In many cases you want to add some input interface to the project, the most simple is to use some push-buttons but what if you need to set some values, some of you might say:  you can do it with up-down, left-right, select button and some clever menu. Yes that’s about right but just imagine the software if you want to set values from 0-9999, or applications like access control, counters.

The most commonly known is the 12 (3×4) type which has off course 12 buttons noted from 0-9 and *, # for extra features. Well so far there isn’t anything clever about it, 12 buttons which need 12 input pins, wrong it needs just 7, the push buttons are organized in rows and columns, each row shares the same horizontal connection, while each column shares the same vertical connection. For a better understanding look at the schematic:

Because of this matrix connection the reading of active button(which is pushed) isn’t so straightforward, each column has to be processed sequentially.

The resistors are external components, there are not included into the keypad but certainly needed, R1-R4 are the pull-up resistor, in the demo application the atmega88′s internal pull-ups are used, R5-R7 are for current limiting.

A few words about the firmware, the keyboard is connected to PORTB, PB0-PB3 are configured as inputs with the internal pull-up active, these will read which row is active, PB4-PB6 are configured as outputs and serve for column selection, their default value is logic ’1′ the same as the row pins default value. The reading is made sequentially, the first column is selected by setting PB4(COL1) to logic ’0′ and the pins PB0-PB3 are red, if no key is pressed the pins remain in their default state: logic ’1′ else their value is logic’0′ we must go trough these steps for each column, once a pressed key is found its space thus value is determined, the function restores the corresponding column select pins default state and exits, this way speeding up the process.

Lets take an example: key 4 is pressed, column 1 is selected by setting PB4 to logic ’0′, when we read PB0-PB3 PB1 will be logic ’0′, since we know which column is active (it would be really funny otherwise since we wrote the firmware) the exact location of the pressed key is determined. Now we have 2 coordinates x(0..2),y(0..3)  x=1 is the row, y=0 is the column the most simple way to turn these coordinates into a value is using a 2 dimension look-up table:

const unsigned char KeyMap[3][4] = {{1,4,7,ESC},{2,5,8,0},{3,6,9,ENTER}}; // values ESC and ENTER are defined by macros

By simply addressing the table with the coordinates we have the corresponding value of the pressed key  value = KeyMap[x][y]; the look-up table can also hold any data of choice like ASCII, or can have other type definition as long as its dimension matches the keypads dimension [x][y].

Since the keypad or any other human machine interface tends to be asynchronous, which means that at any moment the firmware should be able to sense and to handle the input, the entire process or in other words the scanning must be executed periodically. This can be done inside some timer interrupt routine or just by adding some delays to the main cycle, if possible avoid using the mains power frequency or any multiple of this for scanning.

Also between the column selection and the effective pin reading some delay must be inserted, this allows time for the electrical state of the wires, traces to stabilize. After one pressed key is sensed the function exits, thus multiple pressed key will not be recorded although the modification needed to the firmware is quite simple, so consider this a challenge.

I attached the entire AVR Studio project, it is written in C, using the winavr package, it will be easy to integrate into any project. In the near future I will explain how to save I/O pins by sharing the row pins with other functions like driving the 7 segment display.

Matrix keyboard explained: [download]

These days most pharmacies use LED pharmacy crosses posted at their entrance to let people know there is a pharmacy there. The reasons are obvious, they look cool&hi-tech, they can be seen from distance and they can be customized really easy (well, easy customizing pretty much depends on how the manufacturer approaches things).

If you try to search the web about schematics or example codes for this kind of circuit you wont find any, and again i think the reason is obvious, the crosses are quite expensive and so is the profit for the manufacturer. So nobody is gonna post schematics for such a project, unless you’re a hobbyist and you’re having fun with electronics.

The project consists of one ATmega64, three ULN2003 and five 5×7 LED matrix from Kingbright(TA20-11EWA). I had the idea to build something like this but so far i haven’t had the time nor the knowledge to get it done. So i asked Morgoth if he would like to participate in the project. I sent him the LED’s, the drivers and the PCB and he started working. As you can see not many parts are involved , but the secret lies in the microcontroller, it’s the programming that does the job.

In the next pictures you can see the microcontroller board with the ULN2003 darlington arrays:

The circuit was designed to receive messages trough serial interface from a computer and than display them. Morgoth also designed a custom terminal for windows which provides easy access to the display.

