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Old Jan 18, 2009, 12:53 AM
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LED lighting question

I just purchased a pkg of 20 led lights of assorted colors. I planned on usin the clear for headlights and red for tail-lights of course. Anyways I didnt do any research and just tried hooking them up together and pluggin in a battery. bad idea blew all 4 led's. My question is that I know i need to use resistors but if i plan on usign 4 leds. which the manufacturers say use 2-3v. at 10-20mAH. on a 9v. battery which resistor am i gonna need to get, how many am i going to need to get. Do i neeed 1 for each light or will one work for the whole system if i wired them all together? I plan on using a 9v. battery but am unsure as to where and howmany resistors i am going to ned to use and their placement along the wires. I have no idea if they go between the wires, if you just use one for the whole setup, if it goes between battery and first led what, where, how many, etc. Help in any way is appreciated.
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Old Jan 18, 2009, 01:07 AM
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A few links that should help...

http://www.laureanno.com/RC/LED-assembly.htm

http://www.laureanno.com/RC/LED-calc.htm
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Old Jan 18, 2009, 01:17 AM
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RcCrazy
fall river mills
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Quote:
Originally Posted by Tomapowa

Thank you very much. This link http://www.laureanno.com/RC/LED-assembly.htm helped me out very much. Now i know exactly what it is I need to do to make this work, go pick up some new leds and wire a 100ohm resistor into it. Ever so easy gonna have a fun sunday project for my rc.
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Old Jan 18, 2009, 01:34 AM
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Quote:
Originally Posted by CertifyThis
I just purchased a pkg of 20 led lights of assorted colors. I planned on usin the clear for headlights and red for tail-lights of course. Anyways I didnt do any research and just tried hooking them up together and pluggin in a battery. bad idea blew all 4 led's. My question is that I know i need to use resistors but if i plan on usign 4 leds. which the manufacturers say use 2-3v. at 10-20mAH. on a 9v. battery which resistor am i gonna need to get, how many am i going to need to get. Do i neeed 1 for each light or will one work for the whole system if i wired them all together? I plan on using a 9v. battery but am unsure as to where and howmany resistors i am going to ned to use and their placement along the wires. I have no idea if they go between the wires, if you just use one for the whole setup, if it goes between battery and first led what, where, how many, etc. Help in any way is appreciated.
With a good 9 volt source, you can run all four in series with one resistor. Here's how...
Determine what approximate current you'd like to run through them. Look at the data sheets for the LEDs and choose a current well below the maximum rating.

At that current, look at the graphs to determine the voltage across each different LED. (The voltage across the LED doesn't change much after a certain value of current.)

Add the four voltages up and subtract from 9 volts.

Divide that voltage by the current. That is the resistor size needed to run the LEDs.

For example, say you have four LEDs whose voltages at 20 mA add up to 8.4 volts. 9-8.4 = .6v Then .6 /.02 = 30 ohms.

The example points out a problem. As the battery loses terminal voltage under loading (because of increasing internal resistance), the current into the 4 LEDs will go down dramatically because of the small headroom of .6v and as a result the LEDs would get much dimmer. If the 9 volts always stayed at 9 volts (which it won't), there wouldn't be a problem.

So it would be better to run 2 leds with a series resistor in parallel with the other 2 LEDs and another resistor. Use the same method as above to determine the resistor value, but with 2 LEDs in series instead of 4.

In this scenario, you will only be subtracting 4 or so volts from the 9 volts which will give plenty of headroom for the battery losing terminal voltage with time. The back to this is that now the battery is supplying twice the curent. There's no way around this using resistors to limit the current.
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Old Jan 18, 2009, 03:35 AM
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Iam hoping to fit 5 LED's on my plane but powered from the receiver. Is the voltage source ditermined from the battery pack or from receiver output. If receiver whats the best way of finding output at terminal.
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Old Jan 18, 2009, 12:15 PM
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Quote:
Originally Posted by domll
Iam hoping to fit 5 LED's on my plane but powered from the receiver. Is the voltage source ditermined from the battery pack or from receiver output. If receiver whats the best way of finding output at terminal.
On most receivers, the +V and Ground terminals on every servo outputs are tied together (i.e. common), including the connector for power. So, the voltage you use to power the receiver (i.e. battery) is the same voltage each servo will see (no more, no less).

Since each LED typically has a forward voltage of arouind 2.1v to 3v and your power source is say 5 volts (4 cell batt.), you will only be able to drive up to 1 or 2 LEDs of this type in series (2 only if the forward voltages add up to 5v or less). You can though drive many more in parallel, but only if the forward voltage of each LED is around the same (i.e. matched). So, if your plan is to place 5 similar LEDs in parallel (say forward voltage is 2.1 volts and supply is 5 volts and each LED draws 20mA), you can use this calculator to determine the correct current limiting resistor:
http://www.laureanno.com/RC/LED-calc.htm#parallel

You can also get fancy and use a combination of series and parallel arrangements. Since you can only drive 2 series LEDs (2.1v forward voltage) from a 5v source and you want to power up 5 LEDs total, you can drive three paralleled sets of LEDs (2 LEDs in series on two sets, along with a single/last LED). Using this calculator (http://ledcalculator.net/) you would get the following arrangement (see last pic)
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Old Jan 18, 2009, 12:56 PM
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Thanks Tomapowa, Spent all afternoon searching the web and found this link:

http://www.theledlight.com/ledcircuits.html

The pictures done it for me . Unfortunatly takes me a while to absorb data, but add pictures...

