Doppler Radar Sensor

24GHz Doppler Radar Sensor Breakout Board    24GHz Doppler Radar Sensor Breakout Board
Have you ever walked into the supermarket and wondered how that automatic door opening sensor works?  Well its not a camera, its a microwave doppler radar sensor.  These sensors send out a high frequency wave which bounces off nearby objects.  If any nearby objects are moving, the sensor is able to notice that the reflected waves have a different frequency due to the doppler effect.

The innerds of these sensors are widely available to electronics hobbyists and cheap versions can be found on eBay under the names CDM324 and HB100 for just a few dollars each. Although cheap, the output of the sensors is difficult to work with. The sensors output a sine wave whose frequency and amplitude changes depending on the object's speed and reflection. Various DSP algorithms can be used to process this signal into useful information but the programming is not trivial and a simple controller like the Arduino is hardly fast enough to do the job.
24GHz Doppler Radar Sensor Breakout Board
Analog circuits guru Dale Heatherington came up with a circuit that amplifies and filters the sine wave and outputs an analog value that represents the intensity of the returned signal. This analog voltage can be easily read by an Arduino.  Setting a threshold value in the software would let the Arduino quickly decide if an object is there and to take action.  Putting two of these sensors together would allow an Arduino to compare the two signals and decide which direction the signal is coming from.  With such a sensor mounted to the front of a moving robot, the robot could sense objects in the way and decide which way to turn.
24GHz Doppler Radar Sensor Breakout Board24GHz Doppler Radar Sensor Breakout Board
My contribution to this endeavor was to take Dale's circuit and turn it into a compact breakout board.  Two copies of the circuit are on one board, and there are mounting points where two of the 24GHz style sensors (CDM324, IPM165) can be directly mounted to the board.

Connections to the breakout board are as follows:
  • GND, Ground - Connect this to the Ground pin of your microcontroller
  • VIN, Voltage In - This can be 5V or 3.3V depending on what your doppler radar sensors are designed for.
  • !EN, Not Enable - Setting this pin HIGH will turn off power to the sensors. When left unconnected, power is enabled by default
  • A2, Analog Output 2 - This is the analog output for radar sensor U2. Connect to an analog input of your microcontroller
  • A1, Analog Output 1 - This is the analog output for radar sensor U1. Connect to an analog input of your microcontroller
  • D2, Digital Output 2 - This is the digital output for radar sensor U2. Connect to a digital input of your microcontroller
  • D1, Digital Output 1 - This is the digital output for radar sensor U1. Connect to a digital input of your microcontroller
The analog output connections will output an analog voltage determine by the strength of the reflected 24 GHz wave off any nearby object moving towards or away from the sensor.  The analog pins must be connected to a high impedance load such as a microcontroller analog input pin.  The digital output connections will output pulses at a frequency determined by the speed that the nearby object is moving towards or away from the sensor.  For a 24 GHz sensor, the frequency of the pulses will be about 72.3 Hz per MPH of the moving object.  Any of the analog or digital output pins can be left unconnected if not used. 

24 GHz Doppler Radar Sensor Schematic

Layout - Top

24GHz Doppler Radar Sensor Layout Top

Layout - Bottom (as viewed from the top)

24 GHz Doppler Radar Sensor Layout Bottom

Parts List
 Ref. Value Rating Package
 C1,C3,C4,C8 1 uf X5R/X7R, 10V 0603
 C2,C6,C7,C9 10 uf X5R/X7R, 10V 0603
 D1,D2,D3,D4 1N4148  SOD-323
 IC1 TLC3702  SOIC-8
 IC2 LMC6484  SOIC-14
 J1 0.1" Header  
 Q1PFET, DMP2305 Logic Level, 500ma SOT-23-3
 R6,9 10 ohm  0603
 R7,12,13 10k  0603
 R10 15k  0603
 R15 20k  0603
 R3,5 33k  0603
 R11,16 150k  0603
 R4,8 3.3M  0603
 R1 *See Text 1/10 to 1/4W PTH
 R2 *See Text  
 U1,2 CDM324 / IPM165  

Resistors R1 and R2 are optional. Note from the schematic that R1 and R2 are in parallel with R8 and R4, respectively. Leaving these off will result in a high gain amplifier circuit that may sense objects up to 20ft away.  For indoor robotic application it is often better to limit the sensing range to a few feet.  Setting R1 and R2 equal to 470k is a good starting point, but your results may vary. Lower values of R1 and R2 will result in a shorter sensing distance.

I'm sure many of the above components could be substituted but I haven't made any effort to investigate this. The LMC6484 is a great op-amp but is definitely not cheap.  Any decent voltage comparator should work fine in place of the TLC3702, I just happened to have one leftover from my Analog Line Follower project.  The DMP2305 is also overkill for the PFET as the radar sensors only draw about 50ma each and much less than that for the rest of the circuit.  Capacitor C7 is probably unnecessary but you never know when you might end up sharing a power supply with one of those noisy SharpIR distance sensors.

The graph below shows the analog output of the breakout board using two sensors being read by an Arduino and plotted using the Serial Plotter tool.
24 GHz Doppler Radar Sensor Analog Output

The code below can be used to create a graph like that shown above. It also calculates the difference between the two signals averaged over time. This value diffAvg can be used to determine whether an object is seen on the right or left side.

// This demo code shows how to use two microwave dopper radar sensors to determine the direction of a moving object
// The difference in signal intensity returned from two CDM324 sensors is computed and averaged over time
// This requires each CDM324 to be connected to Dale Heatherington's circuit shown at
// This was tested using the CDM324, although the HB100, IPM165, IPM365 should also work

void setup() {

void loop() {
  // Define a location to store a running log of differential sensor readings
  static unsigned int difference[16];
  for(int i = 15; i > 0; i--){
    difference[i] = difference[i-1];

  // Take the readings from the CDM324 sensors
  unsigned int leftIn = analogRead(A1);
  unsigned int rightIn = analogRead(A2);

  // Compute the difference
  difference[0] = 512 + leftIn - rightIn;

  // Compute the average of the differences
  unsigned int diffAvg = 0;
  for(int i = 0; i < 16; i++){
    diffAvg += difference[i];
  diffAvg = diffAvg >> 4;

  // Print to Serial Port. Can be viewed with Serial Plotter by hitting Ctrl + Shift + L (Arduino 1.6.7 or newer)
  Serial.print(" ");
  Serial.print(" ");
  Serial.print(" ");
The .sch and .brd files are attached below.
William Moore,
Jun 3, 2018, 4:24 AM
William Moore,
Jun 3, 2018, 4:24 AM