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Old Oct 13, 2012, 10:17 PM
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Letchworth, Great Britain (UK)
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Question
LED best visibility

I've got strobe and landing lights on a couple of models, one a bought set, and the other home-made using a PICAXE programmable chip and the brightest LEDs I could find.

Neither of them are visible until dusk, and both are only visible when they're pointing almost directly at you. The limited viewing angle seems to be common to all high power LEDs.

So, is there a simple way to improve the viewing angle, please? Reflectors seem to be no use because the light's facing forward anyway, so I'm thinking some kind of lightweight lens must be the solution. Anyone willing to share a successful solution?

Also, regular LEDs seem to max out at 30mA (don't know what that equates to in light units), so is there any other practical device for this kind of application? My shop-bought set is limited to 5v supply, but I can configure my home-made one to run on higher voltages if that will help.

I've considered an array of 3mm LEDs all pointing in slightly different directions, but they don't seem to come in such high power versions as the 5mm, 6mm, or larger ones.
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Old Oct 13, 2012, 10:30 PM
The Fixer
Martin Y's Avatar
Canada, ON, Milton
Joined Jan 2009
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you can find leds with a wide viewing angles. but the wider the angle the dimmer it will look. A simle transistor driver will brighten them up. try digi-key, just pick your color.
http://www.digikey.ca/product-search...r/525057?k=led
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Old Oct 13, 2012, 11:06 PM
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Colorado
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High power LEDs can draw up to several amps (and emit blinding amounts of light) with wide viewing angles (such as this one with 115 degrees Cree XB-D).

For beacon style applications, lenses are available that will give a full 180 degrees viewing angle
17.2 mm Side Emitter Lens


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Old Oct 14, 2012, 01:20 AM
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Letchworth, Great Britain (UK)
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Some good info there, thanks. I think I'll try out some of those lenses, perhaps in conjunction with a multi-LED array.

How can a transistor driver brighten up an LED, Martin Y? They're limited by their V and mA rating, aren't they?

Edit: Using search terms from the info you've given me has revealed a whole new world of high power LEDs with built-in lenses, from my regular UK suppliers, including the Cree models. I'm still curious about "drivers" though, for some of my search results indicate that a driver may be required for some devices -- don't you just apply appropriate + and - voltage?
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Old Oct 14, 2012, 03:26 AM
Stuart
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UK, Cardiff
Joined Dec 2008
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Your PICAXE is limited to driving 25mA to a LED, you need a series resistor to limit the current to this safe value.

For more current output than that you need to use a buffer or driver transistor to drive the LED, and the limit then is really your power supply.

PICAXE manual 3 page 10 shows the general arrangement for using a transistor to drive more current.
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Old Oct 14, 2012, 03:53 AM
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Letchworth, Great Britain (UK)
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Quote:
Originally Posted by srnet View Post
Your PICAXE is limited to driving 25mA to a LED, you need a series resistor to limit the current to this safe value.

For more current output than that you need to use a buffer or driver transistor to drive the LED, and the limit then is really your power supply.

PICAXE manual 3 page 10 shows the general arrangement for using a transistor to drive more current.
Okay, I already use transistors between the PICAXE output and the LED. Even though I often don't need it from the current point of view, I do it as a matter of course so I can change the output device -- such as running multiple LEDs in parallel from the one output. Not being very up to speed on modern electronics, I thought maybe you and some of the advertisers were referring to something else
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Old Oct 14, 2012, 05:39 AM
Flying Saucer Mechanic
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Middle River MD
Joined Dec 2000
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you can overdrive the LED if you pulse it if you are careful it won't hurt it. Also when I was making and selling light kits I advised users to sand the LED to "frost" the plastic. It will display as a bright glowing ball.
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Old Oct 14, 2012, 10:51 AM
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Germany
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A Cree XML LED will work with up to 3A. But you will need to have a pretty good heatsink. I use this type of LED in my better flashlights, and it is awesome.
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Old Oct 14, 2012, 05:04 PM
The Fixer
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Canada, ON, Milton
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I use a NPN transistor for driving the leds. with a 22 ohm resistor to limit current. Here is a night approach system I made. When pulsed the leds can be strobed very brightly without problems of over driving.
104356 (1 min 11 sec)

