It detects stationary and moving people just like a PIR, but it can do that also behind doors and thin walls, by taking advantage of the Doppler effect.
The detection of people, animals and hot bodies in general has been done for years using passive infrared radars, also known as PIR, which work by placing a pyroelectric sensor, so a heat sensor, behind a Fresnel lens, which has the ability to focus on just one point the infrared rays coming from the frontally detected heat, emitted by moving objects within a certain angle.
PIR sensors cover a wide array and variety of applications and represent by now a low-cost solution to protect ourselves from home invasion, automatically activate utilizers when moving people are detected etc., although they have the limitation to be able to detect only whatever can be seen: they cannot detect, even at a short distance, people moving behind doors and windows, so if we employ them in a home security system, they will only be activated when the intruder is already inside the room where the sensors are installed.
In order to have a preemptive protection, we can make use of radiofrequency sensors and, to be precise, microwave sensors, because they can detect people behind doors and even walls as long as they are not too thick or made of reinforced concrete or metal, or walls containing metal plaster reinforcement grids (for bladder walls) or cavity for sliding doors such as Scrigno.
Microwave detectors can be monostable (like ours) or bistable, meaning they are mounted as a couple one in front of the other; in this case, emitter and receiver are placed in separate units. Monostable units define a detection area based on the microwave beam they emit and the coverage can reach up to 300÷400 linear meters, besides, we can configure it to the RF beam to adapt to special situations, so we can lengthen it or shorten it, make it smaller (in order to have a longer beam) or make it wider (which means a shorter distance but a bigger angular coverage).
On the other hand, a bistable microwave detector offers a wider detection era, even up to 1 km, but is very delimited; its applications are on external wall installations, for instance when we want to protect our garden entrance. Bistable sensors are also more prone to false alarms because the emit frequencies, activating and deactivating them in rapid cycles, followed by deactivation of the receiver. The unit uses these time intervals in order to detect movement checking object placement at different times.
In these pages, we propose a project for a microwave detector based on a dedicated module, in breakout board format.
Our sensor is basically a radar based on the Doppler effect, composed of an electronic section described by the circuit diagram, in which we’re going to insert the sensor’s breakout board. The latter is based on the RCWL9196 integrated that interfaces with a radar cavity made with an MMBR941M BJP Colpitts oscillator, which takes advantage of the PCB tracks to obtain the capacities and inductors needed to function properly. Fig. 1 shows the circuit diagram for the breakout board, which can be seen in Fig. 2.
The Doppler effect is a physical phenomenon for which the frequency perceived by someone who listens to a sound wave emitted by a source that is moving in relation to the observer themselves changes its commission value; the typical example is an ambulance, which siren changes tone (it goes lower) as the ambulance goes away, or the sound of a moving train.
The fact is named after Christian Andreas Doppler, who was the first to notice and document it; later on, Hippolyte Fizeau found out that the same effect takes place with electromagnetic waves emitted by a moving transmitter antenna, detected by a fixed receiver.
The Doppler effect is explained considering that as the sound source moves away, the wavelength increases, sound speed being equal.
With that said, let’s move on to the breakout board employed in the project, used to compose a microwave module with a planar antenna created on the PCB and containing an oscillator, a RF mixer and a discriminator detecting the radio wave alteration caused by the presence of a moving person intercepted in the area covered by the sensor itself.
The RCWL9196 integrated contains the mixer and the discriminator for the RF signal and provides, towards its VO pin, which ends on the OUT terminal of the breakout board on which it is mounted, a logic signal whenever there is a detection; specifically, OUT is normally at logical level low and switches from zero to logical 1 (3.3 V) whenever a moving object is detected, then goes back to zero.