Trying to make a simple circuit - LEDs, resistors, etc.

lightsalright · 16-05-2015 2:48pm #1

Hi everyone,

Newbie so please tell me if I've posted this in the wrong place.

I am trying to make a 3x3 grid of LEDs, run off a battery and has an on/off switch of some description. 2" x 2" case.

I can make the housing, but that's about it.

I know from googling that this is a lot harder than it looks, at least to me.

Can anyone help me figure out what sort of LEDs I'd need and any other bits (battery connector bit for example)...? I also assume I need resistors of some description.

Also, I was thinking of using pegboard (?) as the basis for it, is that advisable?

Sorry to be such an idiot.

Thanks!

16-05-2015 3:52pm

Go to Maplins and they will assist and sell you everything for small money.

lightsalright · 16-05-2015 6:30pm

Will Maplins actually help me figure out which parts I need?

Thanks for the advice!

16-05-2015 6:32pm

lightsalright wrote: »

Will Maplins actually help me figure out which parts I need?

Thanks for the advice!

For something simple like that I would imagine they would.
They are a helpful bunch.

lightsalright · 16-05-2015 6:39pm

2011 wrote: »

For something simple like that I would imagine they would.
They are a helpful bunch.

Thanks so much... I feel like a total jerk when I go in and ask for hlp in shops, but in this instance I guess I have no choice...

Why oh why did I never take an electronics class

Thanks again for your help!

Chet T16 · 16-05-2015 9:13pm

Pick what LEDs you want and what battery (voltage) you want to run at and from there you can work out what resistors you need. Then all you'll need is a switch.

lightsalright · 16-05-2015 10:43pm

Chet T16 wrote: »

Pick what LEDs you want and what battery (voltage) you want to run at and from there you can work out what resistors you need. Then all you'll need is a switch.

Thanks.. I literally don't know how to decide what battery I want, aside from size, and how to then divine which resistor... I don't know how they relate basically.

Sorry, I know I'm an idiot.

Thanks for your help!

Chet T16 · 17-05-2015 12:06am

What size LEDs? Normal 5mm ones? 9v battery?

lightsalright · 17-05-2015 1:41am

Chet T16 wrote: »

What size LEDs? Normal 5mm ones? 9v battery?

Well - I don't know. I need to put 9 of them in a 5x5cm square. And I'd like them to be visible from at least 3-4m.

Preferably I'd like watch batteries. 9v are probably too big and heavy.

I don't mind changing batteries relatively frequently. I would say the longest one would ever stay turned on would be about an hour. Maybe less.

I think 10mm is probably way to big. So yeah 5mm would probably be perfect. And some of them need to be red, some white, if that matters.

Thanks for the help!

Nekarsulm · 17-05-2015 1:50am

Buy one of those "Electronic Workshop" kits in a good toy shop. Surprisingly good for learning basic circuitry.

lightsalright · 17-05-2015 9:53am

Nekarsulm wrote: »

Buy one of those "Electronic Workshop" kits in a good toy shop. Surprisingly good for learning basic circuitry.

That's a cool idea, thanks!

spideog7 · 17-05-2015 2:54pm

Watch batteries won't work, they can't provide the current you'll need unless you put one on every LED.

You need 9 LEDs, assuming a 2.5V voltage drop per LED (white might be higher check the data sheet for whatever LED you are using), you could use 2 AA or AAA batteries. They would give you 3V so you would have to wire all of your LEDs in series. Then you could in theory use a single resistor to set your current, but that's probably not a great idea since you'll get a lot of variation between LEDs, better off to use one for each.

What size resistor you use will depend on how much current you want, standard LEDs are 20mA Max, they'll prob look fine at 5mA though. But if runtime isn't critical it doesn't hurt to bump it up to say 10mA.

So if we have a 3V supply and a 2.5V drop in the LED them we need to drop 0.5V in the resistor. Ohms law says V=I*R and we want 10mA so 0.5 =.01*R, basic algebra says R=50ohms. 9 LEDs will draw 90mA total, standard AAA are ~900mAh on the high end so this should last about 10hrs on a 2 AAA batteries but they will start to dim dramatically much sooner than that as the battery voltage will drop close to 2.5V.

