Marking thermal paper

[JohnS]

I have made an "Harmonic oscillator" consisting of two pendulums one mounted with a paper carrier the other with a fine marker pen.
The first pendulum can swing in 2 dimensions, the other can only swing one way.
Beautiful patterns are generated as the swings come in and out of phase and gradually die away.

The system as it stands is not satisfactory for two reasons.
1. The friction between the pen and paper too quickly damps the swing of the pendulums, thus restricting the size and intricacy of the generated patterns.

2. because the pen and paper are mounted on the pendulums they do not move in a plane but in arcs. This means that the pen has to be mounted in such a way as will accommodate the varying distance between them. The springy metal I have used leads to judders and marks and varying depth of line, spoiling the look of the patterns.
I propose to cure both these faults by using a laser to mark heat sensitive paper. Clearly there will be no friction, and I hope a coherent beam will stay in focus over the 0.5 inch or so variation in distance between the pen and paper as the swings occurs.

However I know nothing of lasers and I would much appreciate some advice.

1.What laser should I use.
2. Can it and associated power supply weigh in at around 150g?
3. Would such a system be safe to the eyes of a user or would it need shielding in some way.
I have attached some photos to help make all this clearer. Can anyone help ?

JohnS

 

[ShortyInCanada]

Interesting idea.
First though I will comment on your problems.
A heavier mass on the pendulum and placing the paper on a properly arced base would help solve your issues but that is another discussion I suppose. The ratios involved of resistance to the inertia of the weight need to be reconsidered.

As far as a laser that weighs in at 150g... I would think a nice heavy weight on a pendulum would last longer and produce much nicer output results. The larger the mass the longer it takes to slow down. This also makes it less prone to vibration and undesired oscilation while swinging.
If you also create a curved surface for your "medium" that is heavy enough with it's own pendulum point you could open the creative possibilities to much greater options.
Then you can decide if using some form of photo/light sensitive or heat sensitive paper is needed.
Once you decide on these choices you can determine the level of required safety equipment needed.
That's my two bits.

 

[Cyparagon]

I think it's better to use a system of levers to direct the pendulum movement to horizontal movement.

 

[JohnS]

Thanks for your interest. The pendulums are supported on fulcrums about 3/4 of their length from the mass. which is a cast lead block weighing about 3Kg. The pen and paper are mounted at the other end of the rods. The fulcrum distance is variable as is the position of the mass in each case. This is an important feature as changes affect the amplitude of the swing and the frequency, which in turn affects the pattern. I think this would make the design of a lever system very tricky. I know from experience how sensitive the system is to friction and a lever system is bound to add to that.
As to providing a curved surface to mount the paper on, easy to say but not so easy to do. I only have hand tools. No, I am convinced that some form of "light pen" is the way forward!
JohnS

 

[ApexProxy]

You could always get a small focusable 405nm pointer and use that. 405nm should create the most heat on the thermal paper with the least output. Also, 405nm's tend to hold beam shape for further than most other lasers. The one problem is finding an output that doesn't just set the paper on fire but still marks it. You might want to go with like... 25mW. Taking into account that the paper will be white at first which will reflect most of the light but once its marked it will be black and absorb more light, thus getting even hotter so 25mW shouldn't burn it once its black.

A lever system for at least the paper carrier pendulum would be beneficial.

 

[JohnS]

ApexProxy, many thanks for your reply. The paper and the pointer would be moving in relation to each other which would help with an overheating problem until the amplitudes became very small. I had anticipated a little circuit to detect this situation and shut down the laser. Can the output of a 405nM pointer be modulated by varying the input power?
Are there safety issues to be considered with this type of laser?
JohnS

 

[ApexProxy]

The output of a 405nm diode can easily be modulated by directly controlling the current it is receiving.

The risks from 405nm lasers would be accidentally setting something on fire or eye damage from viewing one at close proximity or accidental reflections. If you have a matte black painted plate of metal under the pendulum system or set it on something that isn't reflective or can be damaged by heat you remove the risk of fire. Also keep a pair of laser goggles designed for 405nm handy when looking at it and you are good to go. I have a link to some goggles that would work perfect for what you are trying to do.