Fighting fIRe with fIRe

By Ryan Winters
Product Manager

In a quiet town where my friends, the Stimpsons, live, brazen home robberies were taking place in mid-day, sometimes while people were home. The would-be burglars scoped out a quiet street, walked right up to the front door and rang the bell. If no one came to the door, they would proceed to kick in a side door and rummage for valuables that could be quickly sold without a trace.

Since stay-at-home neighbors and outdoor pets weren't a reliable deterrent, the Stimpsons decided to install security cameras to keep an eye on the front yard and side-access paths. While the presence of a camera might dissuade a potential thief, they could also provide additional information in the event of a break-in such as time, physical description and potentially a vehicle license plate. The cameras seemed to have worked; shortly after they were installed the neighborhood became quiet again.

LED IR IlluminatorFinished infrared spot light

Then one day there was a knock at the door. It wasn't would-be robbers, instead, Mrs. Flounders, the next door neighbor, had come to complain about the Stimpsons' new truck. She claimed that although it was parked in the Stimpsons' driveway it was slightly over the property line and asked if the Stimpsons would move the vehicle a few inches.

When the property line was examined, it appeared that everyone was on their appropriate side. The Stimpsons figured the root of the problem was most likely that Mrs. Flounders, who typically used the Stimpsons' driveway as an exit from her property, could no longer perform this maneuver.

The dispute went on for months, and even involved the local police; they suggested that the Stimpsons simply ignore Mrs. Flounders. One day Mrs. Flounders noticed the Stimpsons' cameras and felt threatened by the possibility that she was being spied on. In retaliation, she set up her own cameras but pointed them directly at the Stimpsons' house. One camera was even directed toward a bathroom window.

According to local law it is perfectly legal for Mrs. Flounders to set up cameras that look directly at her neighbor's house, even if they are aimed at windows. The burden of responsibility was on the Stimpsons to close the blinds if they didn't like where cameras were pointing. A police officer told them they couldn't do anything to counter the cameras that would cause damage. That's when the Stimpsons turned to me for help.

A laser diode could probably affect the image sensor on the camera, but not only could it cause permanent damage to the camera, pinpoint accuracy is required, and if someone were to glance at the beam, it could damage their eyes. I suggested that they might be able to blind the cameras with an infrared floodlight. If the Flounders' camera used infrared light to help it see at night, then it could be possible to obstruct the camera with more infrared light shined directly at the camera. Some cameras have a "Smart IR" feature where it can cut the IR sensitivity during the day or compensate for the bounce back of infrared light.

An infrared floodlight seemed like the best possible solution so I offered to make one. Although there is a kit that already does this, the 36 LED IR Illuminator, I wanted to see if I could make my own. I picked up a protoboard, 30 IR LEDs, some 75Ω resistors and a 5V power supply. The IR LEDs only require 1.2V, so I decided to connect groups of three LEDs in series and use a single current-limiting resistor for use with the 5V power supply. When I finished assembly, I grabbed my digital camera to check if this new light worked. The digital camera was able to see the infrared light, which appears purple in the camera. REMEMBER, infrared is just outside the visible spectrum and while it is basically invisible to human sight, it can be harmful to your eyes.

Group of three LEDs in series with a resistor. Anodes to 5V rail.

The only downside I could see (or not see) was the amount of IR light. My camera didn't seem to be phased in the slightest by this blast of IR. I figured it was because the modern digital camera was designed for multi-use where it would be used around a lot of sunlight, and there was probably some built-in IR filter that was reducing the effect I was aiming for. I didn't have a security camera like the Stimpsons', so I couldn't be sure it was a design flaw. I picked up the 36 LED IR illuminator kit and quickly assembled it. I paired it with a 12V single-output power supply that I could bump up to 13.2V output. To my surprise, the IR output and effect on my camera from the kit version were no different from the one I built from scratch. To add insult to injury, the kit version was cheaper than my cost of materials, but I still had fun designing my own kit from scratch.

I handed over the IR floodlight to the Stimpsons and we tested it on their own security camera. It seemed the Stimpsons' camera did have some sort of IR or daylight filter because the IR spotlight was just that, a spot of light. At night the floodlight fully washed out the image. I know their goal was to wash out the screen during the day (for obvious reasons), but because we didn't know the capabilities of Mrs. Flounders' camera, the IR floodlight might not have any effect, or it could be completely effective. I guess the only way the Stimpsons will know it works is if Mrs. Flounders comes knocking at the door to complain, or she moves her camera, both of which would be a victory.

So what else could you use this IR floodlight for? Rather than blind a camera, you might be able to help it see at night. Many night vision cameras are very sensitive to infrared light and at a distance of 24 feet, this IR illuminator will light up eight feet in diameter. The brilliant part about the IR floodlights is the light remains invisible to humans, so trespassers might think they're in the clear if you don't have any visible perimeter lights as a deterrent.

How have you used infrared in your electronics projects? Share your stories at [email protected].
Ryan Winters is a Product Manager at Jameco Electronics and a Bay Area, California native. He is mostly self-taught and his hobbies include working on cars and computers, fiddling with electronic gadgets and experimenting with robotics.