In this test phase a serial interface by wire was used to transmit data between the terminal and the ATmega64, but a wireless or bluetooth module could be integrated with no problem.

Also as a note, the LED’s don’t light up really bright, for that to happen you need to use drivers on the positive rail. This also applies if you’re planning to take the project to another level and use bigger LED’s

And now, watch some videos with the LED Pharmacy Cross beeing controlled from the computer:

• LED Pharmacy Cross ATmega source code
• LED Pharmacy Cross terminal source code

The author, Olivier de Broqueville designed this circuit for hi’s son who was dreaming of a small tool able to write symbols or pictures on a screen. The solution adopted was to pilot a matrix of Leds. This way, with only some cheap transistors, common red Leds,  and a 16F628 , the dream could become reality.

Olivier also made a VB application that you can use to design patterns and export them apropriately.

Fancy LED matrix: [Link]

The project uses (2) 5×7 LED Displays, so 10 columns and 7 rows for a total of 70 LEDs.  The matrix works on the basis of scanning the display very quickly. The 4017 chip is used to switch between the columns. The way the 4017 chip works, is that when a signal is sent to a certain pin, 1 of 10 of the other pins is sequentially activated. The way the software works, is it tells the chip to switch columns, which sets one column high.

The software then makes the parallel port turn on the individual LEDs in that column needed. This is done very quickly, many times a second. It’s a fairly easy concept and it turned out the programming was easier than the author had initially anticipated.

And the great thing about this project is that it also comes with a font writer, which is basically a software written also by the author of the circuit to help him quickly design animations.

Computer Controlled LED Matrix: [Link]

Another useful application for a LED dot matrix display, well useful for those who use twitter. The purpose of the project is simple, it scrolls your twitters on the led matrix, this way you don’t have to be in front of your computer to see what’s new. Source codes and info about building it is provided by the author. The matrix is controlled by an Arduino

Physical LED Twitter Scrolls Your Twitters: [Link]

This project’s display is made of a number of tiles, about 2″ square with an 8 x 8 array of color LED pixels. Each tile is individually powered and animated, so your can freely pick them up and re-arrange them. To set up a display, the tiles are placed in a special tray. Animations are downloaded into the tray via Ethernet and stored locally on an EEPROM, or loaded via an SD card. The tray broadcasts the animation to each of the tiles, and then synchronizes them.

If the pieces are left in the tray, the animation can be updated continuously over the Ethernet connection. If the tiles are removed from the tray, they’ll display the animation for several hours with their own re-chargeable battery power. Once the animation is synchronized and running on the tiles, you can pick them up and place them anywhere.

Dynamic Tiled display: [Link] – [Via]

The MAX6953 from Dallas Maxim is a compact cathode-row display driver that interfaces a microprocessor (like PIC or AVR) to four 5×7 dot matrix LED display trough an I2C compatible serial interface. The Max chip includes some nice features like:

• low-power shutdown mode
• segment blinking
• test mode that forces all LEDs on
• 16-Step digital brightness control

This project can be considered both a led matrix controller and a development board for MAX6953. The project presents only the max6953 and the led matrix, if you want it to display anything, you’ll also need to stick a microcontroller in there and write some code. You might want to check this other article 50×7 LED Matrix which has everything included.

MAX6953 LED Matrix or development board: [Link]

The most significant part used in this project is Texas Instruments’ TLC5940 LED PWM driver. It uses a clocked serial data input, and provides 12 bits of PWM resolution on each of 16 outputs.

Normally, the TLC5940 would require one output for each LED element. Since there are three elements per RGB LED, this would add up very quickly; the planned array would require 3 * 24 or 72 TLC5940 chips.

16×24 RGB LED Matrix: [Via] – [Link]

Wow, this is the biggest LED matrix display i ever seen made in a DIY project. Seven 74ABT574s provide the row drive and 10 column transistors with the whole display refreshed 100 times a second. If you consider starting this project, you have to have some experience, its not suitable for beginners because of it’s complexity and lack of information about building it

560 LED matrix display: [Via] - [Link]

The 8×8 LED matrix is a building block. There is no processor or circuitry other then then LEDs and the copper traces. It is simply an eight by eight 64-LED matrix on a PCB. The 8×8 has 16 pins on one edge, 8 connect to the rows and the other 8 to the columns. This allows the maker to use their preference of controlling circuitry.

The way a matrix works can be a bit mysterious at first. Of course building one yourself is the best way to learn

[VIA] – [LINK]