Simple Method to Defeat Outdoor Infrared Motion Sensors |
Overview
Outdoor motion-activated lights are fairly common. These are sensors which are usually placed in people's yards to activate a light upon the detection of any motion.
The actual "motion sensor" is just a common differential infrared detector (8-14 µm wavelength). The output of the infrared detector is slightly amplified then "pulse counted" to translate any motion into detection. The final control from the sensor is usually in the form of a relay triggering the external lighting system.
Most of these outdoor motion-activated lights are designed to only operate at night. They do this by incorporating a light sensitive cadmium-sulfide (CdS) photoresistor to monitor the general background light intensity. The resistance of these photoresistors is normally very high and lowers as the photoresistor is illuminated. The normalized response (wavelength) of CdS photoresistors follows that very closely of the human eye. That's to say, they are most sensitive to the "green" wavelengths between 500-600 nanometers. CdS photoresistors are only slightly sensitive to the 780 nm infrared wavelength available from common infrared laser diodes.
It's possible to trick an outdoor motion-activated light (at night) by shining a green laser onto the Fresnel lens of the infrared motion sensor. The internal reflection of the green laser light off the Fresnel lens will "light up" the internals of the infrared motion sensor. This essentially tricks the infrared motion sensor into believing it's daytime out. A suitable laser was described in GBPPR 'Zine, Issue #89 as the "GBPPR MIL-SPEC Laser Dazzler" project.
This is all easier said than done, though... Aiming the tiny green laser dot onto the Fresnel lens of the infrared motion sensor will require some type of support or pointing apparatus. One of those fancy "joystick" camera tripods from Manfrotto should do just nicely.

Wavelength Response of CdS Photoresistors
Common Laser Wavelengths & Descriptions
InGaN 405 nm Common in Blu-ray DVD and HD DVD drives.
DPSS Green 532 nm Frequency doubled from 1064 nm, green laser pointers.
AlGalnP 635 nm "Brighter" red laser pointers.
GalnP/AlGalnP 650 nm Low-cost red laser pointers.
GaAlAs 780 nm Common in CD players. (non-visible)
GaAlAs Pumps for DPSS 808 nm Pump diode found in green laser pointers. (non-visible)
CO2 10,600 nm Goes through smoke and fog, should also jam the
infrared detector in the motion sensor.
Pictures & Construction Notes

Test infrared motion sensor.
This is a standalone infrared motion sensor I made to trigger a small neon light when motion is detected.
It's based on a standard relay output motion sensor I found at a local hardware store.

Bottom view of the infrared motion sensor.
There are four main on-time settings. Test, one second on, 5 seconds on, and 10 seconds on.
There is also a variable sensitivity setting which basically determines the detection range. Minimum sensitivity is fully-counterclockwise when viewed from the bottom.

Internal wiring of the test infrared motion sensor.
Nothing really fancy, just follow the wiring diagram in the motion sensor's manual.
There is a 10 amp fuse on the main AC input and an output terminal block in parallel with the triggered light wire. This was added for enabling external devices.

Rear view showing the output terminal block.

Internal view of the infrared motion sensor.
The Fresnel lens, which is used to create several "zones" for the infrared detector is on the lower-right.

Internal view of the infrared motion sensor.
The main infrared decector is the silver circular device in the middle. The dark rectangle is an infrared filter ahead of the actual sensor.
The CdS photoresistor is the device to the right of the infrared sensor. It has a little black "hood" to direct the field-of-view to that of the infrared sensor.
The CdS photoresistor is the device we wish to "jam" with the laser pointer.

Overview of the test motion sensor and the GBPPR MIL-SPEC Laser Dazzler from GBPPR 'Zine, Issue #89.

Example infrared motion sensor jamming with the GBPPR MIL-SPEC Laser Dazzler.
The slightly expanded laser beam from the dazzler is very useful for keeping the laser on the target motion sensor.

Real-world testing.
I was able to sneak up on the motion sensor by "jamming" the CdS photoresistor with the green laser dazzler.
Keeping the laser dot on the sensor's Fresnel lens was quite tricky. It would probably be a better idea to setup a laser jammer (with an expanded beam) a hundred feet away or so, pointing at the target motion sensor.