led's

[streetfighterjeff]

guys, the mrs found some battery powered led christmas lights in our local pound shop, they run off 2 aaa bateries so i guess they are three volt. got about thrity blue leds and about ten red ones. if anyone wants some PM me and i'll send some out to you. i'm thinking convoy lights and machine guns.
jeff

[Tankbear]

Good idea Jeff.

Just want to mention resistors. Different colour LED's run at different currents etc. I found when I put a red LED in as a tail light the white headlight dimmed to almost not lit. Ended up putting in 3 x 100ohm resistors in series (300ohm total) to get it right again.

From what I understand LED colour require the most current in the following order
White
Blue
Green
Yellow/Orange
Red

[streetfighterjeff]

never thought of that, good point tankbear

[Tankbear]

I found it trial and error until it looked right, hence 3 resistors in series rather than one of right size.

For the machine gun I managed with out adding any resistor but that might of been because the LED was different spec again.

[Tankie]

All LED's require an inline resistor to control the current. The tamiya and HL multifunction units already have resistors built in which is why you don't need one when connected to the MG feeds on these units. Same is true of the HL headlight feed.

Good idea on the convoy lights - i've yet to see anyone try the front notek light & fancy trying that myself. You could drive them from the headlight feed and switch them so that either headlight or notek is lit.

[Johnf]

Elmod already fitted with resistors also. I already made a very simple convoy light. Have a post on making it on the forum.

[jonger]

Jeff is kindly sending me some of these LED's so I did some research on required voltages so the correct value resistor can be fitted.
White = 3.2v
Blue = 3.1v
red / orange = 2.2v
green / yellow = 2.1v
so red / orange / green / yellow are virtually the same but blue and white need higher voltages.
To calculate the resistor value use the formula R=(Vs - Vl)/I
Where R is resistor value in ohms, Vs is source voltage, Vl is LED voltage and I is current (usually 20ma for LED's)
I have not checked the voltage of the lighting supply I'm going to use but give the following example using 7.2 volts direct from the battery for a blue LED.
(7.2 - 3.1) / 20 = 0.205 which equals a 220 ohm resistor as this is the nearest resistor available rounded up, always round up not down.
Also note that LED's only work if wired the correct way round, the anode (+ve) is the longer leg and the smaller of the metal bits inside the LED, and the cathode (-ve) is the shorter leg with a flattened section and connects to the larger metal bit inside the LED.
Hope that helps.

[streetfighterjeff]

sounds about right bud, as they run off two aaa batteries. get them off to you today bud

[Tankbear]

Great research work Jonger.

Just another thought a fully charge 7.2v battery will be nearer 8.4v so that would need a 270ohm resistor (8.4-3.1/0.020 = 265).

If you used a 220ohm resistor with a full battery at 8.4v the current would be 24mA through the LED. 20% over rated, may not actually make any difference but on paper doesn’t look suitable.