TinyProjector Lab Notebook

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Diary of July 2001


July 9, 2001

After looking for laser pointers and laser modules for some time, I found a few cheap key chain laser pointers to play around (Figure 7).



Figure 7: Cheap key chain laser pointers, sold in many retail and department stores as toys. The price was about $20 originally, but came down to about $5.


I disassembled them by cutting off the outside aluminum tube, and extracted the laser diodes, including lens (Figure 8).



Figure 8: From these key chain laser pointers, the laser diodes are extracted.



I opened a Skyliner™ toy, and extracted the circuit board. Vadim (vadim@ml.media.mit.edu) helped me to connect the electronics of the Skyliner™ to a laser diode. First we tried to connect it directly, but the voltage was not high enough: the Skyliner™ works with 2 AA cells, so 3V. The laser pointer diodes I have need 4.5V.


Then Vadim helped me design a circuit (Figure 9, Figure 10) which uses the output of the LED wires to switch an external voltage on and off, with the help of transistors.


Figure 9: Circuit sketch for laser pointer diode, switched by the output of the LED via a PNP transistor


Figure 10: Breadboard with Skyliner™ electronics (green circuit board), transistors, and battery packs


July 12, 2001

Meeting with Chris: showed him the current prototype that I made yesterday, with some laser diodes mounted on a cardboard casing. The interfacing between the Skyliner™ board and the laser pointer works properly, but the alignment of the laser beams is very bad, and the projection of the Skyliner™ is not visible.


Designed two holders for the 8 laser pointers (Figure 11) on Corel 9: one with eight holes in a row, one with two rows each 4 holes. Laser cut them with 1/8-inch acrylic (100% power, 6% speed). The single row acrylic piece is going to become TinyProjector prototype 1; the two-row acrylic piece will be used later for TinyProjector prototype 2.


Figure 11: Acrylic holders for eight laser diodes; one row (left), two rows (right)


With the laser diodes inserted in the holes of these acrylic pieces, the alignment of the laser beams is better, but still not good enough for a readable projection. Eric Varady (evarady@media.mit.edu, Jacky Mallet's UROP in the Garden) told me that he could help me cut threads, so that the diodes could be screwed in (the diodes come with external threads). He also told me that he could help me cut the thin mirror I have. I tried to cut one piece of the mirror myself manually, but it broke off. I made an axle for the mirror with two paper clips, and tested it quickly. It doesn't look very good: the alignment of the laser beams has to be much better.


The laser diodes from the key chain laser pointers are rather fragile: the 8th laser diode never worked, the 7th (the one we started using with Vadim) has gotten very faint.


I continued the Web search for smaller laser diode modules. The smallest ones (6.4mm diameter x17.25mm long) are very expensive, though ($75), compared to the price of a cheap key chain laser pointer (between $5 and $15)


Figure 12: Small laser diode


Laser diode modules:



July 13, 2001

Did some Web search about prisms that could replace the little mirrors. There are penta prisms for precise 90-degree angle deviation. They would do the job, but they seem to be expensive, and probably over-sophisticated, since we don't need the non-reversing and non-inverting feature.


Figure 13: Penta prism


Penta prisms:


However, after experimenting with flexible mirrors (foil based), it became clear that the mirrors have to be of very high quality so that they do not diffract and diffuse the laser beams too much. Therefore, it has to be either good quality mirrors, or prisms.



July 14, 2001

Tried to get another Skyliner™ at Toys'R'Us: they don't have it anymore. It seems to be available only online, e.g.:



July 16, 2001

Meeting with Chris: showed him the laser cut pieces made of acrylic that hold the 8 pointers: one where the 8 pointers are in a row, one where they are in two rows of 4.


Eric helped me looking for a tool that cuts threads into the acrylic so that the pointers are more aligned. Neither the Media Lab nor the MIT shop had metric ones with the right pitch. We decided that gluing is the only solution, but not to a single acrylic piece: I will glue each laser pointers separately to small acrylic pieces, and then screwing these pieces to a larger frame. So if one diode breaks, it can be removed from the whole and replaced with a working one.



July 18, 2001

Eric emailed his friend Josh who has more key chain laser pointers like I have. Josh said that he has only weak ones left. But that would be fine with me, just for testing.



July 20, 2001

I tried to get more laser pointers from Josh Korn (jkorn@MIT.EDU): he had 4 with him, and I told him that I would take 10.



July 21, 2001

I won an auction on eBay for more laser pointers. They were more expensive than I expected, but still dirt cheap compared to commercial 6mm diodes that cost between $70 and $140 a piece.


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Send me some comments! Stefan Marti Last updated February 23, 2003.

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