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iCufflinks improved battery life code change
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Re: iCufflinks improved battery life code change

by scott_42 on Fri Aug 05, 2011 1:25 pm

Testing is done for now.
Quick synopsis (TL;DR), don't get rid of the resistor if you want batteries to last.

I tested with the full PWM values from 1-255 and then halved them to range from 1-128 (see chart 1). I quartered them as well but ended up not using it because the half ended up disappointing.
Image
chart1 by scott-42, on Flickr

Chart 2 shows the average current versus the resistor value for the different PWM values.
Image
chart2 by scott-42, on Flickr

Lastly in Chart 3, I measured the light output of the LED. If anyone cares, using a Sekonic L-508 spot meter.
Image
chart3 by scott-42, on Flickr

What this is showing is that no matter which resistor value (or no resistor) you pick when using a lower output PWM, it will never match the light output of the full range PWM values with the existing 220 ohm resistor.

Bottom line, if you want to keep the same quality of light output, you can't change the PWM to a lower duty cycle.

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Re: iCufflinks improved battery life code change

by adafruit on Fri Aug 05, 2011 1:26 pm

Oh man this is the best post ever. LOOK AT THOSE GRAPHS! CHECK OUT THAT DATA!

email support@adafruit.com for ANOTHER fabulous PRIZE!

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Re: iCufflinks improved battery life code change

by scott_42 on Mon Aug 08, 2011 11:09 am

So I hear you like graphs and data. Here are a couple of bonus graphs showing the battery voltage over 24 hours runs using CR1220 and CR1216.

Small size for easy viewing (500x500).
Image
battery-plot-small by scott-42, on Flickr

Really big version.
Image
battery-plot-large, not on Flickr because it doesn't like really big images.

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Re: iCufflinks improved battery life code change

by h3llphyre on Mon Aug 08, 2011 12:01 pm

Scott,

A few comments to mull over.

1.) Light meters aren't great analogs to how the human eye perceives light. Lots of great research into how ~60Hz light at 10% duty cycle can appear to the human eye as 100% duty cycle (with LEDs). Weird phenomenon.

2.) Try to find the lumens / amp graph for the LEDs. The high powered LEDs I'm playing with (3W) show a noticable difference up to 350mA drive current, the difference from 350mA to 700mA is mostly noticable, from 700mA to 1000mA (overdriving), almost no difference. Again, the human eye doesn't recognize linear differences. Either way, there is probably a more "sweet spot" for choosing the range of drive current for the LED.

3.) PWM will give you a better range, due to not dropping voltage across the resistor. Due to your low power pursuit, the limited PWM frequency is probably going to limit you.

My hat is off to you sir, this is great watching you dig into this. I look forward to seeing more from you.
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Re: iCufflinks improved battery life code change

by scott_42 on Mon Aug 08, 2011 1:16 pm

h3llphyre wrote:1.) Light meters aren't great analogs to how the human eye perceives light.
Agreed, but they are the only tools I have available to measure light output that is consistent. If only I could figure out how to plug electrodes into someone else's eyes. Reminds me of an old movie quote:
…if only you could see what I've seen with your eyes!
Do you have a recommendation or suggestion on a tool that I could use to accurately and consistently measure the light output of an LED using PWM that would translate the measurement into a human perceptive model?

h3llphyre wrote:2.) Try to find the lumens / amp graph for the LEDs. … there is probably a more "sweet spot" for choosing the range of drive current for the LED.
Here is a link to the the product page of the LEDs I'm using http://www.rohm.com/products/opto_device/led/chipled/smlk34wbecw/ if you care to browse it for the info. Page two of the data sheet has a Luminous Intensity-Forward Current graph, not sure if that is close enough to what you were asking about. Since this is a 3.0V (or less in battery terms) system, there is a limit to the current that can drive the LED for 24+ hours. If you have a suggestion for a change, I'd love to hear it.

h3llphyre wrote:3.) PWM will give you a better range, due to not dropping voltage across the resistor. Due to your low power pursuit, the limited PWM frequency is probably going to limit you.
I'm not sure I follow what you are trying to say here. The design is currently using PWM to drive the LED in a sin wave pattern. The PWM output is currently running at 471 Hz.

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Re: iCufflinks improved battery life code change

by h3llphyre on Mon Aug 08, 2011 3:49 pm

Unfortunately, I do not have any of my "research" material at work. If I remember tonight, I'll dig through some of my stuff to see if I can find a few papers I have on the human eye's response to light.

The relative luminous intensity versus current isn't nearly as extreme as high powered LEDs are, my only point was that the high powered LEDs typically show extreme diminishing returns at the higher drive currents. This isn't the case here (for the most part).

Like I said, I'll dig through some of my stuff tonight, see if there are any goodies I can share for your continued battle towards lower power.
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