Monday, August 10, 2009

Allen organ project - breadboarding the display terminal

As a first task toward making the card reader enhancement, I decided to start breadboarding the display terminal that will go on the outside of the organ to give me access to the alterable stops.

Breadboard with (from top) RBBB serial programming interface; RBBB for driving LED's; 12-key keypad; Arduino with LadyAda prototyping shield; LCD contrast pot and backlight components; 4x20 LCD.

Earlier posts in this series:

As a temporary mount, I soldered male headers on the keypad and display, and plugged them into a solderless breadboard. I then started wiring up the pins. First comes the keypad.

The two end pins on the keypad have no internal connections, and are not counted in the manufacturer's pin numbering. The remainin pins are:

Keypad PinFunctionArduino pin
1Column 26
2Row 05
3Column 14
4Row 319
5Column 018
6Row 217
7Row 116

You can see from the table that I wired these "around the horn",at one end of the Arduino, skipping over pins 0-1 (the serial port) and 2-3 (which support interrupts, not needed to run the keyboard but maybe needed for something else).

I also plugged in the display at this time. Walking down through the list of pins on the display and trying to decipher the East Asian-flavored English in the datasheets, I wired it up:

Display PinFunctionArduino pin
3Contrast adjustSee below
4Register Select13
10DB3See below
16LED-See below

The contrast adjustment went to the wiper of a 10k pot connected between +5V and ground. Initially, I grounded DB3, but we'll see in a subsequent post why that was wrong. Finally, there was a need for backlight control. You'll noticed I skipped over pin 11 in the table above - that's because I needed a pulse-width modulated output for the backlight. I connected pin 11 through a 1k resistor to the base of a 2N3904 transistor. The emitter went to ground, and the collector went to the LED cathode through a parallel pair of 12Ω resistors. (The data sheet showed 6.8Ω in the test circuit, but I didn't happen to have that size in the junk box.)

With suitable programming, this was enough to light up the display, read the keypad and put gibberish on the screen. The next post or two will get us from gibberish to "Hello, world!" and show how the keyboard is read and the display is managed.

To handle them, you'll probably want to download the ZIP archive of auxiliary files from this project's Fossil repository. You'll find the schematic in there under the name terminal/display.sch (Eagle format) and terminal/display.pdf (Adobe PDF).

Next post: reading from the keyboard.

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