The easy solution would be to buy a laser line detector. But that would set me back at least $60. On top of that I would have to buy a new line laser that is supported by the line laser detector. That kind of line laser cost from about $200 and up.
I found out that the detectors usually rely on rotating laser light. Rotating since the detector only detect blinking light. Outdoor a constant light source of a specific color does not make much of a difference compared to the background surrounding light for a light sensor, but by adding a high pass filter to the light sensor, only pulsating light will be detected. Everything else is filtered out.
OK, so I had to adapt my existing line laser to emulate a rotating line laser. I had to make the laser blink at a certain frequency. The line laser I have is this one from Clas Ohlson, Cocraft HL10-S
There is a feature on many line lasers that the laser is turned off if the laser device is inclined too much in any direction. Mechanically the laser emitter is positioned in a pendulum that is hinged in two directions so that it can freely move with gravity. The lower end of the pendulum is hanging down into a hole. If the laser device is inclined too much, the pendulum will touch the side of the hole.
Electrically that will close an electric circuit that in turn will turn off the laser light emitter.
This mechanism I will use to flicker the laser light emitter.
Opening the laser device, I found the soldering pads on the PCB that corresponded to ground(GND), power(Vcc) and the inclination trigger to turn off the laser.
Measuring the signal level on the inclination trigger showed that it was either 0V or Vcc.
Manually forcing the inclination trigger pad to Vcc turned off the laser emitter. Thus, I only needed to create an astable multivibrator that generates a high enough frequency square wave and connect that signal to the inclination trigger. What frequency is needed I still had to find out.
To start experimenting I of course needed a laser line detector. I ended up buying the very affordable, Cocraft D50 Pro edition.
Moving on with the astable multivibrator. I based it on a 555-timer that only need a few extra components to generate a square wave. Here is the electrical diagram and component values I ended up using.
It turned out that the detector start to detect the laser when the beam blinking frequency is over 340Hz. I ended up using a 1kHz square wave with a 67% duty cycle. I soldered all the components directly on the 555 IC, in a "dead bug"-style.
The circuit was enclosed in a white shrink tube and tucked away inside the line laser enclosure.
Testing my new "rotating" line laser with my laser detector outside showed a detection distance of at least 40 meters. All in all it set me back $38 for the line laser, $50 for the detector and less than $1 for the astable multivibrator circuit.
Mission accomplished!
PS. I first planned to make my own line laser detector. But I realised that it is hard to motivate considering the time to develop and component cost, when I could buy one for $50.
