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Question about Dimming LEDs

Toobuilder

Well Known Member
I am using a SPST rocker switch as the autopilot master. This switch features a LED indicator that is activated with power on (internal to the switch) and an external ground. Disconnect the ground, the switch functions fine, the light goes out. Anyway, the LED is extremely bright, which is great in daylight, but I can see this being a problem at night. I?d like to use a simple day/night dimming feature using the nav light switch as the logic. Can I wire a resistor (or something) in line with the ground circuit to dim the LED in the "night" position? If so, a simple DPST relay would provide a two position ground which will give me the day/night illumination level I?m looking for. I know there are LED dimmers out there, but I?m really only looking for a fixed value for a few indicators, so a variable ?LED dimmer? seems to be overkill compared to a single resistor type solution.

Thanks
 
Yes, you could use a resistor in series with the LEDS. You would need a DPDT switch, so that in the off position the ground runs to ground, and in the on position the ground runs through the resistor and then to the ground. If for whatever reason you have the LEDs wired in parallel instead of series you should run a resister for each LED.
 
A resistor will work fine. Try something in the 100-300 ohm range, but you will ultimately have to experiment to find the correct value. Off the top of my head, a 1K potentiometer connected in series, then adjusted for the proper brightness would also work - measure the value of the pot with an ohmmeter to find the correct fixed resistor to put in its place and you are set. A 1/8 watt resistor will probably work OK but a 1/4 watt one would be better.

Mark Olson RV-7A F1-EVO
 
SPST

Why not use a SPST switch wired across the resistor? Switch open, resistor in circuit; switch closed, LED is grounded directly.

If you're thinking about doing this for multiple indicators at the same time, you might want to consider using a zener diode instead of a resistor. It will still need to be rated to dissipate the same amount of power, but a zener provides a near constant voltage drop--which would be insensitive to the number of lights illuminated. I think the AeroElectric Connection may have additional info explaining this technique.

Paige
 
I just wired up a new annunciator panel using LED indicator lights for the -3, and did just as you are wanting to do - a "bright/dim" switch. I am going straight to ground for bright and found I needed about 30K-Ohm to get it as dim as I liked. I tired a pot that I had in my drawer, but it was only 5K, and barely dimmed the LED.

Paul
 
Thanks for the guidance everyone. You gave me the idea to use a DPDT switch on my nav lights as the "router" for day/night logic instead of a relay (less complicated is better!). One half of the switch will be nav on/off, the other circut will be a path to ground for the LED with/without the resistor.

Some of the other threads discussed the LED or dimmers as the source of EMI/RFI... I'm guessing that this is not a concern in this case? (...because of the low power?)
 
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EMI

That is a factor for PWM dimmers. By going analog your generate more heat (not an issue for a handful of LED's) but no EMI
 
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