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Help with adding some LEDs on an RC car?

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Hello and I hope for warm welcome (read: with helpful replies :D).

The car in question is a 
FEIYUE FY06 (google it).
It has a 7.4V 2S battery 3000mAh.
Has two empty connectors on the ESC (35A).

It has space for
4 x 5mm LED on top
2+2 x 5mm LED in front
3+3 x 3mm LED in the back

So I got four (4!) of these sets:
Note white are 5mm and red are 3mm like I need... so with four sets I fill all positions and have 2 red to spare (probably put inside the cabin somewhere).
AFAIK the 4 LEDs are connected in parallel and I don't plan to cut the wires to change the arrangement.
I don't think any resistor is included.
I don't think the seller gives any more info (current or voltage).

The problem is the wiring, the proper way to connect them (in series for each set or in parallel), resistors to use, power source to use... pretty much everything. :)
I also plan to use an on-off switch.

So let me be more specific to help you help me:

1) Use which power source? The LiPo (7.4V, 3000mAh, but would like to be ready for 3S too)? The receiver? (2 empty ports, 5V most probably) A simple 9V alkaline battery? If on the LiPo (or receiver) what kind of drainage should I expect? Noticable?

2) What kind of connection do I make? As I said, I don't want to cut the wires, so I will use the 4 sets as they are (each of them is a parallel set of 2x5mm white and 2x3mm red), but then I am free to choose the best way you tell me to interconnect them. In series? In parallel?

3) Depending on #1 and #2, what capacitors I need? How many and connected where? (a single one for all this or or four individual for each set?)

Can somebody provide me with "stupid proof" info please?

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Here is what I think you have:

1. Some LED light sets that are rated for 5V operation.

2. A R/C car with proprietary ESC/Rx that has two spare servo ports that the manufacturer says can be used to power LED's. Voltage at the spare ports is unknown.

If I had to wire this up I would first use a meter and confirm the ESC/Rx ports have 5V. Then I would install one LED set on it and confirm it works. If successful I would then add the other three LED sets (wire all four in parallel) and re-confirm that everything works well.


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So, not bother with direct-to-battery (main or dedicated), not bother with resistors?

It's your choice what to do. The info I read in your LED link says it can be plugged directly into a R/C Rx, which which would mean they have built-in resistors for 5V use. If you want to use a separate battery (or the main 2S) then you'll need to alter the existing resistors values. You can use this calculator to find the required values: https://www.rc-cam.com/led_info.htm

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Well tried to use that calc, but seems if I add the (supposed) Vf I am well over the allowed numbers (whether I use 5V source, 7.4V or 9V).

Must be doing something wrong.

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First number: 5 or 7.4 or 9 (depending on the setup I'll make)

Second number:
- If I calculate resistor for EACH set (4 resistors total before the parallel connection) then (adding the numbers as "note 1" says) then 10.4 (3.2 x 2 + 1.9 x 2 the first 3mm red found in the list)
- If I calculate a single resistor for the whole setup, even worse, 41.6.

Third number: 20 (a typical current)


"Try Again! The source voltage must be a bit higher than the forward voltage."

So according to this, a 5V can power a... single white LED with a bit of voltage to spare. :P (REALLY doubt that)


Edited by NLS

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So according to this, a 5V can power a... single white LED with a bit of voltage to spare. :P (REALLY doubt that)

That is correct. Example: Vf on a common White LED ranges from 3.1 to 4.0 volts. Typical is 3.4V, so enter that Vf into the calculator. Specify 5.0V source voltage and 20mA current. The resistor required to operate ONE white LED with 5V is 82 ohms.

The calculator did not work for you because your specified configurations are incorrect. To properly change one of your LED sets from 5V operation to another voltage will require fully disassembling the LED wiring harness (cut away the heatshrink that hides the existing resistors) and then replace each resistor with the calculated value for each LED.

But before you do that you'll need to learn a bit about LED's. Please re-read the info on the LED calculator page. Also fully digest this info: https://www.maximintegrated.com/en/app-notes/index.mvp/id/3070

Alternate ideas: Honestly, there are several other ways to solve the problem. For example, there's a chance the existing resistors in the LED set will allow the LED's to tolerate 2S LiPO power (8.4V fully charged). But current will be higher and might exceed safe conditions for the LED's. To avoid trouble you could install a 5V voltage regulator (R/C BEC) on the 2S LiPO and power the four LED sets from it.

Better idea: I suggest you circle back to my original suggestion (see my first post from yesterday). Just try it and if it works you can skip hacking the LED's. Should be OK as long as the R/C car maker was honest about the spare Rx ports being capable of powering LED's and if your eBay LED supplier was honest that the LED's can be connected directly to a R/C Rx.


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