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  #1  
Old 03-09-2011, 06:25 PM
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Toobuilder Toobuilder is offline
 
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Question Question about Dimming LEDs

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
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  #2  
Old 03-09-2011, 06:42 PM
rubber314chicken rubber314chicken is offline
 
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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.
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  #3  
Old 03-09-2011, 06:52 PM
molson309 molson309 is offline
 
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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
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  #4  
Old 03-09-2011, 06:55 PM
PaigeHoffart PaigeHoffart is offline
 
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Default 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
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  #5  
Old 03-09-2011, 08:33 PM
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Ironflight Ironflight is offline
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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
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  #6  
Old 03-10-2011, 08:21 AM
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Toobuilder Toobuilder is offline
 
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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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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
______________
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RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C

Last edited by Toobuilder : 03-10-2011 at 08:27 AM.
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  #7  
Old 03-10-2011, 09:00 AM
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jdeas jdeas is offline
 
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Default 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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  #8  
Old 03-10-2011, 09:00 AM
Bill Palmer Bill Palmer is offline
 
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Thumbs up LED Resistor Calculator

For designing LED applications, here?s a neat little ?LED Wizard? for calculating the resistance values needed for series or parallel arrays:

http://led.linear1.org/led.wiz

Resistance value for a single LED:

http://led.linear1.org/1led.wiz

Have Fun!

Bill Palmer
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