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RF pad (doubles as serial-pad)

combination-controlled, fully customizable radio frequency remote control

The circuit (full schematic here)

I don't like to reinvent the wheel, but I was surprised by the number of components involved in the "Wake up on keypress" application note from the Atmel's web site. You know, I like simplicity, so I dropped off almost everything.

• With appropriate use of the internal pullups, only two diodes (D1 and D2) are necessary to excite the wake-up interrupt with a 4x4 keyboard. Each time you press a key, the chip wakes up, the buzzer bleeps, the key is decoded, and if it completes a valid combination the corresponding code is transmitted.
• The transmitter is a 433.92 MHz hybrid module by Mipot: it is specified for a 5 volt power supply and consumes almost nothing when no data is applied. It can be replaced with similar modules according to your country's frequencies (e.g. 300 Mhz).
• Pin 13 drives a 5 volt buzzer directly. Use a buzzer drawing not more than 10 mA
• The oscillator is made from a 4MHz, three pin ceramic resonator: at wake-up, it restarts faster than a crystal based one
• The reset is of the R-C type: being powered on once every two years, this is perfectly adequate, and there is no eeprom data that could be corrupt by improper reset.

The software

RF pad is written in GNU C. The good news is that this is is an excellent free compiler, ANSI compliant, with legendary portability between many platforms. The bad news is that it is difficult to set up and master at first time (this may be due to the fact it is developed by very clever people). If you have never seen a command-line compiler before and you are unfamiliar with commands like make and compiler flags be prepared for an hard work.

Fortunately, you don't need the compiler to make an RF-pad, and you can customize your RF pad without recompiling the code, just editing a few bytes at program time.

The prototype

I built my prototype from a perf board (veroboard). With so few components involved, a PCB is not really necessary. The board is big enough to host the battery, put in place with biadhesive tape. On the solder side, a 10 cm. long wire forms the antenna. The keyboard is a surplus one from a telephone, using only 12 of the 16 keys theoretically available. The big black buzzer is a 9 volt type instead of 5 volt, but it works as well at lower voltages. The AVR micro is placed on a socket: it could be reprogrammed in-system, but in practice I found faster to remove it and place into the programmer instead of wiring an appropriate connector. The whole circuit is housed in a nice plastic console: the hard part is cutting the keyboard hole. It needs a good shaw, a set of files, a firm vice, and lots, lots, lots of patience to get a clean result.

my prototype, opened

Files available:

Download or view on-line the RF pad schematic (28kB .GIF file)

Download the RF pad sources, executables, schematic in one .ZIP file (53 kB).

Customizing RF pad

Customizing the key matrix

The key matrix of my prototype comes from a surplus telephone and it is like this:

1 2 3
4 5 6
7 8 9
 * 0 #

It's a 4 rows by 3 columns keybaord, so it uses only 12 of the 16 keys handled by RF pad.
However, the code is pre-programmed for a layout like this:

1 2 3 A
4 5 6 B
7 8 9 C
 * 0 # D

You can choose any layout you wish as long as it fits in a 4 by 4 matrix. You can simply omit any key without changing the software. You can assign any ASCII code to your keys based on key's position. The only ASCII code you can't assign is NULL, that evaluates to zero.

If you have the GNU C compiler you modify the keyboard matrix initialization:

//keyboard ASCII matrix, replace your own characters here
//DON'T USE 0x00 as it breaks string compare
const unsigned char keys[MAX_ROWS][MAX_COLUMNS] =
{ {'1', '2', '3', 'A' },
  {'4', '5', '6', 'B' },
  {'7', '8', '9', 'C' },
  {'*', '0', '#', 'D' } };

The following code changes the keyboard from "telephone" (with 1 at top-left) to "calculator" layout(with 7 top-left).

const unsigned char keys[MAX_ROWS][MAX_COLUMNS] =
{ {'7', '8', '9', 'A' },
  {'4', '5', '6', 'B' },
  {'1', '2', '3', 'C' },
  {'*', '0', '#', 'D' } };

(Note that you must initialize all 16 keys even if you don't use them all.)

If you don't have the compiler (or you don't want to recompile) then modify the executable prior to programming the chip. The following example shows how to do the same modifications with Pony Prog.

1. Select the AT90S2313 and load the rf_pad.rom file. Scroll the hex window until you see the keyboard matrix (It is located near the end of the program, address is $34E). Here I highlighted it in yellow:
   
   
   
2. Select "Edit buffer enabled" from the "Edit" menu. This allows you to change the hex or alpha codes by clicking them.
   
   
3. Change the codes to get the following:
   

Changing combinations and RF codes

If you have the GNU C compiler, it is simply a matter of changing some #defines:

//replace here your own ASCII key combination / 12 bit rf codes pairs
//very important: the combination must be the same length as
//the key_combination[] (see below)
#define COMBINATION_1 "111111"
#define RF_CODE_1 0xAAA    //will be AA 0A in code dump
#define COMBINATION_2 "222222"
#define RF_CODE_2 0xBBB    //will be BB 0B in code dump
#define COMBINATION_3 "333333"
#define RF_CODE_3 0xCCC    //will be CC 0C in code dump

 

If you don't have the compiler, you can "hack" the executable as explained above:

• The codes are 12 bits long and default to $AAA, $BBB and $CCC respectively.
  You can find them on the hex dump (left half screen) starting at address $348.
  Note that the codes are stored with low byte first, so e.g. $AAA becomes AA-0A, $427 becomes 27-04, $15B becomes 5B-01 etc.
  
  
• The three combinations default to 111111, 222222 and 333333 respectively.
  They are stored in ASCII form, so it's easier to modify them in ASCII clicking on the right pane. Combinations must be 6 keypresses long.

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