Preparing a 30 mWatt laser pointer to take out a street light

As noted, I was able to use a 30 mWatt laser pointer to take out a streetlight. But, I needed to modify the laser in three important ways to turn it into a practical solution:

  1. it had to be powered with an ac adapter
  1. it had to be mounted to the side of the fence in such a way that it could be aimed and held in position for extended time.
  1. it had to have a long power cable that could reach into my backyard and observatory dome.

image_thumb18I took my 30 mWatt laser apart and hooked it up to a power supply. The top half of the laser has a spring in the center that makes contact with the two 1.5v batteries. The shell of the laser is the second contact.

The center contact is the negative end, the outer shell is the positive end of the laser. Between the external contacts and the laser itself is some circuitry.

I am not sure what the drive circuitry is. The I-V curve, as near as I could tell, was sort of like a diode, a very sharp turn on in current with little voltage increase. The active circuitry did something, but it did not seem to be a very good constant current source.

Rather than try to take it apart and drive the laser diode directly, I decided to just drive the laser through the existing circuitry.

Experimenting using an external adjustable power supply, I found this laser did not turn on until there was 180 mA of current. Then it started lasing faintly.

It took about 2.7v to get 180 mA. At 3 v, I was getting about 330 mA and the laser was very bright. The effective resistance of the laser and circuit was about 3v/0.3A = 10 Ohms. This would determine the current flow in a typical circuit.

I left the laser on at 330 mA of current and after 30 minutes, the intensity had dropped considerably. The laser body was at least 15 degrees C above ambient. The laser itself was probably much hotter. I assume the drop in intensity was due to the increase in temperature.

I decided the current should be cranked down a bit, to about 300 mA, to keep the temperature rise a little lower. It was still pretty bright and would be bright enough to keep the lamp off.

I had a 5v cheap wall wart supply that was rated at 1 A. The ratings are usually based on a 10% voltage drop at the rated current draw. This would have been a source impedance of R = 10% x V/I = 0.5/1 = 0.5 Ohms.

I decided the simplest approach was to add a series resistor to the input of the laser circuitry to limit the current. The power supply output was 5v. With 3v across the laser with 0.3 A, the resistor would have a 2v drop at 0.3 A. This is a resistance of 2/0.3 = 6.7 Ohms.

I looked through my draw and came up with two 10 ohm resistors I could put in parallel with two 1 Ohm resistors in series. This would be 0.5 Ohms from the wall wart and 7 Ohms of resistors for a total of 7.5 Ohms. Close enough.

image_thumb22With the resistors in series with the laser circuitry, when monitoring the current through the laser, I got about 0.29 A through the laser using the wall wart.

I potted up all the cables, connectors and the resistors in a bathroom caulk silicone rubber to protect them from the environment. I added a power plug socket to the end of the cable pigtail so I could plug in the long DC power cable from the wall wart.

The laser tube was glued to a small right angle flange for mechanical support. Onto the base of this, I glued two  ¼-20 nuts. This would allow me to use a small camera tripod for the precise aiming.

Five minute epoxy makes for simple and easy construction.

I built a long extension cord from speaker cables to take the power from the dome to where the laser would be mounted.

image_thumb23The plan was to clamp a small camera tripod to the fence adjacent to the lamp post, next to my neighbor’s backyard. I needed a 40 foot cable to connect from the wall wart in my backyard to the laser.

Once everything was connected, I turned on the laser and used the tripod to align the laser to the photo sensor on the lamp post.

I was not surprised that in about 5 seconds of illumination, the light turned off. Success!