Hello Radio

Overview

Hello Radio is a roughly $10 wireless transceiver — small enough to sit beside a quarter — built at the MIT Center for Bits and Atoms (CBA). For about the price of a sandwich it reaches data rates up to 256 kbps and ranges up to 1 km, bridging the gap between throwaway hobby radio modules and far more expensive commercial transceivers.

What makes it interesting isn't just the price: the whole board is digitally fabricated in-house. The copper traces are milled on FR4 rather than sent out to a fab, so a working radio can go from design file to soldered, antenna-equipped board in an afternoon. The maker's marks are milled right into the copper — the MIT and CBA logos, and the initials of the three people who built it.

Technical Details

Hello Radio pairs two inexpensive parts:

• An Atmel ATmega168 8-bit microcontroller, clocked from a 10 MHz crystal, which runs the protocol and talks to a host computer over an FTDI serial header.

• A Microchip MRF49XA sub-GHz ISM-band transceiver that does the RF work, coupled through a small discrete matching network (a handful of 0603 inductors and capacitors) to an SMA edge connector for an external antenna.

A SOT-23 regulator powers the board, an AVR ISP header allows in-system programming, and two indicator LEDs plus a tactile switch round out the interface. The complete parts list is short — around thirty components, all in hand-solderable 1206/0603 packages drawn from CBA's fabrication libraries.

Fabrication

Because the board is milled rather than etched or ordered, the design ships as image files ready for a desktop PCB mill. The signal traces are cut with a 1/64″ end mill, and the through-holes and board outline with a 1/32″ end mill:

# Traces (1/64" end mill, 0.38 mm)
png_path hello_radio_016-top.png    hello_radio_016-top.path    1 0.38 4 0.5 0.5
# Holes and outline (1/32" end mill, 0.79 mm)
png_path hello_radio_016-cutout.png hello_radio_016-cutout.path 1 0.79 4 0.5 0.5

Build files

Hello Radio is open hardware. The complete design and firmware for revision 016 — assembled and documented by Shelby Doyle — are included here:

• Schematic — PDF · EAGLE .sch

• Board — PDF · EAGLE .brd · library

• Assembly diagram — PDF

• Bill of materials — text

• Firmware — source (C) · HAL header · Makefile · compiled .hex

• Bootloader — ATmega168 @ 8 MHz

Credits

Hello Radio was developed by Brian Mayton, David Cranor, and Rehmi Post at the MIT Center for Bits and Atoms. Revision 016 was assembled and documented by Shelby Doyle.