Driving 20 leds with battery

[iteoschi]

Hello everyone,

first of all, sorry for my english, i'm italian and i'm sure that i'll make some mistakes

As the title says, i need to drive 20 leds with the smallest battery possible, seems easy uh? Well, the problem is that i need to make them work for around 100hrs before recharging the battery.
I was thinking about using arduino for some light effects, and since arduino offers PWM it can be a good way to save energy, as well as programming some light effects.

My question is: is there a way to drive thoose leds for around 100hrs using 2 or 3 of this batteries? (https://www.sparkfun.com/products/13189)

 

[TEEJ]

What LED at what drive levels?

 

[iteoschi]

What LED at what drive levels?

I want to drive 20 white leds, i don't really have a specific model or brand, i'm looking for a not-too-bright level of illumination, they will be used for a lamp that will be used in a little club. It needs to produce enogh light to lit a small area, but not too much.
I want to use this concept for a future flashlight with the goal of having a freaking long battery life.

 

[SemiMan]

After the Arduino you would be lucky to get 200mw for the light

 

[iteoschi]

After the Arduino you would be lucky to get 200mw for the light

Wow is arduino so power-hungry? Since I don't have a lot of space I was thinking of using just the bare chip (with the crystal and caps) to also reduce power.
So yea, the question stays the same: is it possible?

 

[SemiMan]

Not hungry but we are talking 100 hours. If it draws 20mA at 5v, or 100mw, then that is 10w-h or 1/3 total battery.

 

[SemiMan]

Its 30w-h ... In 100 hours .... Or 300milliwatts ... Simple math. Is that enough for you?

 

[HKJ]

Not hungry but we are talking 100 hours. If it draws 20mA at 5v, or 100mw, then that is 10w-h or 1/3 total battery.

If you know how to program small microcontrollers, you can put them to sleep most of the time, reducing the current consumption to nearly nothing.
Doing things like sleeping 0.9second working 0.1second is very beneficial for power consumption.
With leds it depends on how bright you want them, for some led 1mA is enough current.

 

[SemiMan]

Its for a lamp in a "little club" ....1ma is unlikely to cut it. 300mw on a small tabletop may.

 

[TEEJ]

If you don't need a lot of light, why 20 LED?

It seems random...but, is there a reason for that particular number of LED?


If you gave an idea of about HOW bright, at what distances, you wanted the light to illuminate, it would also help.

 

[iteoschi]

If you know how to program small microcontrollers, you can put them to sleep most of the time, reducing the current consumption to nearly nothing.
Doing things like sleeping 0.9second working 0.1second is very beneficial for power consumption.
With leds it depends on how bright you want them, for some led 1mA is enough current.

Thank you for the detailed answer!
Yea, I read something about putting arduino in sleep mode. That way I can reduce power consumption, thank you for the advice. Some LEDs can draw as low as 1mA? Can you tell me which led are you talking about? (like the brand or the model)

 

[iteoschi]

If you don't need a lot of light, why 20 LED?

It seems random...but, is there a reason for that particular number of LED?


If you gave an idea of about HOW bright, at what distances, you wanted the light to illuminate, it would also help.

The easiest way give you an idea of the amount of light I need is to explain what I'm making. I'm making a ring of light, think about a tube bent in a circle that needs to contain an arduino and the batteries needed in order to drive 20 little leds glued on the outside. (obviously is much more sophisticated but I want to give and idea) I want to illuminate a small area, suffused lighting, so I prefer lots of leds that are not that powerful over fewer leds that are very spotty and powerful. The amount of light should be equivalent to one of this spheres: http://www.terredumonde.it/shop/10-342-thickbox/sfera-luminosa-led-lampada-b30.jpg ... Sorry for the lack of terms but as I said I'm Italian and it is quite difficult to explain for me.

 

[iteoschi]

Not hungry but we are talking 100 hours. If it draws 20mA at 5v, or 100mw, then that is 10w-h or 1/3 total battery.

Yea that's quite a bit of power. But I think that I put the controller in sleep mode (as HKJ suggested) we can lower that power consumption by a lot. Tomorrow I'll test the program and see how much it draws.

 

[arek98]

You could use 20mA 3mm or 5mm LEDs. Assuming you using 3 Li-Ion batteries in series, you could wire them in strings of 3, add 20mA linear driver (something like that) to each string, connect few strings (e.g., 6 for 18 LEDs) in parallel.

You should also look in one LED option with diffuser. Even ~20 LEDs will need diffuser otherwise individual spots may still be visible. It may actually be easier to diffuse one LED assuming is in center (spheres in photo you linked).

 

[SemiMan]

Each 5ma led is 60mw from a 300mw budget ... Best case for 5 LEDs.

 

[iteoschi]

You could use 20mA 3mm or 5mm LEDs. Assuming you using 3 Li-Ion batteries in series, you could wire them in strings of 3, add 20mA linear driver (something like that) to each string, connect few strings (e.g., 6 for 18 LEDs) in parallel.

You should also look in one LED option with diffuser. Even ~20 LEDs will need diffuser otherwise individual spots may still be visible. It may actually be easier to diffuse one LED assuming is in center (spheres in photo you linked).

I was actually thinking about changing the battery setup. Since the frame i'm using is a tube bent in a circle, it's more convinient to use more, but slimmer, batteries. (something like this: http://www.batteryspace.com/sanyo-l...7-5-aa-size-3-47wh-0-28---un-38-3-passed.aspx) This way i can fit more batteries in this tube, and since it's quite small (1.9cm in diameter) i'll have less problems with the curvature.

Looking at the driver i saw that the minimum voltage is 5V, i think that i'll need to find another driver that supports 3.7 volts, and maybe that has an integrated PWM dimmer.

In this image you can se (more or less) the final setup. The yellow dots are the leds. The light should be diffused from there, and since the leds will be flush to the casing (because the idea is taht you will not see them) i think that the diffuser it's not really indicated. I'm planning to use 20 leds in order to diffuse light in a semi-organized manner, with the final goal ov not seeing the origin (in this case the leds) of the light. As always, sorry for the lack of wording

 

[HKJ]

Some LEDs can draw as low as 1mA? Can you tell me which led are you talking about? (like the brand or the model)

Standard 3mm white led. I uses some for indicators and at 1mA they where very bright to look directly at, but if you want to illuminate and area it is probably not enough.

 

[iteoschi]

Each 5ma led is 60mw from a 300mw budget ... Best case for 5 LEDs.

Yea 5-6 leds is not much. You said that some led can consume as low as 1 mA. If we can reach 12-15 leds it's enough, maybe using 3mm leds.

 

[iteoschi]

Standard 3mm white led. I uses some for indicators and at 1mA they where very bright to look directly at, but if you want to illuminate and area it is probably not enough.

Yea but maybe using a good quantity (like 15-20) the light produced could be enough, as i said i don't really need much light, just enugh to see the actual circle "glow" and producing a bit of light.

 

[HKJ]

Yea 5-6 leds is not much. You said that some led can consume as low as 1 mA. If we can reach 12-15 leds it's enough, maybe using 3mm leds.

The best way to test is to buy 20 3mm leds, solder them together in a serial/parallel configuration and test with a variable bench power supply. Then you can see if they can do the job and how much current they need for it.
When you know the amount of current for the leds, you need to look at the driver and how good efficiency you can get.
It is much easier to select batteries when you know how much power you need.