The Right Circuit Turns Doppler Module Into A Sensor

Can you buy a working radar module for $12? As it turns out, you can. But can you make it output useful information? According to [Mathieu], the answer is also yes, but only if you ignore the datasheet circuit and build this amplification circuit for your dirt cheap Doppler module.

The module in question is a CDM324 24-GHz board that’s currently listing for $12 on Amazon. It’s the K-band cousin of the X-band HB100 used by [Mathieu] in a project we covered a few years back, but thanks to the shorter wavelength the module is much smaller — just an inch square. [Mathieu] discovered that the new module suffered from the same misleading amplifier circuit in the datasheet. After making some adjustments, a two-stage amp was designed and executed on a board that piggybacks on the module with a 3D-printed bracket.

Frequency output is proportional to the velocity of the detected object; the maximum speed for the sensor is only 14.5 mph (22.7 km/h), so don’t expect to be tracking anything too fast. Nevertheless, this could be a handy sensor, and it’s definitely a solid lesson in design. Still, if your tastes run more toward using this module on the 1.25-cm ham band, have a look at this HB100-based 3-cm band radio.

22 thoughts on “The Right Circuit Turns Doppler Module Into A Sensor

  1. I didn’t think GBW product was calculated over the whole circuit, just per op-amp? Therefore I would have thought with a gain of 125.5 and an upper cutoff frequency of 999Hz this would yield a minimum GBW product of 125KHz -> a 1MHz opamp would have done just fine, no need for 50MHz!!

    1. You can buy those, they just cost a lot more.
      More or less all of the cheap sensors use circuitboard traces as the resonator (CDM324 and all the “1.3GHz”, “2.4GHz” and “5.8GHz” doppler motion sensors on ebay). So the frequency is fixed.

      You could add a varactor in there to make it tunable.

      1. I want to measure liquid levels in large metal fuel tanks and I wondered if this sensor could be used to measure RF absorption and calculate from there.

        Do you know of a sensor that might be suitable.

  2. What I really want is a module with scanning radar decent resolution and great boresight ranging, doppler would be cool too, much prefer phased array but mechanical scanning is enough. This looks like it could possibly be the start of this, maybe at least for a slow moving vehicle or drone.

  3. I played with those as well. You can also get the respiration and heart rate of a person, provided it is aimed at the chest and the person is rather still.
    I wonder if you could do it with the newer ones which are using the BISS001 ic which are simpler and cheaper.

  4. Isn’t 24GHz also the frequency used by police speed traps? Would it be possible in this case to disable the module TX section and use the RX only as a carrier detector?

  5. The sensor is “fast” enough to detect higher speeds, but the object has to be close enough (1-5 feet) distance. And of course your amp/signal circuit has to be designed for detecting higher doppler values.

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