Nuts and Volts - July 2010

outputs, respectively. Each diode’s brightness is affected by changing the PWM duty cycle of the preferred channel. For each channel, the duty cycle can range from 0xFFFF (LED full off) to 0x0000 (LED full on), but is limited in firmware to a minimum of 0x4FFF to help limit LED current consumption. A single resistor, R6, serves as the current-limiting resistor for all three diodes within the package. By doing so, I could remove the need for two additional resistors (one resistor for each diode), but this means I can only have one color on at a time to achieve consistent brightness control. I used U1‘s Timer 1 to create an RGB LED multiplexing routine which would enable each individual LED in a cyclical fashion every 1 mS — fast enough to be unnoticeable to the human eye.

Microphone

MK1 is a Knowles Acoustics ( www.knowles.com) SPM0408LE5H SiSonic analog amplified surface-mount

Badge Type R1-R3 A2..A0 I2C Address All N/A 0 (Broadcast) Uber R1, R2, R3 000 1 Goon R1, R2 001 2 Contest R1, R3 010 3 Speaker R1 011 4 Vendor R2, R3 100 5 Press R2 101 6 Unused N/A 110 7 Human DNP 111 8 Table 1: I2C addresses of the different badge types.

MEMS microphone. Over one billion of their MEMS microphones — mainly used in high volume consumer products like mobile phones, laptops, digital cameras, and headsets — have been sold since their launch in 2003. Like D1, the package comes in a rear-mounting configuration, so it’s mounted on the back side of the PCB, and a small through hole is aligned with the microphone’s sound input port. The device footprint is very small, measuring approximately 4. 7 mm long x 3. 7 mm wide x 1.25 mm high, and is the only part on the badge that can’t be soldered by hand (there are no accessible pins; only solder pads underneath the package).

The microphone features adjustable amplification which is set with an external resistor (optional) and a capacitor. The DEFCON 17 badge sets MK1 for its maximum gain of 20 dB via C3. Internal amplification is especially useful, since I avoid having to externally pre-amplify the microphone signal before passing it to the microcontroller. The MC56F8006 does have two internal programmable gain amplifiers that I possibly could have used with a non-amplifying microphone, but since I was locking in the hardware design much earlier than completing the firmware, I didn’t want to commit to that. MK1’s OUT pin connects directly to ANB8 of U1 — a single 12-bit A/D channel. Since the microphone requires very little current to function (between 100 uA and 350 uA), I used a general-purpose I/O pin, GPIOC5, connected to the microphone’s VCC line to power the microphone directly. This let me completely shut off power to the microphone when it wasn’t in use, and then enable the microphone only when the badge was awake.

DEFCON 17 badge schematic.

Complete source code, schematics, audio, video, BOM, and other documentation for the DEFCON 17 badge is available on my website at www.grandideastudio.com/ portfolio/defcon-17-badge/. Additional supporting content and graphics are on the NV site at www.nutsvolts.com.

Joe Grand is an electrical engineer, hardware hacker, and president of Grand Idea Studio, Inc. ( www.grandideastudio .com), where he specializes in the invention, design, and licensing of consumer products and modules for electronics hobbyists. He can be reached at joe@grandideastudio.com.