Today, I’m gonna show you how to unlock a hidden feature of your Nerdkits LCD screen. Well, officially we don't include a back light. I’m goanna show you how you can use a little bit of analog circuitry plus the digital power of the microcontroller to back light the LCD you already have included in the USB Nerdkit. There are the two major types of back lights for LCDs. One is LEDs, which stands for Light Emitting Diode and the other EL, which stands for Electro Luminescent. EL back lights tend to be more efficient and have more uniform lighting than LED ones. But they require some tricky circuit to drive.
If you heard about Organic LED displays or OLEDs, that's basically an array of electro luminescent strips struck down to the size of individual pixels. So far, every kit we’ve shipped and continue to shift includes an LCD with an electro luminescent back light. This might change in future, but for now you all set. Electro Luminescent back lights are hard to drive because they require 100V A/C at a 150 HZ. All we have on board is the 5V to power your micro controller, but we can generate the high voltages by using an inductor.
An inductor is just a coil which that energy in a magnetic field and it tries to keep the current running throw it steady. But if we suddenly stop any current from running by opening the switch, the voltage across the inductor will raise very quickly to try to maintain the flow. We can capture that voltage by using a diode and store it in a capacitor. That's basically how our circuit works. By pulsing the transistor Q1 on the left very quickly, we build up a big voltage across the capacitor C1. Then, pulsing transistor Q2 on the right much more slowly, we periodically let this voltage be applied across the electro luminescent panel or not. Just a quick safety warning, that voltage spike can get very big. In our circuit, we are talking about a couple hundred volts. Definitely avoid touching it and protect yourself and your computer. Know where the high voltage nodes are and stay away.
And here is the demo of the working circuit. One nice thing about doing with micro controller is that you can actually fully control the brightness in software. Here, I’m changing the frequency that we are driving the Q1 using the USB connection of the Nerdkit. Now we’ll pan over the LCD. Well, I’m changing the frequency and you can see that I basically have nice smooth control of the brightness. Now, let’s turn off the lights and the back light feature is really especially useful when it’s dark.
So, hopefully we’ll have some videos showing cool applications of this functionality in coming months. Hopefully that was a fun first taste of analog powered electronics. For the source code, schematics, parts list and more detailed instructions, take a look at our website www.nerdkits.com and if you have more ideas for the videos like this one, send them in our feed back form.