Arduino draws a circle on an LED Matrix that moves around according to readings from a 2D Accelerometer.

Objectives:

* Learn how to draw a circle using simple Maths

* Learn how to use the LOL shield for the Arduino

* Learn how an accelerometer works

**Signing Up**

## Step 1: What you need

* Arduino UNO

* LOL shield (http://jimmieprodgers.com/kits/lolshield/)

* Accelerometer (http://www.oomlout.co.uk/accelerometer-3axis-adxl335-p-247.html)

* Breadboard for wiring up e.g. (http://www.oomlout.co.uk/prototyping-bundle-for-arduino-ardp-p-186.html)

* Some Wires

## Step 2: Wire it up

Wire the Accelerometer as follows:

+3V

GND

X => A0

Y => A1

Z => N/C (Not Connected)

Note: The LOL shield does not come with Headers (as shown in the picture), attached to the topside.

I bought some here: http://www.amazon.co.uk/gp/product/B004RASBVY/ref=oh_details_o06_s00_i00

## Step 3: Install the LOL Shield Libraries

You can get the link to them from this page:

http://jimmieprodgers.com/kits/lolshield/

## Step 4: Let's Start With a Circle

This circle will eventually move around as we tilt the device. But first things first....

We'll illuminate individual pixels on the LOL shield using the LOL library command:

*LedSign::Set(x, y , 1);*where x and y are the coordinates on the shield of the LED we wish to illuminate

Our job is to figure out how to calculate the x,y values that make a circle.

In Pseudo code we do it like this to draw just one quarter (Quadrant) of the circle:

**for x = 0 to RADIUS**

y=f(x); // This means y is some function of x. We haven't said what function yet

plot(x,y)

endy=f(x); // This means y is some function of x. We haven't said what function yet

plot(x,y)

end

So what's the function?

**r^2 = x^2 + y^2**where x^2 means "x squared" or "x to the power of 2"

We know

**because it's the for loop iterator;**

*x*We know

**because it's the radius,**

*r*So we re-arrange to find

*y***// sqrt() means square-root**

*y = sqrt(r^2 - x^2)*When you code it up you get a circle as shown in the picture.