Nuts and Volts - July 2010

the Propeller’s multi-processor capability. It consists of the “Debug LCD” object and the “servoEngine” object. Both objects are run in separate cogs (processors) and can be operated independently and/or simultaneously. The way the software works is that the program launches the objects and then checks for a button press. It then displays on the LCD “Lockdown in 10 seconds.” It counts down and if the button is not pressed in 10 seconds, the servo moves 90 degrees (for continuous rotation, this took a lot of trial and error). The box is then locked and the device must be reset to use the RFID card. To turn the device on and off, you (obviously) use the power switch on the front.

When the button is pressed, the LCD displays “Lockdown Cancelled” and the Propeller switches to the RFID scan loop. The RFID code is from a demo posted in the Propeller Object Exchange but has been modified to fit the needs of this project. The one notable part of the program is the small section in between a servo on and a servo off command:

Waitcnt ((clkfreq/20)*4 + cnt)

This line is for repositioning the servo after startup. Upon startup, the servo would give a little “twitch” caused from the sudden spike of the power supply. The servo didn’t turn more than two degrees, but it was enough to lock the box (locking because it was always twitching clockwise). This waiting period (also a matter of trial and error) was designed to twitch the servo back into its leave the negative intact. Strip and tin the wires. Separate the positive and negative wires, and pull them apart so they’re easier to work with. Connect the positive from the battery side to one of the contacts on the SPST switch. Solder the positive wire from the barrel connector to the other contact on the SPST switch. Plug the barrel connector into the board. For a permanent solution, solder the wires directly to the board. See Figure D and Figure E for a view of the connections.

■ FIGURE C.

BILL OF MATERIALS
QTY DESCRIPITION
1 Wooden box
1 Pistol lock and key
1 Servo
1 RFID reader (serial)
1 LCD 2x20 backlit
1 14” LCD extension cable
2 Three-pin header
1 Propeller protoboard (USB or not, doesn’t matter)
10 Three inch wires
1 Normally open pushbutton
1 10K resistor
1 SPDT switch
About 12” of wire
12” x 12” sheet of sheet metal
Hot glue
(Optional) a breadboard and four 10-pin female headers

RESOURCES

A board and pre-programmed microcontroller will be available soon at www.gadgetgangster.com.

The Arduino blog that inspired this: http://arduini ana.org/projects/the-reverse-geo-cache-puzzle.

For more information on RFID technology, check out these links: http://electronics.howstuff works.com/gadgets/high-tech-gadgets/ rfid.htm and http://en.wikipedia.org/wiki/ Radio-frequency_identification.