Mathieu Stephan's CDM324 Doppler Backpack Is a Compact Speed

Self-described "electronics geek" Mathieu Stephan has designed a "backpack" for the ICStation CDM324 Doppler motion sensor — improving an earlier effort with extended range, integrated speed measurement calculation, and an on-board display.

"When sending an RF [radio-frequency] tone at a given frequency towards a moving target, the reflected signal's frequency will be shifted. This is the reason why a fire truck's siren has a higher pitch when the truck is going towards you than when it is still," Stephan explains of the project's operation. "What's nice with the CDM324 [module] is that it actually outputs that frequency shift, so you 'just' need to measure that frequency to compute the moving object's speed!"

Stephan had already designed a "backpack" board to make working with the CDM324 sensor easier, six years ago — but this newer variant offers a selection of improvements. Not least of these is an on-board twisted-nematic (TN) LCD display, allowing the device to show an object's relative speed in kilometers or miles per hour — selectable with a DIP switch — without the need for any external hardware bar a power source.

Other improvements in the board include the use of a fast Fourier transformer to conver the CDM324's output into said speed reading for display and a new automatic gain control (AGC) circuit which, Stephan says, considerably expands the detection range over the original design. A UART bus also provides access to the measured speed — though "it required adding a microcontroller to first digitize [the] analog signal," Stephan admits, referring to the redesign's on-board ST Microelectronics STM32F301.

"As there was still some board space available during the layout stage, I also broke out a full I2C and SPI bus to 2 expansion connectors," Stephan adds, "and that's on top of the existing UART bus for MCU flashing and the spare IOs [Inputs/Outputs]/reset/boot signals! I therefore fully hope to develop some expansion boards that would plug to that module.

The full project write-up is available on Stephan's website, with design files and source code, including a Python graphical user interface, published to GitHub under the permissive MIT license; fully-assembled boards can be ordered from Stephan's Tindie store at $29.50, plus $9.50 for an optional expansion board.