HB-100 / GH1420 10.25GHz Microwave/Doppler Radar Motion Sensor w/Arduino

I picked up some of these modules out of curiosity, as these modules can detect the motion and speed of objects through walls or other non-metal obstacles.


Overview:

The modules themselves are fairly simple in use, since they output a signal with the frequency/pulse width indicating the speed, and the amplitude/voltage indicating the strength of the reflected signal.

The speed is very simple to calculate once the frequency of the incoming signal is detected:

km/h = hz / 19.49
mph = hz / 31.36

These HB-100 modules require a pre-amp circuit to boost the IF signal, so I used an LM358N and the circuit at https://hackaday.io/project/371-hb100-radar-shield to boost the intermediate signal.

The output of the circuit sits at about 2.5v when quiet, and moves up & down to represent the received AC waveform.

In pure digital form, this would be very easy to detect using interrupts, but with an amplified analog signal, weaker signals may be missed.

Problem:

Tests with this radar shield using standard frequency counters/libraries seemed to show that interrupts and edge-detection have limited sensitivity to weak signals. They also provide no means for detecting the amplitude of the signal. It seemed that using the on-board ADC may provide better results in this frequency range. (0 to 4000 Hz or 0 to 205km/h)

Goal: 

Detect the frequency and amplitude of low frequency (0-4khz) signals from HB-100 modules, while potentially increasing the sensitivity of waveform detection using the HB-100 & circuit as linked above.

Result:

The resulting code uses the Arduino ADC to sample incoming signals using very simple peak-to-peak detection.

Signals are detected when the waveform transitions above or below a set "midPoint" which represents the 0v value of the AC waveform. Typically, it will be ADC_RESOLUTION / 2.

This is very similar to the edge detection used for interrupts and digital signal detection, but using the ADC to detect weaker transitions.

ADC Resolution / VCC == Volts per Increment ( 1023/5v = 0.005v )
Sensitivity of 10 == peak-to-peak detection of +/- 0.05v signals


Technical Info:

The ADC is driven in free running mode, taking samples at about 38.5khz while looking for specific deviations in the measured voltage that correlate with a peak-to-trough waveform.

The results are summed in a single variable, and returned either as a single result, or an average, depending on how often the results are read in relation to the received signal.

Information about the signal, including frequency, amplitude and number of samples allows more complex analysis.

Conclusion:

The code was developed in a very short time, and can still be considered a work in progress, although it seems good as-is. It tests well up to about 2-4khz with another Arduino producing square wave output. When used with the radar module, it is very sensitive, and allows the sensitivity of the device to be defined in software, and/or results to be filtered.

Code & Examples:

https://github.com/TMRh20/AnalogFrequency