I plan to add a White 16 LED Circular Light Board with Lead from HK to my RC night flying plane. It will be used as a bright landing light. But I want it on at a lower, dim level during normal flight and turn it high and low with a switch on my TX.

At HK it says the lights are 9-11 volt with 16 LED elements that total to 7watts. I used that information to calculate amps and then with the amps, calculated resistance. Here is that math.



I took that information to an electric circuit simulation program and built the following circuit. The resister is to step down voltage when the switch is open and have the LED lights on at a dim level. The switch will be a Turnigy Receiver Controlled Switch that, when activated, will short across the resister delivering full voltage from the 3S LiPo flight battery to the LED's to make them go bright. Here is that circuit with a 35 ohm resister in the voltage drop position and the switch open and closed showing the voltage at the LED.



My problem is first, that I do not know that my math is sound for this application. And, second, I do not know what voltage is needed at the LED's for the dim light condition. I did more simulations and built the following graph. The graph shows what voltage is available to the LED at different resistance values on the R1 step down resistor.



I think the lights will come on at around 4.5v and that my dim light condition might be around 5v which is in the 35-40 ohm range for the R1 resistor.

What are your thoughts on the integrity of this circuit for use as described? Any hints on the voltage needed to light these lights at a low level? I was thinking of getting a potentiometer or rheostat to find the brightness I want and then purchase the resistors to make the circuit to put into the plane. Or, if the potentiometer is light enough, to just use it in the circuit.

Thoughts? Comments? Suggestions? All are welcome.

Freddy

7W of LED is going to be VERY BRIGHT. Think small floodlight.

One other consideration is the wattage rating of any resistor you use. If the final choice of resistor drops the voltage by about 5V the wattage it will require to dissipate will be about 3W. It may get hot.
For experimental purposes a rheostat would be better than a potentiometer which may not be able to dissipate the required power.
Have you thought of using a very small brushed ESC instead? Could be smaller, lighter and get less hot.

Yes, 7 watts will be bright. That is why I want it on low brightness. High brightness would only be to light the runway on landings.

To avoid overheating the resistor I can use several, like 5 to 7, 5 ohm resistors in series spreading the heat across each.

As for using a brushed ESC, I have one and could use it but my TX is a Spektrum DX7 and I have no knobs on it to control brightness. I could possibly use auxiliary channel 6 and use the TX settings to set the output to the radio like I would if it were a servo. Just set the end points for the 2 settings for low and high brightness. At least, I think that would work. I have never tried it. Is that feasible?

My current night flyer has a on/off switch but that is all so my experience with this is very limited.

Will the brushless ESC work like I described by setting the endpoints to control the brightness?

Freddy