Proximity sensing is a very common application in electronics. There are several ways to accomplish this. The most common way is by using a PIR sensor. PIR Sensor senses the change in ambient infrared radiation caused by warm bodies. I have already covered this in my Tutorial No. 5: "PIR Sensor Tutorial - With or Without Arduino". However, since PIR sensors detect movement from living objects, they can generate false alarms. These sensors are also inefficient in hot environments, as they rely on heat signatures.

The other common methods of proximity sensing involve, using reflected ultrasonic or light beams. Using these sensors, the intruding object is detected by the reflected beam back to its source. The time delay between transmission and reception is measured to calculate the distance to the object. In this tutorial, we are going to look at another method of proximity sensing using "Microwaves" and "Doppler Effect".  In my hand is an inexpensive RCWL-0516 Microwave Radar Motion Sensor. The RCWL-0516 microwave sensor detects "any movement" from "any object" and does not rely on heat, making it more reliable in hot environments. I am going to use this sensor to create a Geo-fence around my house to detect motion and get notifications.

What is the Doppler effect?

The RCWL-0516 module uses a “Doppler Radar” that makes use of the "Doppler Effect" to detect motion and trigger proximity alerts.
So, before understand how the RCWL-0516 sensor works, let’s understand the Doppler Effect.

The Doppler effect, is named after the Austrian physicist Christian Doppler, who described this phenomenon in 1842. He described the change in frequency observed by a stationary observer when the source of the frequency is moving. The sound's pitch is higher than the emitted frequency when the sound source approaches the observer as the sound waves are squeezed into shorter distance (bunched together), which can be heard as a higher pitch. The opposite happens when the object moves away from the observer, causing the sound waves to become lower in frequency and lower in pitch (spread out). As a result, the observer can hear a noticeable drop in the pitch as it passes.

This holds true for all sorts of waves, such as water, light, radio, and sound.

How Does The RCWL-0516 Works?

Like the PIR Sensors, these sensors also detects only movements within their detection range. But instead of sniffing the blackbody radiation of a moving object, these sensors uses a “Microwave Doppler Radar” technique to detect a moving object.
 
Doppler microwave detection devices transmit a continuous signal of low-energy microwave radiation at a target area and then analyze the reflected signal. The target’s velocity can be measured by analyzing how the target’s motion altered the frequency of the transmitted signal.

Due to Dopplers effect, the frequency of reflected microwave signal is different from the transmitted signal when an object is moving towards or away from the sensor. When a car approaches a speed trap radar, the frequency of the returned signal is greater than the frequency of the transmitted signal, and when the car moves away, the frequency is lower. This is how a speed gun calculates the speed of the car.

Technical Specifications

The technical specifications of this sensor are listed below:

• Operating Voltage: 4-28V (typically 5V)
• Detection Distance: 5-7 Meters
• Maximum Current Drawn: ~ 2.7 mA
• Operating Frequency: ~ 3.18 GHz
• Transmission Power: 20 mW (typical)/30 mW (max)
• Signal length: ~ 2s
• Regulated Output: 3.3V, 100mA

RCWL-0516 Module Pin outs

The RCWL0516 module is a single breakout board with the following connections:
 
3V3 : it is the "output" from the onboard 3.3V regulator which can be used to power external circuits. Remember, this is not an input pin. This pin can provide up to 100mA of current.
GND : is the ground pin.
OUT : is the 3.3V TTL logic output. This pin goes HIGH...