Montreal 2012 night flight (5 min 22 sec)
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Old Oct 14, 2012, 09:51 PM
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Letchworth, Great Britain (UK)
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Thanks guys. Just one more question pending further research into the manufacturers' spec sheets:

When you talk about safely overdriving LEDs with pulses, what length of pulse are you typically talking about, what duration between pulses, and what percentage over the rated current?
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Old Oct 14, 2012, 10:02 PM
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Colorado
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The proper (and the most efficient) way to do it is to use a LED driver IC. Take a look at Zetex Design Notes
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Old Oct 16, 2012, 06:38 PM
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Oregon, USA
Joined May 2010
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Another way to go is the linear regulator route given the 5V supply. Here is a cheap high power led already mounted Here They also have the bare AMC7135 linear regulator chips for CC 350ma. The AMC7135 Vdd pin can be PWM'd with the Picaxe. Lots of other similar flashlight minded sites with all manner of led parts and pieces. You need plenty of heatsinking though if run at full power for any amount of time.
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Old Oct 17, 2012, 03:29 PM
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I got my LED's at the following:

www.spudgun.com/led.htm

He offers 5, 8 and 10mm LED's. The 5mm are 1/2W and the 8 and 10mm are 1W LED's. The 8 and 10mm can be seen in broad daylight and are great for adding Nav or landing lights.

These are the brightest LED's I have seen. They operate on 4.8Vdc and can be connected to the RX power supply or an external power supply.

You can't beat the prices anywhere

Larry
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Old Oct 28, 2012, 06:51 AM
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Letchworth, Great Britain (UK)
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Follow-up questions ...

Okay, so I've got some white, red, and green Cree LEDs, and I have to agree they're bright. But I've never worked with surface-mounted components before, or components this small producing so many watts, so I need some practical help.

First off, I managed to knock off the attached half-dome lens from one of the LEDs while soldering wires to it. It still works, though I'm not sure what its visibility angle is now. Do I need to provide further protection (a coating of clear polyurethane or epoxy, for example) to hold things together better?

Secondly, I've soldered short 24g wire leads onto the LEDs so I can use them like my "normal" 3mm and 6mm devices. What is the best technique for putting them onto PCB board with a hand-held soldering iron? I'm guessing (haven't tried it yet) I make sure the copper strips on the board are bigger than the LED, pre-tin them, and then sit the LED on them while applying the iron to the bit of strip that's protruding around the LED

Thirdly, I've seen mention of heat-sinks. For the moment I'm just using my LEDs as strobes, so I don't think they'll overheat. But what about when I want to have landing lights running near their rated current limit of 1000mA? How do I go about dissipating the heat if I need to?

Lastly, for my temporary rig I've simply put resistors in line to achieve about 500mA current from nominal 5v input. For the moment the only ones I could find locally are huge 3W wire-wound ones, though I've now sourced some smaller 1W ones which will be sufficient watts for the green and white, but not for the red, which needs at least 2W. My strobe circuit will still work if I reduce the voltage to 4v (though it then won't be able to work directly off the receiver or a normal BEC) to reduce the resistor wattage, but is there a more elegant electronic solution to individually drop the voltage in each individual LED so that they can all operate off the same voltage, rather than a rather large resister in each line?
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Old Oct 28, 2012, 08:37 AM
AndyKunz's Avatar
Illinois
Joined Sep 2001
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Wire them in series. Each LED will, like any diode, have a drop voltage that you can find in the datasheet. It's the same one you used to compute the drop off 5V when selecting the resistor.

Andy
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