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Tri Color LED Circuit Question

Toobuilder

Toobuilder

Well Known Member
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.
 
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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/moreinfo/through-hole/rl5-rgb-diffused-tricolor-led/776/#/tab/Overview

Discuss.
 
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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
 
cfiidon said:
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
Click to expand...

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.
 
If you want to use 2 switches and 1 LED (BiColor) you're going to need some CMOS logic or at least some DPDT relays. If the float switches were DPDT you'd have a chance but they aren't, SPST. If you want to switch an LED on/off with a SPST you'll need logic.
 
Toobuilder said:
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.
Click to expand...
Right, by orienting the switch float one way or the other you can make it close when "fuller" or "emptier" than the mounting location. (as long as Mr. Gravity is pointing the right way!)
 
This is what I was talking about. No CMOS or relays or other stuff should be needed. Common anode RGB LED, leave the blue disconnected.
20140811170913479_0001_zps4d3f7ca3.jpg
 
just an FYI ... use two independent LED's ... if you have someone on board that is color blind ... the multi-color LED will be meaningless.
 
fl-mike said:
This is what I was talking about...
Click to expand...

Hey Mike- I purchased the LEDs and float switches and have been playing with them on the bench. I have the common anode tied to a 1k ohm resistor (not the 850 ohm you specified, but that's all I had on hand), and a dead Odyssey battery (10.5 v). I can get the red segment to light, but not the green or blue. I understand that red is the lowest power of all the LED colors... Is the higher value resistor and dead battery dragging the voltage down so far that blue or green don't light off?
 
I probably should have specified two different resistor values in the cathode legs to optimize the current in each LED to 20mA. Rev A!
I'm surprised you are not seeing anything on the green, but green and blue do have a higher forward voltage than the red. Parallel two 1k resistors and try again. That should be safe for a 10V supply. Let's try and get light, then you can finalize the values.
 
I would add a 5.1v Zener diode in parallel with the LED's to ground to help regulate the voltage seen by the LED's.

:cool:
 
LEDs operate on current. Change the current and you will change the luminace. You need two resistors one for each led. The resistor is selected by first picking the desired current. 20mA is a good starting point. Different LEDs have different operating currents. The resistor is determined by (Vbatt - VLED)/20mA. VLED is based on the forward voltage of the LED. No Zener is required for this type of circuit. Look at the datasheet for your LEDs to get the actual values for VLED and the current
 
I purchased the LEDs from the link shown by Mike, specs here

If I'm reading it correctly, the overall fwd voltage is 2.6 max, 2.0 typical.

Yet reading further, this is the spec for the red only, while green and blue require 4.0 and 3.5 for max and typ.

What is the significance of this disparity?
 
Toobuilder said:
What is the significance of this disparity?
Click to expand...

Physics....
i.e., the characteristics of the materials used to make the LED colors. Just one of those things. (blue was a really tough one to figure out)

See if you can get the green to light first, and then we can add some features to the circuit including some protection from reverse voltage spikes, etc. No need to complicate things until you get the proof-of-concept working.
 
What I mean is why list the "overall" value of the LED at a lower level (2.6v) than one of the individual elements (green =4.0v)?
 
Because to set the current you need to know the Vf of each led. Look at my earlier post. To set equal current for different Vf values you will need different resistors.

One other thing the Vf is at the rated current. It's not a voltage rating per say. I usually use the max voltage for the resistor calculation that way if the Vf happens to be on the low end of the rNge you won't exceed the current rating.
 
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