Friday, 20 May 2022

Laser-line Oche for Dartboard

When I built the dartboard cabinet last year, I put a piece of tape on the floor to act as an oche (throw line).

As they say, there's nothing more permanent than a temporary solution, and a year later, the tape was still there, so now it's time to do something about it.

Years ago, back when I first started posting my projects on social media, one of my first Instagram posts was about a (now long abandoned) project to create a 3D scanner:


In this process, a regular laser diode - like one from a laser pointer - is aimed through a transparent cylindrical object. This acts like a prism, refracting the light, creating a line.

The same principle I used back then, is what I'm using this time around. Although this time I'm using a piece of acrylic rod rather than being cheap and trying to use a stem from a broken wine glass...

The Diode

The laser diode that I'm using comes from an old Nintendo Wii lightgun accessory, and was the one I started using in the laser scanner project.

With all the plastic shroud removed, this is what is we're left with


 The Casing

I initially went down the rabbit hole of making a 3D printed case, but in a moment of inspiration, realised that a short section of copper pipe would be perfect.

The pipe is about 1.5 inches long, and we're using two end caps.

One simply has a small hole drilled for the wires to come out of the back

A plastic washer was used to create a mount for the laser diode in the pipe.

The second pipe end cap is used to create the lens assembly.







The lens assembly

A hole was drilled into the end cap, bevelled at the outside for a nicer finish. The hole should be no wider than the diameter of the acrylic rod.

A small piece of transparent acrylic rod was cut, and sanded to make a friction fit in the end cap perpendicular to the hole that was drilled.

You can see how the rod bends the light, making the drilled hole look square.

This lens assembly can be push fit over the end of the rod.


The wiring

The diode was powered by 2 AAA batteries, so I found a 3.7V phone charger as the closest contender for a power source. However, it's not just a case of connecting the diode to the charger.

It's important that laser diodes are driven correctly as current fluctuations can easily damage them. In some cheap laser pointers, control of the current is sometimes limited by the specification of the batteries that are used.

I was expecting this to be the case with this one, but after some probing, I determined that I was lucky, this laser contained some control circuitry (behind the yellow shrink-wrap in the diode picture).

So the only extra circuitry I had to add was a resistor to drop the voltage closer to the 3v that a battery would provide, and adjusted the resistance of the battery so that the current was also similar.

For safety I did this by starting with a high value, which provided virtually no light output on the diode, then reduced it until the brightness was suitable.


The mount

The mount is 3D printed (Source), and attaches to the top of the bottom compartment of the Dartboard Cabinet. The body of the laser is attached with a copper pipe clip, and is simply a case of holding the laser so the line is drawn the required distance away, and then the bolt tightened.

This can take a bit of trial and error, as the tightening of the bolt can unintentionally move the line.




The finished oche


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