Obviously my calculations are based on several assumptions so you need to pick a few things before you can fully calculate what you need. If I were you I'd buy a variety pack of resistors, they're dirt cheap and you probably will need to experiment a little to get the LEDs to look just how you like.

lightsalright · 17-05-2015 4:49pm

spideog7 wrote: »

Watch batteries won't work, they can't provide the current you'll need unless you put one on every LED.

You need 9 LEDs, assuming a 2.5V voltage drop per LED (white might be higher check the data sheet for whatever LED you are using), you could use 2 AA or AAA batteries. They would give you 3V so you would have to wire all of your LEDs in series. Then you could in theory use a single resistor to set your current, but that's probably not a great idea since you'll get a lot of variation between LEDs, better off to use one for each.

What size resistor you use will depend on how much current you want, standard LEDs are 20mA Max, they'll prob look fine at 5mA though. But if runtime isn't critical it doesn't hurt to bump it up to say 10mA.

So if we have a 3V supply and a 2.5V drop in the LED them we need to drop 0.5V in the resistor. Ohms law says V=I*R and we want 10mA so 0.5 =.01*R, basic algebra says R=50ohms. 9 LEDs will draw 90mA total, standard AAA are ~900mAh on the high end so this should last about 10hrs on a 2 AAA batteries but they will start to dim dramatically much sooner than that as the battery voltage will drop close to 2.5V.

Obviously my calculations are based on several assumptions so you need to pick a few things before you can fully calculate what you need. If I were you I'd buy a variety pack of resistors, they're dirt cheap and you probably will need to experiment a little to get the LEDs to look just how you like.

This is hugely useful!

When you say I need a resistor per LED and that it needs to be wired in series, does that mean batteries, resistor, light, resistor, light, resistor, light, etc.

And where would a switch fit into this...?

Thanks so much for the help!!

mawk · 18-05-2015 8:49pm

Something like the pictures attached

one schematic, one practical

mawk · 18-05-2015 8:58pm

the reason to have individual resistors is due to the natural variation in the LED construction.

If every LED was totally identical you could just use one resistor about where the switch is or somesuch and it would be ok. but in reality some will be have a lower forward voltage meaning they will be able to draw more current at a given voltage. this means that for a given voltage one led will be brighter than the others. in your case the issue is that the different colours have fairly different forward voltages.

In short youll need one resistor value for each colour and the ability to swap out resistors in case some are too bright/dull.

as mentioned above a grab bag of resistors is cheap.

spideog7 · 18-05-2015 9:38pm

Mawk got in ahead of me but I'll attach the drawings I did anyway.

A) Is as mawk drew it, simple and effective, the calculations are what I did in the earlier post, you can tweak the current to each LED individually by adjusting what resistor you use (remember different colours will appear much brighter or duller even with the same amount of power going to them).

Is a bunch of parallel LEDs, here the calculations are the same as above (assuming the LED forward voltage is the same for each LED), which typically means they're all the same colour and type. However even with LEDs of the same colour and type you'll likely get some part to part variation which means you'll get a slightly different current in every current path, which in turn show up visibly (definitely don't use this method with different colours).

C) Is a bunch of series LEDs, a better way to go if you have a higher voltage because your resistor has less power to burn off (more efficient and more resistor friendly). In this case the LED voltage in the calculation becomes the sum of all of the individual LED voltages. And since there's only one current path the same current has to go through all LEDs, again this probably won't work for mixed colours (you'll probably need different current through the different colours to get the brightness to appear matched). The disadvantage if that this won't really work without a higher voltage, with a 3V supply you probably won't even get 2 LEDs to work reliably in this configuration).

D) (Not Shown) Is a better way to do it and uses a constant current supply instead of the more standard constant voltage supply, it's actually really simple using either a pair of transistors or a variable voltage linear regulator. But it's probably overkill for your application.

Your best bet is to pick the LEDs first, then work around their requirements.

mawk · 18-05-2015 9:47pm

I'm lightning fast like that ;-)

Here is a fairly practical photo of 'B' as I happen to have the thing to hand

Trying to make a simple circuit - LEDs, resistors, etc.

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