LED and PixelPulse

LED stands for "light-emitting diode," a fairly succinct explanation of what they are and what they do. Like all diodes, they let current (made up of electrons) go in one direction. Unlike most diodes, they convert some of the energy the electrons have into photons of light. This particular LED is a 5mm wide red LED, made by CREE. If you own a nice LED flashlight, it probably has a CREE diode inside. If you're particularlly curious, you can find the datasheet for this LED here.

Let's hook up the LED to your CEE and get a feel for exactly what it does and how to use it.

As LEDs only let current flow in one direction, a convention has developed for making sure they're installed correctly. The long lead of the LED is almost always positive, called the anode. The shorter lead is called the cathode. Bend the leads to form a 30 degree angle and then stick the Anode of the LED into channel A, and the shorter lead straight into ground, next to it. Bending the leads a bit will ensure that the LED makes a good electrical connection with the CEE's output header.

With your CEE plugged in, open a new tab in Chrome and pull up PixelPulse.

The basic interface of PixelPulse exposes the two channels of the CEE as two pairs of graphs. The top two graphs show channel A's voltage and current, respectively. Hit , and let's get rolling!

Channel A can either set voltage, set current, or be disabled. It starts out disabled. To set a constant, click on , and will appear. The panel meter will change to reflect the value, and the last ten seconds of activity will be shown on the graph.

Click the number "2" on the plot. Did the LED light up? Try dragging the circle that showed up. Look at how current changes compared to voltage.

Try the same thing with current - click a number (half way between 0 and 100?) and drag the circle that appears. Look at how voltage changes when you're driving current.

Let's look at this a little closer. Click . This will bring up another graph, showing the relationship between voltage and current in channel A.

Click on the voltage numberline and drag the slider from top to bottom a few times, slowly. Look at how the plot shapes up. Park the dot at two volts, and let's make the computer do the dragging for us.

The source panel exposes a ton of useful functionality in a small space. Click and select .

That dropdown menu lets you select from sinusoidal, triangular, or square waveforms. You can also adjust the frequency, center voltage, and amplitude.

With the voltage vs. current plot open, your screen should look like this:

The voltage vs. current plot should resemble the screenshot above, showing an the exponential relationship between voltage and current. This exponential relationship, determined by the behavior of the semiconductor junction, makes it difficult to drive LEDs with voltage. A small change in voltage, a mere few millivolts, can cause a change of dozens of milliamps, decreasing the efficiency of light output and increasing the risk of burning out the LED. This is why LEDs are commonly referred to as "current-driven devices" and usually used with either a constant-current supply similar to the set-current mode of the CEE or a "current limiting resistor" - a concept that will be discussed later on.

Christmas Light

Remove the LED from your CEE, and grab the christmas light.

Christmas lights are a small version of the traditional lightbulb, known as an incandescent bulb. Crude, but effective, they convert electrical energy to heat, bringing a small piece of tungsten to white hot.

Let's check it out.

Pull the plastic straight off the glass bulb and straighten out the leads. Insert the bulb into the CEE just like you did with the LED - note that as the christmas light is simply a fancy resistor, it has no polarity. You can plug it in either way.

Source just enough current for the bulb to start glowing, about 100mA, and then use that current as the offset for a low-amplitude square wave. 100mA offset, 30mA amplitude, and 0.3Hz is a good place to start.

Play with different frequenices and amplitudes and try to keep an eye on both pixelpulse and the lightbulb at the same time. Notice anything interesting?

As the glow fades, the filament cools down, and the resistance decreases. To keep a constant current, the CEE sources a lower voltage.

Tungsten is said to have a positive temperature coefficient - resistance increases with temperature. When you have a device like the CEE, you can "peek" at the temperature of the filament. This effect is used in hot wire anemometers, commonly used to monitor airplane velocity.