10 leds running off 2 AA batteries

[Donwill]

I purchased a string of 10 Leds powered by 2 AA batteries connected in series giving 3 volts or thereabouts presumably. I was a bit sceptical about the life of the batteries so I left them on

24 hrs to check, I was surprised that they lasted over a month which makes them very useful as party lights in a damp conservatory, let's say running them for 4hrs a day which should make them last for 6 months perhaps.

I got to thinking about dimming them, a series resistor would surely do that, but it would reduce the battery life by warming up the resistor. Now you can dim filament bulbs by using an astable multivibrator and varying the mark to space ratio, relying on the chopping frequency and the time constant of the filament to provide a non flickering and dimmable light. Doing that to an led would cause flickering of the light but would the relativly slow response of the eye smooth out the flicker? as in film movies which I think have to be in excess of 25 fps in order to fool the eye. In the interests of battery life, are there any dimmers of this type available for leds?

Cheers Don

[Andy Burns]

Not perceptible if you do it at several kHz

[brass monkey]

A 555 timer will prolly do the job

course, the cost of your time in making it would buy a good few batteries ;)

[Adrian C]

A bit of slightly opaque paint dabbed over the LED's would be even cheaper.

[<me9]

It would be easy to knock up electronics do do so.

A lot of vehicle lights are dimmed by pulsing them. If you observe LED rear lights and turn your head there is often a flicker. When the same lights are used as brake lights the flicker disappears as they are then fully on. The frequency used must vary as some lights are more noticably flickery than others.

I'd go for at least 100Hz to minimise the flicker. Much higher is easy to achieve.

[Donwill]

Unless you'r an OAP like me. :-) Cheers Don

[Donwill]

but a little difficult to adjust the brightness perhaps :-) Don

[Frank Erskine]

Sounds a fun place to have a party :-)

[Adam Aglionby]

Pulse width modultaion, subject of several U.S. patents in relation to LED dimming but thats a rant for another time, 555 as suggested can do it, fly swimming slowly in ointment is V drop, only have 3V to play with , drop 0.7V across a semi junction, even low Rds Fet will drop a bit.

Couple of strings in series to get voltage up and then dim them.

Cheers Adam

[Clive George]

At which point maybe the resistor may be as efficient, depending on current.

[mick]

This gets to be fun if you are restricting your supply to 3v. Here's a dimmer for 12v:

may be able to get it to work using a CMOS 555 (which works down to about 1.5v) and, maybe, a MOSFET such as 2N7000 instead of the transistor to give better efficiency at the low voltage. You could decrease C1 to increase the frequency if you get any flickering. R2 can be omitted if a MOSFET is used.

[Harry Bloomfield]

Donwill explained :

It works, but for LEDs you need to increase the switching frequency to be much higher - say around 1Khz. The filament in a lamp also takes some time to heat up and cool down, giving it some persistence.

[Andy Champ]

Go up in the kilohertz range, or you'll annoy some people.

As many car manufacturers are currently doing with cheap circuits to save a few pence, sometimes on cars costing 100k.

Andy

[Owain]

I was thinking of getting some cheap battery LED Xmas lights but was worried about battery consumption, if they will run for a month continuously (on Tesco Value batteries?) that means I can stick them up somewhere inacessible on 1st Dec and leave them on.

Owain

[Steve Walker]

Yes, I'm fed up of having a string of red squares or circles across my vision as I look from one side to the other behind an LED lit car! The red man on newer pedestrian crossings can have the same effect.

SteveW

[Tim W]

Steve Walker wibbled on Sunday 22 November 2009 20:47

There's simply no excuse for it. I was messing around with AVR microprocessors and RGB LEDs, doing PWM on each element with a fixed cycle period and variable mark/space. No problems running in the many kHz or even

10's of kHz.

Now, if for some reason this is problematic over metres of rough car wiring looms, then the answer (if they want to be clever) is to build the lamp controllers fairly local to the lamp clusters and slick them on one of the control busses. Doesn't have to be one per lamp, could be 2 or 4 for front/back or corners, plus another set for dash and interior.

I assumed they did something like this anyway to cut down on the number of heavy copper conductors running everywhere, but I haven't looked hard enough at my car to inspect the wiring looms to the lights.

If that is undesireable for them, (ie they are cheap or want to be simple) then they would be better sticking with simple bulbs (which could be LED bulbs in the same format with the same drive).

[Steve Walker]

I must admit that I thought that as more and more cars use canbus the lamp controllers would all be local to the lamps. As you say, this reduces the weight of copper that is needed.

As for the the frequency, indeed, there is no excuse for using such a low rate.

SteveW

[Calvin Sambrook]

Hang on a minute, many LEDs won't turn on at 3v as that's quite a low Vf. Not only that but the normal arrangement for multiple LEDs is to put them in series chains. You could put them in parallel but there are issues. I suspect then that the 3V from your batteries is steped up to something rather higher.

LEDs require a current source rather than a voltage and there are some nice little ICs available which do all of the step up and current control in one package, you may well have one of these in the lights you've bought. Have a look near the batteries, if they're cheapo it may be a small black blob rather than a chip as such.

If the lights use an IC to do step up and control then no, a resistor won't work, or at least not in the way you think. Even for simple resistor in series with LED circuits you wouldn't reduce the battery life by increasing the resistor value. You would waste power of course by heating the resistor but the overall current draw would be less so the battery would last longer.

Again anything like this will screw up an IC controller, if that's what you've got.

Movies are 24Hz frame rate *but* they are double or treble shuttered to avoid flicker. 24Hz is objectionable and unwatchable, 48/50Hz is right on the limit and anything you'll see in a UK cinema will reach your eye at

72Hz.

[Adam Aglionby]

Was thinking that but presume OP has red, green and amber chains, red ,amber and green LEDs will run off sub 3V, blue and white need 3V+ to light and typically have 3 AA batteries.

B&M Homestores have a good range of both mains and battery operated LED sets this year.

Cheers Adam

[Donwill]

The leds are wired in parallell and I have looked, and checked again after your email for a black blob or any indication of some kind of electronic gizmo that might control current or whatever, I am pretty sure there is nothing there. the leds have transparent plastic housings and I can see nothing in the housings that might indicate that they have anything significant in there either.

I don't think there is one, it's just a simple translucent battery holder which includes a switch, no sign of a "black blob" there either. The possible explanation that occured to me is that they have a constant current diode built into the leds during the manufacturing process and is visually quite undetectable.

OK, thanks, I hadn't realised that, I was thinking to set the multivib to 1KHz at least.

I purchased them on ebay and also bought a string of 20 and the only discernable difference between them is that the 20 string has 3 AA batteries in series. I eventually cut off the battery housings and connected the two strings in parallell powered by a Nokia mains charger ACP-7X which were being sold off cheaply, if I remember correctly for 99p, the charger got a little warm after 10mins use so I inserted a 2ohm power resistor in series which reduced the current sufficiently to keep the charger temperature within bounds. The subsequent light output was reduced obviously but still adequate for my purposes. This was just an experiment as I did not want to introduce mains voltages to the still drying out conservatory (damp).

Thanks Don