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LED Array Flashlight
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Please be positive and constructive with your questions and comments.

LED Array Flashlight

by elfrocampeador on Wed Jul 22, 2020 9:29 pm

I'm designing a fun intro to electronics project for a friend who has shown some interest, and I'd like to make it a flashlight.
Ideally I'd use something like a cree at some several hundred rated lumens, but the supporting infrastructure needed
for one of those is infeasible, I think, for an intro project (LED drivers, Heatsinks, etc). Is it feasible to build a handheld flashlight
of a normalish handheld size (perhaps ~30-35mm head) with an array of relatively normal LEDs like https://www.adafruit.com/product/754
or would that not be enough light?
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Re: LED Array Flashlight

by adafruit_support_mike on Thu Jul 23, 2020 1:25 am

Yeah, small LEDs (officially called 'indicator LEDs') will work for a simple flashlight.

An indicator LED emits about the same amount of light as a candle, but in a different pattern. A candle is what's known as an 'isotropic radiator', meaning it sends the same amount of light in every direction. If you put the candle in the center of a spherical shell, the inside of the shell will get the same amount of light everywhere.

LEDs are what's known as 'lambertian radiators', meaning the amount of light you see is independent of the angle relative to the LED's surface. An LED inside a spherical shell would send the same amount of light to the half of the shell in front of it, and none to the half behind it. Concentrating the same amount of light into half the area would make the half that's light twice as bright as the surface lit by a candle.

Then, to make things more fun, LED manufacturers mold the transparent coating around an LED into the shape of a lens. There are several different shapes, but the standard rounded-end LED focuses most of its light into a region about 120 degrees wide, and with more light going to the center than to the edges of that region.

So.. even though indicator LEDs emit about the same amount of light as a candle, the light is focused to be maybe five or six times as bright as a candle directly along the axis of the LED package. If you add more lenses or a reflector, you can concentrate the light even more.

To make things even more fun, the human eye responds to light logarithmically, which means we see ratios of light more than absolute amounts. The amount of light at noon on a clear summer day is about ten million times as much as the amount on a clear moonless night, and we can see reasonably well in both once our eyes have time to adjust. So a single white indicator LED will do a fairly good job of lighting objects out to about five or six feet on a dark night.

Adding more LEDs will give you more light to play with and more range, but you have to remember the inverse-square law: if you double the diameter of a sphere, its surface area increases by a factor of four. If an isotropic radiator (a candle) is at the center of the sphere before and after you double its diameter, the amount of light leaving the candle doesn't change. All the light leaving the candle will hit some part of the sphere, so the total amount of light across the surface of the sphere remains constant regardless of its size. The surface of the 2x sphere has four times the area of the 1x sphere though, so any point on the 2x sphere only sees a quarter as much light as a point on the 1x sphere.

So if you want distance, trying to collimate the light.. make it stay in a cylinder whose cross-sectional area never gets bigger.. pays off better than making the light source brighter.

An array of LEDs approximates the cross section of a cylinder, so making an array does have some benefit.

The upshot is that indicator LEDs will work well enough to be interesting, and there are a lot of variables and options you can play with in parallel with the pure electronics.

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Re: LED Array Flashlight

by elfrocampeador on Thu Jul 23, 2020 1:49 am

Hmm. How does an array benefit useful output relative to a single brighter LED? The relatively narrow optimal beam width of traditional LEDs is convenient for a simple DIY as a reflector is less necessary (though I suppose a crude reflector could be fashioned out of foil or something). I'm trying to estimate what I need to produce a reasonably useful flashlight hopefully without the need for tremendous supporting hardware IE: No driver (which is pricey and a bit of a black box, so not tremendously useful as an instructional tool) or heatsink (also a bit pricey) etc. I plan to 3D print a suitable housing and design small custom PCBs for the electronics (which is remarkably cheap through OSHPark).
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Re: LED Array Flashlight

by adafruit_support_mike on Sun Jul 26, 2020 3:24 am

In spherical cow terms (old physics joke), an array of LEDs approximates the cross section of a cylinder. You get a certain amount of collimation that's less sensitive to dropoff from the inverse-square law.

If you have a 2x2 array arranged in a square, with each LED emitting a cone of light, the cross section of each cone becomes 1/4th as bright at twice the distance. But in the region where all the cones overlap, those four quarters add up to as much light as you'd get from a single LED at half the distance.

The more LEDs you add to the array, the more overlapping cones you get. The larger you make the array, the more you get a beam of light moving straight out from some LED with no angular deflection to produce inverse-square dropoff.

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Please be positive and constructive with your questions and comments.