Wednesday, March 12, 2008

Red, Green, White LEDs

Our original circuit design used a dual-colored LED (red/green) as an idicator to signal the crossover of a voltage on the comparitor (if that makes any sense). The problem, as seen in the graph below, was that for a given voltage, the currents across the different colors varied significantly. This caused a problem in that when the red/green LED switched colors, it would cause the white LED on the end of the shaft to dim or go brighter. For obvious reasons, this is not good. We need a constant light source.

So I decided to use two discrete LEDs in hopes that I could put different resistor values in series with each of them, thus providing equal current drain.

This last week we got the LEDs and I built the circuit and it works. Previous current drains were up in the ~20mA (for green) and ~30mA (for red). Now it's closer to 6mA for both colors.

Just a note: The opamp swings different amounts in both directions. I don't know if this is a problem or what exactly is causing it. My guess would be that the resistor values on the voltage dividers aren't exactly equal, resulting in the power supplies being skewed.

So I hooked the two leds up (separately) to a variable resistor, set the resistor value to yield an appropriate3 intensity and current measurement, and then measured the variable resistor to figure out what value of discrete resistor I should use to yield the desired results.

Red: ~8k
Green: ~3.3k

Using these values, like alluded to above, I measure the current drain of the entire circuit and found it to be very stable.


Now a note about white LEDs. The white LEDs we have are super bright and they only draw 1-2mA (compared to ~3mA for the red and green ones). It makes me wonder if we should (can) use a coupled of white LEDs with cheap filters over them (think red cellophane) instead of the colored ones.

No comments: