Computer rendering of prototype board. Created with Altium Designer 6.9; components modeled in SolidWorks 2009
Many small projects seem like they would benefit from low-power, low-bandwidth wireless connectivity. Commercial modules such as the excellent XBee series of devices exist, but are relatively large, expensive, and seem better suited to tinkering with the technology than integration into a finished project. Single-chip RF transceiver solutions are small and inexpensive, but require a fabricated PCB for every design. My goal with this project is to develop an inexpensive and small RF transceiver module that is flexible enough to use in the prototyping stages of a project while not being so general and large that it’s wasteful to use in a finished work.
- Communications Protocol: 802.15.4, provided by the AT86RF230 transceiver chip and MAC implemented in firmware
- Microprocessor: ATmega168 or ATmega328 (same footprint, more memory)
- Antenna: 2.4 GHz board antenna design from TI application note
- Matching network: Integrated 2.4 GHz chip balun for Wi-Fi and Bluetooth applications
- Interfaces: RS232 and I2C exposed at board edge, SPI exposed in programming connector
- Additional I/O: 2 digital GPIO lines with PWM, 3 GPIO lines with analog inputs
- Dimensions: 1.25″ by 0.6″
Key Design Goals
- Low cost: $10-20 per board
- Hand-assemblable (minimal component count, low component density, and oversized pads for the 0402 components)
- Possible to integrate into an existing microcontroller-based system in addition to providing sufficient I/O and computing to drive external hardware, sensors, etc. (It’s basically a low-cost 8-bit wireless mote.)
- Small size
- Inexpensive board fabrication: 2 layers, 0.006″ minimum trace/spacing, 0.015″ minimum hole size
- Can be paired with an FTDI TTL RS-232-to-serial module ($15) to provide a computer-to-802.15.4 interface.
The boards are completed and functional, but I haven’t had a chance to properly document them here yet. For an example of the boards in use and firmware/software code, please refer to this project on my MAS 863.10 page.
Partially assembled prototype board.