# Older blog entries for jkkroll (starting at number 6)

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 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:

Syndicated 2007-02-12 20:53:01 from robotguy.net/Blog

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

Syndicated 2007-02-12 08:02:05 from robotguy.net/Blog

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.

Syndicated 2007-02-11 23:46:14 from robotguy.net/Blog

Watson, come here. I want to see you!

I finally managed to send some data over the RF link. Somehow I managed to break the printf function, so I only saw the data on the scope as it clocked out of the transceiver on the SPI bus, but it was the correct data. Here’s a hint if you are using the NRF24L01 chip. Go to the Nordic website and make sure you have the latest datasheet. There is a step by step guide to the minimum config for enhanced shockburst mode. Once I had all of my generic SPI problems worked out, it just fell into place. Now to wrap it all up into a nice library and install it on a robot!

P.S. If the title didn’t make any sense, try here. Be sure to check out his notebook.

Syndicated 2007-02-09 16:58:00 from robotguy.net/Blog

20 Apr 2007 (updated 20 Apr 2007 at 00:50 UTC) »

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