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Resistor for LED in main gun

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Resistor for LED in main gun

Old 07-05-2007, 12:02 PM
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magickapps
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Default Resistor for LED in main gun

Hello All,

Could anyone tell me was resistor i will need if i put an LED in the main gun instead of the airsoft unit (i'm connecting the led to the main board so i will work when you press K on the controller).
Also is it possible to to use the up motion on the controller ( the one that would fire the BB) to lower the gun, If so could someone tell me how do do it?
Thanks.
Magickapps.
Old 07-05-2007, 12:43 PM
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123Splat
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Default RE: Resistor for LED in main gun

Anything between 470 and 1K ought to do O.K. Lower value will make the led burn brighter, but you stand a higher chance of burning out the led.
Old 07-05-2007, 02:17 PM
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darkith
 
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Default RE: Resistor for LED in main gun

It depends on the voltage drop of the LED used. You can punch in the numbers here http://www.theledlight.com/resistancecalculator.html
The supply is probably 5v (if you're using the mod from http://www.dougstigerparts.com/led_cannon_flash.htm), or 7.2v if you're using some other method.
Current is typically 20ma unless you have a high output LED. If the LED is only on for a brief flash, you can bump up the current to get more light, but don't overdo it.
A good starting point would be 120 ohms, and dropping it do get a brighter flash (maybe 50 ohms)

There's no easy way to change the controls on the tank to get up and down on the stick. You could take the airsoft and current elevation signal, and feed them into a h-bridge in order to get "proper" up-down elevation control. It would take 4 transistors or FETs. I believe this was discussed in YHR's thread on his excavator (do a search).

David
Old 07-05-2007, 04:06 PM
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magickapps
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Default RE: Resistor for LED in main gun

Thanks David and splat,
Yes i am doing the convertion layed out in doug's page, the LED i'm using in a 5mm superbright (orange). I think i will stick to leaving the up and down the way it is. oh one last question if i go for the 120ohms is that all i need to ask for as the guy behind the counter is isn't the best of help (why they call it Great Britain i will never know).
Many thanks
Vince.
Will post pictures soon, if i ever get time to finish them.
Old 07-05-2007, 05:16 PM
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123Splat
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Default RE: Resistor for LED in main gun

If he is any good at all, he'll ask "what wattage?". 1/4 Watt or less (1/8W) will do fine. What du ya say Darkith, sound 'bout right?
Old 07-05-2007, 06:00 PM
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darkith
 
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Default RE: Resistor for LED in main gun

Yup, 1/8 watt would do. I tend to stick to 1/4 watt when I'm buying more than 1 as they're more useful.

D.
Old 07-06-2007, 09:00 AM
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Default RE: Resistor for LED in main gun

Most standard LED's are rated for 30ma continuous or about 1 amp impulse. Which means for 5 Volt you could use 160 Ohms for maximum continuous brightness, OR for the daring if the light is on for a fraction of a second, as little as 10 Ohms could be used!

I would go with the continuous rating simply because high impulse will eventually lessen the life of the dielectric in the LED.
Old 07-06-2007, 09:49 AM
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darkith
 
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Default RE: Resistor for LED in main gun

There's also a voltage drop through the LED, so, the voltage through the resistor will be reduced. If the LED is ~2 V drop (typical cheap RED LED), the resistor see 3V.
3V / .025A = 120 ohms. If the LED is a higher drop (e.g. a typical white LED) it's probably closer to 3.2/3.6 V, it'll need something closer to a 68 ohm resistor.

Oh and yeah, the current may be different. I tend to use 25 as it's a borderline max output from PIC microcontrollers. 30 should be fine for most (non-industrial half-life) designs.

D.

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