Power/drive unknown amount and colors of LEDs using DC voltage from AA batteries

Question

The Plan

In a 3d printed game board tiles that I'm designing, there are game pieces that will be placed down by players on specified locations. I'd like to have these pieces light up with LEDs. I've bought a bunch of pogo pins from amazon and the plan is to use those to pass the DC voltage from the game tile to LEDs in the pieces. Every player will have different color pieces and the LEDs' color to match the player's color. Along with these player pieces, I'd like to add additional LEDs to the game board tiles for aesthetic purposes. At the present I'm looking to use Chanzon 0603 SMD and Chanzon 1206 SMD LEDs.

The Research

In my research, I understand that they have a forward voltage that must be met to turn on, and sending too much voltage can kill them. I also know that the LED's need to be current limited with a resistor or else they'll burn out. Since I'm looking to power the LEDs with batteries, the supply voltage will drop over time as the batteries die and I should be using a constant current led driver with will accept a higher voltage and current limit the supplied power to be what the LEDs are needing.

I've found lots of examples on how to make a constant current LED driver using things like mosfets and n/p transistors. However, in all the examples I've come across they all have something in common; the amount of LEDs being driven by the example circuit are fixed which means that the calculated resistor value only works for the given required milliamps of the LEDS being used. Also, most of them usually say to not mix types/colors of LEDs due to the different forward voltages and current requirements. These examples don't help much as with my intended usage, the amount and colors of LEDs will change based on what game pieces are placed on the playfield.

The Question

What would be required for a circuit that's designed to power/drive an unknown amount of LEDs of differing types and colors from a battery voltage source that will drop over time?

I'm looking for some general knowledge targeted at my use-case as well as pointing me in the right direction so that I can learn how to design the circuits myself. It's very possible that I'm lacking a basic or key understanding of electrical components which has caused me to go down the wrong path in my journey of self education.

Answer 1

Basically you would need to have a separate constant current source for each LED, or string of LEDs in series.

In order to use a constant current source, the loads have to be in series, you can't have loads in parallel.

You could put a CCS in each game piece, then have them attach to a voltage source in the board.

Probably a better approach would be to use a voltage regulated supply and then you would only need a resistor for each LED.

Answer 2

If you start with a higher voltage, the current (brightness) change will be small as the battery degrades. This should be acceptable for your application (a constant current source is overkill).

Run your LEDs at a 1-5 milliamps, modern LEDs are plenty bright at low currents.

Example with 6V batteries:

Resistor for 2 mA, assuming LED drops 2 V: (6 - 2)/0.002 = 2 k

When battery degrades to 80% of original, current will be: (6*0.8 - 2)/2000 = 1.4 mA

Example with 12V batteries:

Resistor for 2 mA, assuming LED drops 2 V: (12 - 2)/0.002 = 5 k

When battery degrades to 80% of original, current will be: (12*0.8 - 2)/5000 = 1.52 mA

The LED voltage drop will change some with current, but the calculations are much more complicated if you consider it.

LED voltage drops will vary from about 1.4 to 2.8 V, depending on color. Use a different value resistor for each color. Or, you could use the same resistor if you can tolerate some brightness difference. Again, starting with a higher voltage will minimize the brightness difference.

AA batteries are about 2000 mAh. So if you have 10 LED at 2 mA, they will last about 100 hours.

Answer 3

Don't be confused by the use of LEDs for illumination (as a light bulb). In those cases the LEDs are run at maximum extremes to get the most light.

You are seeking the more traditional use of LEDs as indicators, and for that, you simply need a voltage a bit higher than the highest LED (4.5 to 5 volts is about right) and then you use a resistor in series with the LED to limit its current. A red vs blue vs green LED will take a different voltage, and you handle that by choosing different resistors.

I have my doubts that the pogo pegs will be satisfactory. A harder but cooler approach is to use an inductor, where the piece has a coil of wire in it, and that is magnetically coupled to a coil of wire in the base. You must use a high frequency AC for that, which you'd need to make. However, adjusting the number of turns of wire will allow you to adjust the voltage seen by the LEDs. If the AC voltage is near the LED voltage, you can wire the LEDs "back to back" oriented opposite directions, and one LED or the other will light up. That plus a resistor and that's all a puzzle piece needs.