## [StabiloRose] Placing the LEDs

After many tries, with “real” strips or with paper strips, we have a solution \o/

Near to the equator, the strips will be placed according to the parallels. Near to the poles, where this placement is no longer possible, we will place them in parallel to a meridian (see picture below). To connect these different sections of strips, we will make holes to solder wires, which will be connected together inside the ball.

Near to the poles the LEDs will be in staggered rows. The white strips come from an older project. Ours have an higher density, as you can see on the corner in the second picture. Therefore, you have to imagine that there is another LED on each connector.

## [migB] RGB LED Strips returns

We finally got our LEDs allocation over migB. We are going to use the same principle as explained in a previous post (article and video). We will also put one RGB LED on each pole to reduce the obscurity. The main issue with this allocation is the conversion of coordinates. With aligned LEDs the conversion is quite easy. Spherical coordinates can be easily projected into a Cartesian system which will govern our strips.

We also thought about a spiral wrapped around migB or parallel circles mapping a sphere. However it is very difficult to manage coordinates.We thought about a mapping matrix to deal with led position and solid angles, but it requires a calibration and a high similitude in the way the strips are mounted in the sphere. More difficult, imagine you would like to color the LED which point out a certain direction. It becomes really hard. We chose simplicity over homogeneity. This spacial allocation will also strongly help the understanding of the displayed symbols by the player.

The numbers of LEDs will be between 50 and 100 for each migB according to the final size, in order to keep the price and consumption reasonable.

## [migB] RGB LED strips

Today, my partner and I worked on leds for migB. We had to find a way to connect properly about a hundred of leds. The precise number is not defined yet. As we say, sleep on it. I hope we’ll have clearer ideas about that tomorrow. Two challenges here : get the leds controlled by the micro-controller and find an intelligent way to allocate them on the sphere.

The first one was quite easy (Thank you Alexis ! ). We are very likely going to use RGB led strips. These devices are really amazing. You may cut them, extend them by soldering and you control them with at most 2 wires. It facilitates our implementation. There will be less wires inside the balls, less solder and less pins used on the micro-controller. No need to use a driver either. So, is everything perfect now ? Not perfect, but it could have been worse. Actually, we can’t use multiplexing with these strips. Except with the DotStar series from Adafruit, but we don’t have precise values yet. RGB led strips act like shift registers. You push the configuration of the farthest led, then the configuration of the previous one, of the previous one and so on. The longer your strip is, the lower the refresh rate is. This model permits to keep the configuration even if the bus is disconnected. Unfortunately, as there is no multiplexing, all leds are powered on all the time (with a PWM modulation, but you get the idea). Therefore the consumption can be quite high. 7A @ 5V with the 144 leds NeoPixel strip from Adafruit ! This consumption is obtained when all leds are white and the brightness is maximal. As we will probably never meet this configuration, the battery is going to last more than 1 hour. With less leds used simultaneously and a reduced brightness, we will limit the consumption.

The allocation of the leds on the sphere is not easy. We would like to have and homogeneous distribution and aligned leds. That’s impossible. We found a led ball made of PCBs (and a video). Here, the alignment is privileged. It is good to display images or symbols, but the poles are in darkness. Not funny. It is difficult to find a projection which keeps the aligment. We have to make more tests.

To be continued…