The solar-powered security light spent many years in the sun, charging its batteries to protect from the darkness. It is highly valued for its reliable and selfless vigilance. The unit is completely self-contained and requires no electrical wiring. It was perfect for temporarily replacing wired security lights when they were disconnected during a recent house remodeling project. The solar light also spent time strapped to a tree to surprise nocturnal raiders in the garden.
Of course, to work at its peak, the solar charger needs at least 6 hours of direct sunlight to prepare for the next nighttime watch. But after 15 or more years in the sun, the plastic enclosure became yellowed by UV light. That should be an easy fix: apply some masking tape to the passive infrared (PIR) detector lens, LED flood array, and solar cell, then spray on a couple of coats of glossy white paint. All went well until the PIR detector lens was touched. It was very brittle and disintegrated into Fresnel dust.
Sadly, I figured that the lens was unique and manufactured specifically for the PIR detector enclosure of that old light and couldn't be replaced. Oh well, since it was working before, the unit could at least be kept for spare parts.
The Lens is Indeed Unique
The usually flexible frosted plastic detector lens is pretty special. It doesn't just protect the electronics and optics from the weather, it also concentrates and directs IR energy onto the internal thermopile sensor using a custom pattern of refracting grooves (a Fresnel pattern) molded onto one side of the thin plastic. The lens pattern provides greater sensitivity for detecting movement than would be possible through a clear window, increasing the range from 1 meter to almost 15 meters. The lens also concentrates the field-of-view to approximately 120 degrees.
Cue The Great Search Music
On a whim, I searched DigiKey for IR lens and found a section for Sensor, Transducer Accessories with a PIR Lens Accessory Type. That was welcome news. We now have 68 round, rectangular, white, and black lenses to choose from.
And there it is! Thin, flexible, and size compatible for less than $3.00. I bought two.
E6000 to the Rescue
From my arsenal of sticky stuff, the best adhesive for the job was E6000, the arts and crafts relative of Shoe Goo. Not only did it bond perfectly with the slippery lens plastic, it also provided a water-tight seal around the edge. A toothpick worked nicely as an applicator. I started by attaching the left edge, securing it with a small clamp. After the glue cured, I attached the center followed by the right edge, after allowing the glue to cure for each step in the process. Afterwards, a bead of E6000 was added to the inside edges to reinforce the attachment and to seal the lens window.
