LedyBug

[treg]

Quick description :

-A small, cheap (10€ target), PCB with a rgb led(s). Interaction via accelerometer (and maybe capacitive touch).
-Will be used as a way to learn playing with accelerometers
-For now, 3 different applications that I'd like to try :
*The LedyBug : The led behave like the led on a mac (breathing while it sleeps), and react to acceleration (wake up if shaked, turns red if free-falling, ...)
*Electronic dice : you shake it displays random number
*Light painting : change the color with acceleration

Current decisions :

-The MCU might be a pic18fxxK50 : rather cheap for something with USB included, and can do usb without external crystal (saves space and money). Maybe be fast enough to use the pwm for light painting.

Current questions :

-How to power the thing : 150mAh lipo designed for rc planes and a charge controler IC seems to be the best option, but it comes with a cost (3€ at least). Another problem is that swinging around the lipo battery (for light painting) won't be as safe as using coin cells (but 7 rgb leds won't be easy to power with coin cells).

I found a project which is suprisingly close to what I want to do, but does not have onboard leds and is not build around an accelerometer : http://rayshobby.net/?page_id=562#squarewear

treg.

[t0m]

Coin cells can easily handle peak currents of 30 mA which is fine for a single LED. Coin cells have internal resistance of about 15 ohms so the best way of using them to light LEDs is to put a low ESR capacitor (e.g. tantalum) in parallel with the cell and pulse the LEDs in sequence. 7 RGB LEDs would be a stretch because the duty cycle would be cut by a factor of 21.

http://www.ti.com/lit/wp/swra349/swra349.pdf

[treg]

For light painting, I'd need as much much continuous current as possible.

But maybe other applications can cope with lower intensity using several cells in parallel is good.

-> Would I need to put 21 capacitors after the 21 transistors, or just one before the transistors ?
-> You say that "Coin cells can easily handle peak currents of 30 mA" : What kind of cell coin are you speaking about ? I looked into several datasheet and they all gave nominal discharge current in the 0.03-0.1mA range, and did not give the max discharge current. (As I tryed to find a link to the datasheet, I just found some lipo CR2023 with 40mAh capacity, 3.6V and 15mA max discharge current, that may suit !)