10 leds running off 2 AA batteries

[Donwill]

Found this also:

might be interesting to play with. Cheers Don

[Calvin Sambrook]

I'm still struggling to imagine how these are arranged. If they are in parallel then each LED will have a wire going to it so that's 10 wires running along the "string" plus the return which can be common which makes

11 in all. Each LED will require a current limiting resistance of some sort. They will not share a limiting resistor as a mis-match between Vf of individual LEDs leads to runaway conditions and dead LEDs so there will be 10 resistors. This is however a poor way to design battery powered LED lights as If will vary significantly with battery terminal voltage, bear in mind that a fully charged AA cell may be anywhere between 1.2V and 1.5V depending on its chemistry.

It's just never done this way, in fact the only case where I've seen LEDs controlled with a resistance commercially is in keyring lights where the relatively high internal resistance of two button cells is relied on to limit the current through a single LED. Your chain simply can't be using the internal resistance of AA batteries as that can be very low in some cases. In fact as a fun experiment try fitting NiCd or NiMh cells, either it won't light because they have a lower V or the LEDs will pop because they have a very low R.

What voltage is the mains charger? If you have access to a multimeter why not measure the current.

[Donwill]

The leds have a thin cable, transparent insulation, and having two conductors in it; each conductor is connected to a leg of the 1st led and then carries on to the next led, so in effect they are in parallel. I found the invoice, and have a ref to the advert on ebay, you can see on the pic how they are wired.

I tried using my own camera but it wouldn't focus closely enough to show the detail of the led connection.

I think I did at the time, but they are disconnected and back on the battery packs now. I think it was (from memory) approx 350 mA (30 leds) and the voltage was just over 3volts. Power supply rated at 3.7 Volts,

355mA

Cheers Don

[Clint Sharp]

In message , Calvin Sambrook writes

Don't be daft. It's two wires running from the battery holder.

Think simple, the guys that make these things just use one series resistor for all the LEDs, it's not good practice but it does work after a fashion.

If you want to do it right, yes they do. If you want to sell the lights for a quid in the local pound shop (where I got mine) then they don't.

In practice, I bet the mismatch between LEDs of the same batch is tiny.

It is. As you say, it's not the right way but it is done.

[Calvin Sambrook]

Uk, I must remember to get out into the real world occasionally (apparently the Real World is where the pizza delivery man comes from). I fear you are right having seen the ebay item referred to.

Nasty but cheap.

[mick]

Yeah - I saw that one too. I decided that a) it was probably a bit of overkill (you don't really need 0-100%). b) it might be difficult (if not impossible) to get working from 3v.

I also considered using a PIC chip as those will work down to 2v. It would be possible to use software PWM to drive a LED output MOSFET and an analogue input from a "brightness" control. The problem is efficiency - it probably isn't very... ;-) One of the little 8-leg chips like 12F683 would be the animal. This approach has one advantage over the others - you can make the brightness control linear (to all intents and purposes), no matter how the LEDs behave. You can also do silly things. :-)

[Clint Sharp]

In message , mick writes

Why use software PWM when that chip has it in hardware? Trivial to set up and control with a pot. I reckon you could buy the chip and a capable transistor cheaper than the equivalent non micro components needed to do PWM.

[Clint Sharp]

In message , Calvin Sambrook writes

Oh I'm right, I bought a few sets because I can't buy LEDs that cheap.

Definitely.

[mick]

Oh yeah... so it has! I should have checked the spec sheet first, shouldn't I? :-)

I might have a go at building this just for fun!

[Clint Sharp]

Did it last night, works well.

Using the internal clock and an external transistor (BC639, first one I found) it's very simple to code. I reckon the cost of parts is well under 2 quid and I built it into the battery holder.

[tahrey]

replying to Donwill, tahrey wrote: That's basically the way most LEDs are dimmed in the first place. And generally unless you're extremely sensitive, flickering at mains rate (which, through a bridge rectifier, is essentially 100Hz, thus equal to all but the most excessively high-refresh CRT monitors of old) should be imperceptible when the diodes are stationary or only moving slowly relative to the eye. I was in Homebase earlier looking at some light fittings and only realised that one stand was displaying a mix of halogen and LED when I went to take a picture with my phone and saw some of the lights strobing very gently on the screen. Having been regularly annoyed by low-refresh CRTs back in the day, I tried to determine whether there was any flicker visible to direct viewing, but couldn't make any out even with the corner of my eye.

So if you were to use a 555 or other simple vibrator, so long as you tuned it to operate at least somewhere into three figures that would be more than enough to exclude flicker. If it's not visible with high brightness picture tubes of 15 inches diagonal or more, it won't be visible with small, dim LEDs. My phone itself has an OLED screen which probably refreshes somewhere around

120Hz (it's certainly not 60Hz) and the only evidence of flicker is if I wave it around rapidly in pitch darkness. Most of the time you completely forget it's not a regular LCD with a steady backlight.

I would question why you want to do this, though. I ran similar sets of lights over christmas, and found them neither to be too bright for comfort, nor to have such short battery lives or high voltage demands that running them off "spare" batteries (non-rechargeables that had run too low to work in at least one other more demanding device; freshly topped-off rechargeables would have run a good bit longer) became an annoying performance of repeatedly switching them out. Are they really too intense, or is changing the batteries every 3~4 weeks (which for christmas lights is essentially once a year) that much of a hardship that building a dimming circuit is worth the hassle?

FWIW, if the problem is that they produce too intensely focussed little points of light, I found that with a bargain bucket chain of multicoloured diodes, which I think were initially supposed to come with star-shaped diffusers on the end but turned out to lack them. A bit of pocket-money modification turning them into a lantern string, by buying a pack of white plastic party cups from Poundland, cutting small holes in the bottom of each cup and poking a diode through each, proved to have a rather pleasing end result, with the light being nicely diffused around the entire cone of the cup and giving the whole string a softer, more traditional incandescent lantern type appearance.

Another mod that may be worth trying, if you have spare battery holders sitting around, is to just run additional pairs of AAs in parallel, or hooking the switch up to a pair of Cs or Ds instead. The overall bulk and weight of the contraption still won't be particularly massive. In extremis a 3V mains transformer, or a Poundland USB charger with a suitable dropper resistor (or one charger wired to run two chains in series, though I'm not sure if they produce much light with only 2.5v applied, my own tests suggest 2.6v is a bare minimum; alternatively omitting the resistor and seeing if they stand up to being overdriven to higher brightness, as they're probably underdriven by design anyway) could be wired up to power it without any batteries, or even a butchered cheapo solar-charging garden light or two for "free" non-mains juice.

[tabbypurr]

You're replying to a post from 2009. Do yourself a favour and use a sane interface to this newsgroup rather than that semifunctional website. Google groups works. Here is news:uk.d-i-y

NT