Taking out a streetlight with a laser pointer

imageThere is a very obnoxious street light located behind the dome of the Addie Rose Observatory. It is almost in the sight line to Polaris.

This make most of the views around the celestial north pole inaccessible.

The solution is to take out the light when I want to observe north.

While some of my friends suggested a B-B gun, or an arm mounted sling shot and a steel ball bearing, I wanted a process less permanent. After working through a dozen alternative ideas, I settled on shooting the light out with a laser.

imageIt turns out that many street lights have a photo sensor, usually located on their north side, away from the southern, direct sun.

In the close up picture to the left, you can see the small round circle on the base of the light. This is the photosensor. It is probably just a large area photoresistor.

When the light level drops below some pre-set threshold, the resistance goes up and this triggers a circuit that turns on the streetlight.

What if I could fool the streetlight into thinking it was daytime?

I tried shining the brightest flashlight I had on the sensor, but there was no impact. I tried shining a red laser on the sensor spot, but there was no impact. The light stayed on.

I was thinking of trying to place a mirror near the light to shine its own light on the sensor, but I realized this would just cause the light to go into some sort of oscillation.

As a last resort, I tried a 5 mWatt green laser pointer. No impact.  I was pretty confident some power level would work, it was just a matter of finding the lowest power (safest) that would still reliably turn off the light. I had a 200 mWatt laser available but didn’t want to bring out the “big guns” unless I had to.

And if this didn’t work, I knew where to get a 2000 mWatt laser if I really needed it.

imageSure enough, I mounted a 30 mWatt laser on a camera tripod and aimed it at the photo sensor. In less than 5 seconds, the lamp turned off.

The laser stayed on all through the evening and the light stayed off.

I’ve used this device a number of times to turn off the street light and shoot northern shots.

Now I have a stable technique to turn off the streetlight whenever I need to.

I get a few funny looks from my neighbors, but I’m used to that.