Hacking an old radar gun to interface with a laptop

[Gregory Charvat] decided to see what he could do with this old Police radar gun. It is an X-band device that broadcasts continuous waves and measures the Doppler shift as they echo back. He cracked it open to see if he could interface the output with a computer.

After a little poking around he’s able to get it connected to a 12V feed from his bench supply, and to monitor the output with an oscilloscope. He established that it draws about 0.5A in current he built a companion board which uses AA batteries for power, and provides an audio output which can be plugged into his laptop’s audio-in jack. This technique makes reading the device as easy as recording some audio. From there a bit of simple signal processing lets him graph the incoming measurement.

In the video after the break you’ll see his inspection of the hardware. After making his alterations he takes it into the field, measuring several cars, a few birds, and himself jogging.

24 thoughts on “Hacking an old radar gun to interface with a laptop

  1. I would be careful; there have been some stories floating around about radar guns causing cancer. Maybe not true, but when running any type of RF device out of spec that emits wavelengths that can damage biological structures, well….don’t stick your head in front of it :P

    1. X-band (high microwave) is well below the energy levels of ionizing radiation. If it were larger, I would say to be worried about RF burns, but I doubt it’s producing output power anywhere near dangerous levels.

    2. Your cell phone transmits more power than a police radar gun. Those guns have a tiny oscillator diode that can only handle 10-100 milliwatts. You don’t need much power for doppler.

    3. It depends. If the oscillator is a tube (eg. a magnetron), then you need to be a bit careful as overvolting microwave tubes will cause them to emit X-Rays.

      1. no way this machine has a magnetron!

        X-band magnetrons are quite rare, especially for this application, and they require high voltage.

        this type of radar has a gunn diode, which is nothing but harmless.

  2. Where can you get a radar gun?

    If you are concerned about radiation effects, visit the ARRL for publications on the subject.

    1. Hot Wheels used to have a nice little 10 GHz radar gun. Check your local toy store or eBay.

      Also, older motion-activated door openers and alarms used a similar 10 GHz Gunnplexer.

  3. At least lower-power radar modules are available commercially relatively cheap. There’s a german company (http://www.innosent.de/) who sells the most basic module – the IPM-165 – for roughly 6 EUR a piece; more advanced modules (with integrated amplifiers, quadrature transceivers and FM) are more expensive (~50 EUR).

    1. this is not a reflectometer, no time is involved.
      it only uses doppler shifts to measure a speed map or what’s behind.
      Two objects moving at different speed require an FFT analysis to discriminate the two signals.

      the device sends ~10 GHz via its gunn diode, and the received signals are autodownconverted by mixing them with the sent signals, then detected using a diode. very simple.

  4. You can also operate the radar gun “in reverse.”

    By modulating the Gunn oscillator source with a audio tone equal to the Doppler shift of your target speed, you can “spoof” the readout on a police radar gun.

    1. yeah you can use a speaker or a piezo and a frequency generator in the audio range to make a doppler rader like this show a moving target or “speed”. It’s effectively doing the same thing as the set of tuning forks that come with radar units to confirm their operation and to calibrate them. Some people have tossed about the idea of mounting some very powerful piezo horn speakers, like you’d find in a sports arena or concert hall behind the grill of their car or some of the the “surface speakers” to make their car resonate at a frequency to fool police radar. Thats a bad idea though because police radars have an audio output so the cop can “listen to your speed” and a good traffic cop can tell by that frequency if your speeding or not, some can even tell you the clocked speed based on that tone all by itself. A lot of LEO’s turn the audio off because it’s annoying to listen to, but all it takes is one cop listening to his audio and it will be perfectly clear that something fishy is up! You would also need to dump a massive amount of audio power into your car to be able to pull that off at any kind of range. You’d probably be deaf before you got enough sound to cloak your ride against a CW radar like this. RF jamming a CW radar without some serious military hardware is pretty improbable as well. Getting the kind of power down range at the frequecies police radar (depending on the band) work at would involve microwave tubes that I can assure you – you cannot afford unless you have about $100k laying around.

      1. A magazine, Modern Electronics(?) back in the ’80’s(?) had an article where a radar detector was modified to detect oncoming radar and retransmit the jamming signal. Not $100K, but a bit more expensive than just the radar detector. But if you were caught, you could face a $15K fine or something like that.

  5. I have a few door sensors, and have played with the audio point. Is there any way to discriminate coming and going in this pulse stream? I would like to use as hand motion control.

    1. You need a setup with a quadrature (I an Q) output.

      Some Gunnplexers already do this, but you may have to make your own setup. It’s done by splitting the incoming RF in two via a 0 degree spitter then feeding two mixers with one of the LO signals fed via 90 degree phase shift. The IF outputs of the mixers are then in-phase and quadrature-phase.

      The phase shift between the two determines whether the target is incoming or outgoing.

    1. quoting: “”This charge has been made by several police officers and state troopers in suits against their departments and radar gun makers. As yet none have prevailed. “”

      Yes, some military personnel died from radar exposure. not not roadside radar! It was a kilowatt/megawatt radar as you can see in airports and war ships! The death can occur within seconds.

      But…

      The device described in this article is using a GUNN diode, which barely reaches a few milliwatts.
      Given the fact that radars radiation is non-ionizing, you can assume that they have the same effect as microwave as used in ovens.
      A microwave can “kill” (e.g. a kitten, IIRC, some microwave oven manufacturer was sued because of that), because it radiates some kilowatts of radio power.
      A wifi router cannot kill, because it radiates some milliwatts.
      Both are using C-Band radiation (2.45 GHz).

      See also Kaj’s answer.

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