Reducing the sensitivity of a PIR sensor?

Question

I have several standard BISS0001 based PIR sensor modules, but lack any sensitivity trim pot.

I am hoping there might be a way to reduce range/sensitivity to a very small field. Currently they are detecting motion about 15-20 feet away.

Does anyone know of a way to reduce the field as low as maybe a 1 foot radius?

I am using them with an Arduino Mega 2560 or an Arduino Uno.

I have tried placing various objects / films in front of the sensor.

Running the sensor through a tube (cardboard or PVC) seems to effectively reduce the sensor to a very narrow beam of detection, however I also need to reduce the overall range, as even with through the tube the sensor fires off at 15-20 feet away. I have tried placing plastic wrap or the reflective static free plastic that electronics are shipped in over the end of the tube, these do not seem to do the trick.

Answer 1

You can attenuate by using thin sheets of polyethylene. Polyethylene is the same material that the PIR lens is made of. It will pass, but attenuate the wavelengths you are interested in. Find something around .015" and begin stacking until you hit your desired range.

Here is a source: http://www.mcmaster.com/#polyethylene-plastic-sheets/=hc4uvj

Answer 2

You can either do this physically using an IR opaque material as Jason suggests, or electronically by modifying the sensor.

Most (all?) of these small PIR detectors seem to be based around a BISS0001 chip. These have two amplifier stages for taking the signal from the PIR FET. If you take a look at the example circuit on page 4, most of the detectors I have seen follow this almost exactly, down to the component labels being the same.

The resistors don't always seem to have the same values on the boards. But by simple observation, you can see that for opamp OP1, it is operating in non-inverting mode. The gain is R7/R8, 2M/47K or approximately 20x.

OP2 is in inverting mode and the gain is R5/R6, 1M/10k or approximately 100x.

I would adjust the values of R7 and R8 to reduce the gain and make the device less sensitive.

Alternatively, you could adjust R1 (connected to the source of the PIR detector) or try reducing Vref (which sets several of the voltage references inside the chip).

All of these require removing and adjusting components though, so maybe the physical way is easier.

Answer 3

I have had the same issue with a PIR sensor I bought online (cats, and even the wind was setting it off). I have placed a coffee can around the sensor cut off the bottom (metal lined, not plastic) and placed the lid on it. The lid is a semi-clear. I just did this a few minutes ago, will let you know how it works.

OK after some field work, and suggestions on this page, here is what I did:

1. Cut off the threaded end of a pill bottle
2. Duct taped a used toilet paper tube around the sensor
3. Placed the pill bottle inside the T.P. roll

The IR sensor is transmitting through the bottom of the pill bottle (no modifications). I can slide the remainder of the pill bottle in and out of the empty T.P. roll for a finer adjustment.

The wind is no longer triggering it, and when the pizza guy shows up in my driveway we will see if it goes off properly.

And abandoning the coffee can idea!

OK, so I cut off the bottom, more distance, better isolated beam.

Hope this helps.

Answer 4

black film may be transparent to Infra Red ??

If you can access the load resistor across the PhotoDiode, (PD), then you can change the gain of the receiver. Since light generates current, gain is proportional to R. PD is always reverse biased with R to V+.

Or if using BISS0001 chip reduce 47KΩ in spec sheet. or feedback resistor between pins 16,15. http://www.ladyada.net/media/sensors/BISS0001.pdf

I remember designing a light sequencer in the 70's using a cheap electret mic inside the drummer's kick drum. Needless to say it was too sensitive. So I taped the aperture and the string of spot lights which spelled the band's name sequenced with the beat of the music. It worked well until they changed the sign to 1kW spot lights, and then the triacs burnt out.

Answer 5

I worked around this problem by using a prescription pill bottle and foil tape (the kind used for sealing heating ducts):

1. Cut off the top and bottom of the pill bottle.
2. Wrap the foil tape around the cylinder with a bit hanging off the one end
3. On the end with a little extra foil tape, connect that towards the PIR sensor

In my case the cylinder creates a field of view approx 2 or 3 feet wide and about 12 feet long (maybe less).

Answer 6

The module you have is the old RadioShack sold, manufactured by Parallax, BISS0001 based modules, which lacked the sensitivity trim pot. The way the newer ones handled it was by placing a large trim pot in series with a fixed resistor. This is connected to pin 12, 2OUT (2nd stage Op-amp output). It is marked RL2 on the schematic below. The series resistance would be between 470kΩ and 1.4MΩ depending on the trim pot.

In a Keyhole light I have based on the same IC, the resistance used is 2MΩ. The light is only sensitive a few feet away. You can attempt to change this resistance on your module.

The Better option is simpler. Remove the Fresnel Lens.

The lens provides multiple benefits to the sensor, by dividing the field of view, providing a wider angle, and focusing IR into the sensor. No lens, no benefit. This should be enough to both narrow it's view, and weaken it's sensitivity to distance. If you want a single foot of distance, then try this before taking a soldering iron to the module. The Lens should pop right off.

Answer 7

Put a layer of Scotch tape should be fine.

Answer 8

Partially block the sensor window, directly

Remove the lens and put aluminum foil tape over a portion of the PIR sensor window, directly. The sensitivity is directly reduced by the area blocked. The edge of the tape should be placed parallel to the long edge of the sensor window. Replace the lens, if desired.