You answered my question on running differnet forward voltages though of which iam greatful.

Switching was my next question; using rudder control to turn on/off lights, then found this link:

http://www.rcgroups.com/forums/showt...landing+lights

And right at the end of post, by modfly;

http://www.rcgroups.com/forums/showp...6&postcount=26

Bingo!! Ahhh my dream coming true
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Old Jan 18, 2009, 06:45 PM
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Personally, as I said in some other post, i just ballpark a resistance value for about 25 milliamp current (fairly safe with most LED's), find a trimmer in the proper range and tune it by lowering the resistance gradually until my multimeter reads the proper current and the brightness is what I was aiming for. Once done I test the resistance on the trimmer and get closest value resistor I can find (rounding by excess). So far it's worked for me
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Old Jan 22, 2009, 07:57 PM
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Quote:
Originally Posted by Tomapowa
On most receivers, the +V and Ground terminals on every servo outputs are tied together (i.e. common), including the connector for power. So, the voltage you use to power the receiver (i.e. battery) is the same voltage each servo will see (no more, no less).

Since each LED typically has a forward voltage of arouind 2.1v to 3v and your power source is say 5 volts (4 cell batt.), you will only be able to drive up to 1 or 2 LEDs of this type in series (2 only if the forward voltages add up to 5v or less). You can though drive many more in parallel, but only if the forward voltage of each LED is around the same (i.e. matched). So, if your plan is to place 5 similar LEDs in parallel (say forward voltage is 2.1 volts and supply is 5 volts and each LED draws 20mA), you can use this calculator to determine the correct current limiting resistor:
http://www.laureanno.com/RC/LED-calc.htm#parallel

You can also get fancy and use a combination of series and parallel arrangements. Since you can only drive 2 series LEDs (2.1v forward voltage) from a 5v source and you want to power up 5 LEDs total, you can drive three paralleled sets of LEDs (2 LEDs in series on two sets, along with a single/last LED). Using this calculator (http://ledcalculator.net/) you would get the following arrangement (see last pic)

What if you have 41 3mm LEDs all with a resistor preinstalled and you would like to use a seperate power supply and some kind of sequencer. What would you suggest.

HC
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Old Jan 24, 2009, 08:50 PM
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Quote:
Originally Posted by Happy camper
What if you have 41 3mm LEDs all with a resistor preinstalled and you would like to use a seperate power supply and some kind of sequencer. What would you suggest.

HC
The Mini-Flash LED Sequencer has 4 output channels (max. 600mA each) that are fully programmable using your PC and free software.

Unfortunately, those LEDs you have with built in resistors will not work with anything but probably 12 volts. The Mini-Flash as-is outputs only the supply voltage, usually 5-6 volts. If you really wanted to, you could modify the circuit board a little allowing you to power the Mini-Flash using your receiver (5-6v) but power the LEDs (transistors) with a higher, external voltage, like that from a 3 cell LiPo pack (11-12 volts).

For higher current capabilities, the Maxi-Flash would make a good solution (designed for some one for driving 150 Luxeon LEDs on an R/C blimp).
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Old Jan 25, 2009, 10:21 AM
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Quote:
Originally Posted by Tomapowa
The Mini-Flash LED Sequencer has 4 output channels (max. 600mA each) that are fully programmable using your PC and free software.

Unfortunately, those LEDs you have with built in resistors will not work with anything but probably 12 volts. The Mini-Flash as-is outputs only the supply voltage, usually 5-6 volts. If you really wanted to, you could modify the circuit board a little allowing you to power the Mini-Flash using your receiver (5-6v) but power the LEDs (transistors) with a higher, external voltage, like that from a 3 cell LiPo pack (11-12 volts).

For higher current capabilities, the Maxi-Flash would make a good solution (designed for some one for driving 150 Luxeon LEDs on an R/C blimp).
I tried one of the LEDs on a 2 cell Lipo and it lit up ok. I was told that maybe I could use a 401 decade timer to flash the LEDS? Are you familiar with this component?

HC
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Old Jan 25, 2009, 11:28 AM
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I think you mean decade counter like the SN7490.
http://www.hobbyprojects.com/digital...e_counter.html
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Old May 19, 2015, 02:56 AM
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I've bodged some SLED lights and have been using them one form of another for over a year. The latest version is just a little smaller.A few more details here: http://www.pbcheap.com/led-light-flashlights The range is pretty good too. I'll try and get some new photos of real world" use this week
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