Perfect as a remote replacement for access control and for switching on/off burglar alarms, it emits an RF code every time you digit the correct combination on its keyboard. Three different combinations for three different actions can be entered at program-time. The keyboard layout is configurable, too: you can use any matrix up to 4 rows by 4 columns, assigning your own ASCII codes to each key.
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.
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.
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