Tri Color LED Circuit Question
 

Electrical wizards ? (that?s everyone except me)

I want to use a single tri color LED as a status indicator for an aux fuel cell. The logic for this circuit is driven by 2 ?open/closed? float switches; 1 each placed at the ?empty? position and the ?5 gallons remaining? position.
Desired functionality is as follows:

More than 5 gallons remaining (both switches floating) = Green

5 gallons or less but not empty (?empty? switch is floating; ?5 gallon? switch is not) = Yellow

Tank empty (no switches floating) = Red


There are no doubt plenty of you who can do this circuit in your sleep, but tri color LED?s are a bit beyond my grasp. Is there an off the shelf source for such a driver circuit? Perhaps I can cannibalize some cheap ornamental yard light? Are there plans available on the Web? Can you point me to a book so I can learn?

Thanks for your help

Tri-color LEDs are RGB, so your really only need bi-color. Red + Green = Yellow.

Are the switches closed or open when "empty"?
Not difficult, but would be simpler if you used three discrete LEDs.

I suppose you want dimming for night ops?

See, I'm learning something right away!

I'm looking for a single LED because I plan on placing it on the fuel selector cover plate in allignment with the selector "arrow". Multiple LEDs would be confusing, I think.

Dimming is not a problem with single LED's as in the past I've simply wired a resistor in line with the nav light circut. Nav lights on, LED is about half bright. The multi color units may not respond to such a simple trick, however.

The float switches I've looked at can be positioned (or are available) to be open or closed, so I think I can do what I want there.

Well, it would seem that one of each switch type is what you need. Consider that the fuel is going to be sloshing, so at the transition levels things are going to be blinking. That is where hysteresis comes into play in designs. More complications!

Here is my simplest solution:
The "5 gallon" switch is open whenever the level is above 5 gallons. One switch terminal is tied to ground, the other to the cathode of the Red LED. The Red LED anode is tied to the + panel lighting power via a current limiting resistor.

The "Empty" switch is closed whenever the level is ABOVE the empty level. One switch terminal is tied to ground, the other to the cathode of the Green LED. The Green LED anode is tied to the + panel lighting power via a current limiting resistor.

Anytime the fuel level is above "empty", the Green LED is powered. Anytime the fuel level is below "5 gal", the Red LED is powered. For the region between "5 gal" and "empty", both Red and Green are on = Yellow. If the level is "empty", only the Red LED is powered. If the level is above "5 gal", only the Green LED is powered.

BTW, you want a common Anode bi-color (R/G) (or tri-color) LED. Then you only have one anode connection to deal with. http://www.superbrightleds.com/morei...#/tab/Overview

Discuss.

Well that sounds easy enough!

I'm not concerned too much about the sloshing issue and it will still give me an indication to how close I am to my limits- "solid" green and an alternating green/yellow (or yellow/red) are all valuable info for my purposes. My aux tank is going to be a fairly tall "hopper" style anyway, so the level should stay fairly solid.

Thanks much for the advice and link.

Someone with more knowledge than I may chime in, but I was under the assumption that any switch or float in a fuel tank is powered with a voltage very low so as to preclude arcing.

I know on my Bonanza the float rheostats are not sealed. But I believe they are fed with about 1V from the gauge mechanism.

Food for thought when you design your circuit.

Don

Good point, but typically, these switches are magnetic. The electrical stuff stays on the outside.

Those float switches are sealed reed switches. The only part that touches fuel is the plastic.

First of all there are two types of bi-color LEDs, two pin and three pin. The third color comes from turning on both LEDs. For what you are going its probably easiest to use a 3 pin LED. In this type of part the Red and the Green LEDs can be controlled independently. That will make getting the correct control function easier.

To help you with your circuit I need some more clarification. First of all by floating do you mean that the switch is open. Normal terminology for switches would be open or closed. I'm a little confused by your description of the switch arrangement or maybe its just the way you are saying it.

I think you could connect the empty switch to the Red LED and then use the Green LED for your more than 5 gallon indicator. Getting the mid state to work is where more clarification is needed.

Yes, by "floating", I meant that literally - probably a poor description. anyway, it is in regards to the fuel level. I didn't define open or closed because I'm not sure what the circut needs. With these switches I think I can configure them to open or close when high and dry or submerged.

My goal really is to tell at a glance of the fuel selector where the fuel is (or isn't)

Green means I have "at least" 5 gallons (enough for a safe go around or 30 minutes of cruise flight):

Yellow means I have less than 5 gallons and could run out any second;

Red means dry, danger, do not select, etc.