Mattel makes a real radar gun, on the cheap

Able to measure scaled miniature Hot Wheels speeds along with real-world speeds (mi/hr or km/hr) at the flick of a switch, the radar gun uses an impressive but inexpensive component set to create a true Doppler radar measurement device. I had a couple of my kids put it to the test, and the results were close to indicated speedometer speeds. Not bad!

The gun's case is a two-piece shell enclosure, and the screws holding it together have formidable cover plugs. After a small injury, a little sweat and some frustrated muttering, I got at the gun's internals: an emitting/receiving microwave assembly and a separate board holding all control, signal-processing and user interface circuitry, Figure 2.

(Click to Enlarge Image)

The Doppler principle employed is straightforward. An emitted microwave signal is bounced off the object whose speed is to be measured, and the frequency-shifted return signal is mixed with the transmitted signal to produce a difference frequency proportional to relative velocity. With the 10.5-GHz microwave signals used in Mattel's gun, the mixing product provides a low-frequency difference signal used as a proxy for speed; the 21-GHz doubled frequency output from the mixer does not appear to be used in any way.

Starting at the emitting element, an inexpensive soldered metal tube waveguide focuses a pulsed microwave signal generated from a dielectric resonant oscillator (DRO) and launched from a patch antenna. The same waveguide and a second set of patch antennas pick up the return pulses. Others curious about the HWRG operation (see www.austech.info/showthread.php?t=135840 &page=1) have measured an 8-kHz, 10 percent-duty-cycle, 4.75-volt signal hitting the DRO.

The generated microwave bursts are sent out the antenna/waveguide and simultaneously over to an input on the mixer, whose other input is the return (shifted) pulse signal. As with the antenna elements, signal traces on the microwave board are carefully designed RF distributed structures, providing inexpensive, etch-based filters and impedance-matching networks.

An Atmel ATMega88 provides almost all logic and processing for the HWRG, monitoring switch inputs, implementing the LCD interface and processing the mixed-down return signals for translation to a speed readout. An STMicroelectronics MC33078 dual op amp presumably provides front-end amplification of the mixer's output before going to the Atmel part, which has memory and an A/D converter on board.

The HWRG retails for $30 and likely costs $10 to $15 to manufacture. It lacks the power and precision of police radar, but it's based on the same physics and analog-design principles as its industrial-grade counterparts.

About the author
David Carey is president of Portelligent ( www.teardown.com). The Austin, Texas, company produces teardown reports and related industry research on wireless, mobile and personal electronics.