a better approach.

My only concern is that the range of voltage between the LEDs activating here may be a little higher than I'd ideally like, and I'm not sure what the behaviour would be there. Despite that I'm tempted to pick up a few of these to try, possibly an IC which has it's low threshold at 3.9V, which I think would give me a red LED below 3.9V, and a green LED over 4.05V.

And if I find there's too much inaccuracy around the threshold I can just make two separate circuits. One using an IC which outputs active low and the other using one which has an active high output. Using two ICs would also let me fine-tune the overlap (even with my lack of knowledge!), so I could have a red LED illuminate any time the voltage drops below 4V, but with a green LED if it's over 3.9V.

Could I ask if any of you can foresee any problems with using the circuit above, or with my idea of using two ICs?

Thanks all!

Myx

 

It looks like there is 0.2 volts between turn on and turn off.

 

Well, just as I'm looking to start buying the parts to make this, look what I find on eBay for £1.10:
http://www.ebay.co.uk/itm/RC-Lipo-B...sGames_RadioControlled_JN&hash=item3ce447cb97

Amazing what you can import from the far east these days, I guess you've just got to know what to look for...