Last year, I took a look at some mostly low-cost passive infrared (PIR) motion sensor modules. Then recently, I became more aware of cheap microwave radar motion sensor modules. The popular version of the pre-wired module currently available on eBay is the RCWL-0516 microwave radar motion sensor module. The compact module holds all essential electronics, including an on-chip integrated low-voltage regulator. It is very easy to use and trivial to splice into the roost of a little project idea.
This sensor module has been designed as an alternative to the common PIR motion sensors widely used in burglar alarms and security lights. Like the PIR sensor, this sensor also detects only movements within its detection range. But instead of sniffing the black body radiation from a moving person, this sensor uses a “microwave Doppler radar” technique to detect moving objects. It has a sensitivity range of ~7 meters. When triggered, its TTL-level output (OUT) pin will switch from LOW (0 V) to HIGH (3.3 V) for a finite time (2 to 3 s) before returning to its idle (LOW) state.
Supply Voltage: 4–28 VDC (tested with 5 V at my lab)
Operating frequency: ~3.2 GHz (observed by me at my lab)
Transmit power: 20 mW (typical)/30 mW (max)
Sensing Distance: 5–7 m (>3 m measured at my lab)
CDS — Sensor disable input (low = disable)
VIN — 4- to 28-V DC supply input
OUT — HIGH (3.3 V) motion detected/LOW (0 V) idle
GND — Ground/0 V
3V3 — Regulated DC output (100 mA max)
This flexible sensor module can easily be used in conjunction with many microcontrollers and even without a microcontroller at all. It can handle power supply inputs anywhere from 4 to 28 V. The output pin can be utilized for a multitude of tasks, such as for driving an aural/visual indicator or even linking with the I/O of any 3-V microcontroller for further processing. During construction, avoid any metal part in front of the sensor module. Similarly, always keep a minimum of 1-cm clear space in the front and rear side of the module.
Surprisingly, detection distance and output timeout of the module can be adjusted by adding passive components in their respective solder pads located at the rear of the circuit board (see next figure). There is also a provision to add a light-dependent resistor (LDR/CDS), and a sensor disable input pin is available to defeat the ambient light sensing option if necessary.
A word of caution: I could not find an official English datasheet of this module, so I’ve had to make some guesswork here based on the machine translation of Chinese to English. I’m not responsible for any damage that this could cause to your sensor module and/or interfaced microcontroller!
The electronics of the module consists of two equally important sections: a microwave frequency transmitter/receiver/mixer based on the MMBR941M high-frequency NPN transistor and a much-lower-frequency section based on an IC — RCWL-9196. Technically, the microwave section resembles a “Colpitt oscillator,” with the requisite inductor (and capacitors) made by circuit board traces. The inductor (~10 nH) is the S curve trace on the top surface, and capacitors are the ring structure on the bottom surface and also the rectangular block to the left of the S curve.
Before delving into anything, I recommend that you become familiar with the hardware and initial setup/run procedure and try to do some little experiments. Although you can use RCWL-0516 with just a power supply and an LED connected, I added an electromagnetic relay driver circuitry to control some external loads when a valid motion is detected. Here, note that the 1-kΩ resistor (R1) is not necessary as the module already held a 1-kΩ resistor between the OUT pin and actual output pin of the 16-pin onboard chip (RCWL-9196). Now to the schematic of the first test circuit:
The electromagnetic relay (RL1) in the circuit is driven by a standard BC547 transistor (T1) and there is a “relay on” indicator (LED1) that is when the relay is in an active state. If desired, you can also use other relays with a different supply voltage rating. However, in that case, the power supply input (now 5 V) will have to be changed (with some other minor modifications, of course). The 2-pin header (JP1) is reserved for the future and is practically usable only when a light sensor is connected to the module.
For experimental purposes, this circuit can be constructed on a perfboard/breadboard. A 5-pin header (only three are required for the first test) can be used to connect the radar module. Shown below is the random snap of my quick test setup (also watch the test video):
In principle, the Doppler effect1 is a change in the frequency picked up by a receiver from the signal reflected by a moving object. In Doppler effect radars, to detect a moving object, an unmodulated (CW) signal can be used. The receiver of the sensor processes the transmitted signal with the received signal reflected from a target. Due to the Doppler effect, the mutual speed of an object related to the antenna causes a frequency shift. It can be simply estimated that the Doppler frequency (which is the beat frequency obtained in receiver) is the number of the half-waves of the signal frequency passed by the target per second. A higher speed will produce a higher Doppler frequency. Such a system, with a provision for detecting signal phase, can also indicate the sense of target movement: Escaping objects generate a lower frequency than that of the probing signal, while approaching objects generate a higher frequency.
The Colpitts oscillator is a popular type of LC oscillator invented by Edwin Colpitts in 1918. The figure shown below depicts a typical BJT-based Colpitts oscillator with the tank circuit in which an inductor L is connected in parallel to the serial combination of capacitors C1 and C2. The frequency of the Colpitts oscillator depends on the components in its tank circuit and can be calculated by a simple formula (see figure). For example: If L=27 uH, C1=1 nF, and C2=15 nF, then F=1 MHz. Note that the Colpitts oscillator can be tuned either by varying the inductance or the capacitance.
In my opinion, the RCWL-0516 microwave sensor is a powerful alternative to the common PIR sensor, but this model has very limited (scant) documentation, making it difficult for beginners. In addition to reading the “Chinese” material collected by me, I also did some research to aggregate as much information so someone can use it quickly. While, at the moment, I don’t see a use for this sensor module apart from its signified application of motion sensing, it can easily be adapted to add or modify functions!
Reference 1: An Overview of Microwave Sensor Technology/Jiri Polivka, Spacek Labs Inc.