I have already discussed many PIR based motion detector applications in this website, however all these applications requires the human presence to be constantly in motion in order to keep the PIR detecting their presence, this appears to be a big drawback which prevents these units from sensing a constant or a stationary human occupancy.
However the above explained drawback has a reason behind it. The conventional PIR sensors work by sensing the IR signals from a human body through a couple of parallel slots on their front lens, and its internal circuitry activates only when the IR signals cross between these sensing slots ("visions").
The crossing of IR signals across the sensing slots enables the PIR circuit to translate the info into two corresponding alternating pulses, which is in turn is rectified to generate the triggering voltage at the out pin of the PIR
This implies that if the IR source is stationary, it won't prompt the PIR module to produce any trigger across its output pin. It also implies that the IR signal from the source should somehow keep crossing across the given PIR detection slots in order to enable it to sense a given human being within the zone.
It seems there's no direct or simple remedy for this, because the PIR modules cannot be modified internally for this, which cripples the unit from detecting stationary human presence.
However a logical observance tells us that if its a varying IR source that may be required to keep the PIR module activated, then why not force the PIR itself to be in a constant motion instead of the subject.
The concept can be visualized from the following GIF simulation, which shows an oscillating PIR module and a static human being in the detecting zone.
Here we can see how an oscillating PIR adapts to the issue and transforms itself enabling the detection of even static IR subjects.
This becomes possible because through its movement the PIR module transforms the stationary IR source into a continuously changing IR imaging across its two receiving slots.
Although the idea looks complex, it can be actually simply solved using a slow oscillating PwM controlled motor circuit.
We'll learn the entire mechanism and the circuit details in the next post.