Spoka – Episode IV – A New Hope
OK, so there’s nothing here about hope. I just couldn’t resist the Star Wars reference.
I did however succeed in implementing PWM on the RGB LEDs last night, so I now have 24 bit color for each eye. It turned out to just be a simple interrupt routine. Of course this solution is brute force, and takes up a *!@$load of the processors time (it interrupts every 256 clock cycles). Oh, well. I’ll worry about optimizing it later if I need to. Here’s the interrupt routine if anyone is interested.
ISR(TIMER0_OVF_vect){
if(pwmCounter>rred) LEDPORT|=R_RED;
if(pwmCounter>rgreen) LEDPORT|=R_GREEN;
if(pwmCounter>rblue) LEDPORT|=R_BLUE;
if(pwmCounter>lred) LEDPORT|=L_RED;
if(pwmCounter>lgreen) LEDPORT|=L_GREEN;
if(pwmCounter>lblue) LEDPORT|=L_BLUE;
if(++pwmCounter>254){
pwmCounter=1; //reset counter
LEDPORT&=~(R_RED|R_GREEN|R_BLUE|L_RED|L_GREEN|L_BLUE); //turn off all
}
}
After a tiny bit of playing around with the color scales, I ended up implementing the first one in the list in the previous post, simply because I could do it programmatically, rather than as a large lookup table. I’m not really happy with the map though. I’d bet the linear optimized map or rainbow looks better (check them out with this java applet). Here’s my colormap function:
void scale1(unsigned char value){
if(value<128){
SetRGB((255-(2*value)),2*value,0);
}
if(value==128){
SetRGB(0,254,0);
}
if(value>128){
SetRGB(0,(255-(2*value)),2*value);
}
}
I also ordered a Mirf with a tiny ceramic chip antenna so that it would fit inside the light. Hopefully iy will be here before the weekend, since I’m taking a 4/5 day weekend to work on the project.
Spoka Pt. III – Choosing current limit resistors & color mapping
I was just realizing that I didn’t put values in for the LED current limiting resistors on the schematic, so I thought I would add a little detail regarding their values and how I chose them. I decided from the start to limit the current through each LED to 15 mA. Each PIN on the AVR can source 40 mA so no problem there, but the entire chip can only source 200 mA total. 7 LEDs X 15 mA = 105 mA. Should be no problem. Plus room to spare when the device is AC powered.
First and easiest was the red. I placed my meter in diode check mode and measured the voltage drop across the LED (Vf) as 1.74V. Measuring the battery output as 3.8V, we get a margin of (3.8-1.74=2.06V) which must be limited to .015A. Dividing the two we get (2.06V/.015A=137Ohms). Putting a 220 and a 330 in parallel gives me (220*330)/(220+330)=132. Close enough.
The blue and green LEDs had too high of a Vf for my meter to read in diode check mode. I knew however that the blue LEDs in the original circuit had a 20 Ohm resistor, so I figured that anything larger should be OK, so I scrounged around and found some 47 Ohm resistors. I soldered these to the LEDs and applied the power, then used the meter to measure the voltage across them. I got 3.16V for the green and 3.07 for the blue.
Green: (3.8V-3.16V=0.64V) (0.64V/0.015A=42 Ohms) Just leave in the 47 Ohms.
Blue: (3.8V-3.07V=.73V) (.73V/.015A=49 Ohms) Again, just leave in the 47 Ohms.
Worst case voltage should be 4.8V when the AC is plugged in. This will give (4.8-1.74)/132= 23mA for the red, (4.8-3.16)/47=35mA for the green, and (4.8-3.07)/47=37mA for the blue. All within the per-pin limit of 40mA. Total output would be (3*23)+(2*35)+(2*37)=213mA. Hmmm, I wouldn’t put out a production unit with numbers like that, but for a hack, it’s not too far off.
I was explaining to my wife the kinds of things I could do with this when it is PC controlled, like changing color according to the speed of a download. This reminded me of an article I saw a while back, “Rainbow LED Indicates Voltage with Color.” This article gives a nice mapping of value to color for the range of 0-255, but it is centered around 128. In other words 128 gives black (all off), <128 gives bluish tints, and >128 gives reddish tints. This might be handy for some things, but not for others. So I did some research on “color scales” and here are a few interesting links:
http://www.ks.uiuc.edu/Research/vmd/current/ug/node76.html
http://www.cs.uml.edu/~haim/ColorCenter/ColorCenterColormaps.htm
Spoka – Part II
(Continued from previous post..)
After removing the LEDs, I marked the outside of the shell to indicate where the eyes and nose were on the rubber cover. Then I modified a small prototyping board to fit into the space, aligning the eyes with the marks made previously. (Notice that the marks are on the same side of the shell as the hole for the plug. Whoops. I had to redo them on the other side)
Then wire the new pcb up, program it and install in the housing for a test (notice my chip-clip programming adapter. It’s handy)…
Red Green Blue White Mouth
Spoka project – Begin
I was at Ikea the other day (they just built one in town, so i get to go all the time) and saw a rechargeable nightlight that I thought would be cool with some RGB LEDs. As I was working on the RF link yesterday, I realized that I could make a wireless, pc-controlled RGB LED light. So I headed over and picked up a Spoka from Ikea. This thing is just begging to be hacked. It comes with a 4.8V 400mA wall wart, and an internal 3.6V 550mAh NiMH battery. The silicone rubber outer cover diffuses the light nicely.
Once you get the plastic shell out of the silicone rubber cover (it takes some work), remove the bottom by releasing the tabs on either side. Then split the shell by releasing the tabs at the top (circles), and prying lightly. There are two alignment posts on each side (arrows) that are a snug fit.
After a little circuit tracing, I had the schematic:
I should be able to take out the LEDs and place my circuit in the same spot in order to keep the battery/AC/rechargeable portion working.
I am planning 2 RGB LEDs (That’s all I have at the moment) behind the eyes, and a red LED behind the mouth. I’ll use a small AVR (maybe a 2313) and the MirfV2 with a ceramic chip antenna for the RF.
