I am writing this Instructable because Bot1398 used an Arduino and I've never used them. I have nothing against them, however, when I started using microcontrollers about 3 years ago I was looking for speed and power for a specific application. I settled on the PIC Microcontroller series by Microchip. It is amazing what you can do with microcontrollers, you can let your imagination run wild and with a little creative programming achieve exactly what you want. There are a number of programming languages out there as well but when I started I wanted speed and to know exactly what was going on at each and every step, therefore, I chose Assembly Language since it seemed a lot like BASIC that I learned on the IBM PC Jr. back in the 1980's when I was growing up. That is what this Instructable entails: PIC Microcontrollers and Assembly Language to use an LED (Light Emitting Diode) to detect the amount of ambient light and display the numerical measure of this light along with a bargraph on an LCD screen.
Why This Works
When we attach an LED to a Microcontroller we are usually doing that to light it up. In this case, however, we are actually applying a positive voltage to the negative end of the electrode. This is so we can take advantage of something called parasitic capacitance. Applying this positive voltage will build up a small charge (resources on the internet say within about 100-200 nanoseconds) within this parasitic capacitance of the LED that we are using. The actual capacitance is not very important but online resouces say that it is around 10-15 pF. We will then switch the pin of the microcontroller attached to the negative end of the electrode from an OUTPUT to an INPUT and wait for the charge to drain. When charge drains enough the pin will now read a Logic Level 0 or Low state where we will turn off the timer and use this value to calculate the moving average and display the results on the screen. We will then take more readings and display them on the screen at the interval that is written in the assembly code. In the finished code, I have it set to take one reading about every 20 or so microseconds. This occurs 50 times so that an updated result is displayed on the screen approximately every second or so. The time it takes to get a reading will vary a little bit because it takes longer for the charge to drain in a dark environment and this occurs faster in light environments. The discharge rate of the LED capacitance is somehow directly related to the number of photons or amount of light striking the LED light generating elements. Neat isn't it. :)
Now that we know how it works, let's move on to Step #1.
Step 1: Gather Components and Assemble the Circuit
PIC Microcontroller (I used the 18F4550)
LCD Screen (2x20 or 2x16)
Now you should be able to follow the schematic to assemble the circuit on a breadboard or any other way that you like to put your circuits together. This one is fairly easy and has minimal parts. I did attach two different schematics, the one has the light measuring LED connected to two different microcontroller pins and the other one has this LED with the cathode (or negative end) connected to a microcontroller pin and the anode (or positive end) of the LED connected to ground. This seems backwards but, remeber, we are not using the LED to produce light, but to measure it instead. That is why it is connected in this way. In my pictures you will see that the LED is connected to two pins of the microcontroller. This is because I wanted to be able to use the LED to produce light too, in order to check to make sure it works when the program is started.
If you do choose to connect the LED to 2 pins of the microcontroller you will need to keep this in mind for it to work. I could not find this documented anywhere so it took a little experimentation to get it to work. In order to be able to light up the LED both microcontroller pins will need to be set as OUTPUTS. Then, to make the LED shine, you set the anode (or positive end) of the LED high so that current flows through the LED to produce light. At first I thought that I would need to set the cathode (or negative end) of the LED as an INPUT but this did not work.
Now that you have the componenets, put them together on the breadboard by following the schematic you want to follow. When assembled, it should look somewhat similar to the pictures below. I am using a 5 Volt Regulator to supply 5 volts to power the circuit. I use a 'wall wart' that supplies about 9 volts to the regulator.
The LCD screen that I am using is a Newhaven Display NHD‐C0220AZ‐FSW‐FTW COG (Chip‐on‐Glass) Liquid Crystal Display Module. I used to use the HD44780 LCD displays that many other people use. You can definiately use the HD44780 displays without any problem and there is a lot more source code out there to make them run. The Newhaven display had no source code but sitting down with the data sheet and using the HD44780 source code, I came up with the code to make it work perfectly. I like the display for a few reason. The contrast is very easy to set and so far I have not had to change it at all during the different seasons, it always remains clear and very easy to read. It also has a smaller footprint and was exactly what I needed for a huge project that I am working on where available space is at a premium. It is smaller and than the HD44780 displays and still displays 20 characters on each of the two lines. Finally, it is less expensive than the HD44780 displays as well. I picked a couple of these up from Digikey and at only a price of $10.25 each. Check them out here: Newhaven LCD Display at Digikey. Keep in mind that this display requires from 3.3 to 5.5 volts so keep that in mind when choosing your power source. The source code that is supplied should also run the HD44780 displays without any problems.
This is just the way that I have it set up. You can place the components to diffent pins of your microcontroller. Just keep in mind which pins of the microcontroller you connect to the positive and negative end of the light sensing LED. You can also wire up your LCD screen to operate in 4-bit mode instead of 8-bit mode as illustrated. That would require four less connections in the circuit but makes your programming just slightly more complicated.
On to next steps...Writing the Program.
Step 2: Programming - Where All the Really Hard Work Comes In...
I also have a PIC 16F917 microcontroller available so I will attempt to modify the code for the 16F series of chips available as well. (IN PROGRESS)
The code is also written for a clock speed of 8 MHz. You would have to modify the code to obtain the correct delay times if you use any other clock speed (i.e. 20 MHz). I also took advantage of using the microcontroller's internal oscillator as the clock source too. The schematic shows an external oscillator or crystal for those of you who would rather use an external clock source. If you need to change the Delay Subroutines you can find an extremely useful webpage to produce the code that you need. That is at: Microchip PIC, ASM Delay Code Generator.
What I will do is display some of the main sections of the code that is essentially what makes the LED take the measurements, etc. The source code that I have supplied has been heavily commented so that if you have any PIC Microcontroller experience at all you should be able to follow what I was doing when writing the code. Feel free to download the code and follow along. There are two versions here. The first version is programmed to use a 20x2 character LCD display, the second uses a 16x2 character display instead.
That is pretty much it. Next, let's program the microcontroller to do our bidding...
Step 3: Program the Microcontroller
That is pretty much it. I hope you enjoyed the LED LYT METER and my take on detecting light using PIC Microcontrollers using Assembly language programming and a Moving Average code to display the readings on an LCD display. Feel free to use the code in your own projects and drop a note and maybe even a picture if you do decide that you think it is worth something to use. You could use it to turn LED accent lighting on an off depending on how dark or light it is, monitor light over a 24-hour period and graph that from day to day and month to month, or whatever else you might imagine. Have fun with it and I'll include some more code over the next few days to include the PIC 16F Microcontrollers as well.
You can find the program code attached to this page as well. Experiment with different LEDs and have fun with it, that's all I ask. This is a fun hobby and so much more.