TinyProjector Lab Notebook

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Diary of September 2001


September 20, 2001

Replaced three defective diodes with new ones.


After long brainstorming with Kimiko, I decided to use a hot glue gun to attach the diodes. The advantage of hot glue over epoxy resin is that hot glue can be melted afterwards to adjust the diodes. So I glued all diodes to the acrylic holder (8 diodes in a row) (Figure 14) and calibrated them with a template on the wall, about 1.5 meters away. The calibration is not perfect, just as good as possible: it is very tricky.



Figure 14: TinyProjector prototype 1



Problem: Projection is not readable. Possible reasons:


(1)   Getting the rotation speed of the mirror right is very tricky: if it is too slow, the human eye doesn't integrate the dots into a 2D matrix. If it is too fast, the dots become lines.

(2)   Even if the right speed could be achieved, the "dead time" (inverse of duty cycle) of a full turning mirror with one or even two surface is large. E.g., if the desired projection angle is 60 degrees, then the lasers are unusable for 83% of the time! Therefore, it is not clear if a rotating mirror with one or two reflecting surfaces would work at all, without synchronizing the repetition rate with the mirror’s rotation speed.

(3)   Writing is mirrored on the wall, if the Skyliner™ board is used without modification.


Possible solutions

(1)   Use a dedicated PIC chip, and increase the blinking speed remarkably (reduce the time per dot). Or even better, make it adjustable: potentiometer on one A/D input of the PIC.

(2)   Synchronize the rotating mirror with the character repetition rate of the lasers, possibly with a photo diode and an LED behind a hole in a wheel mounted on the mirror.

(3)   Instead of one (or two) reflecting surfaces, provide three or four or even more, perhaps mounted on the outside of a tube. Like that, the dead time of the lasers would be reduced by the factor equal to the amount of surfaces per 360 degrees. Downside: the rotating element gets big.

(4)   Instead of a 360-degree rotating mirror, use a mirror that does a left-right sweeping movement of, e.g., 45 degrees. Obviously, it has to be synchronized with the lasers (forward and backward writing). This is mechanically difficult, and would probably create vibration problems.



September 21, 2002 and following days

Determined the formula for calculating the projection angle given the amount of mirrors on a 360-degree tube.



n = number of mirror surfaces per 360 degrees

p = projection angle

r = rotation speed of the mirror assembly, in Hz, for an estimated refresh rate of 4Hz




Table 1: Number of mirrors vs. projection angles and rotation speed (Hz and rpm)

Mirror surfaces

Projection angle

Rotation Speed (Hz)

Rotation Speed (rpm)



4 Hz

240 rpm



2.7 Hz

162 rpm



2 Hz

120 rpm



1.6 Hz

96 rpm



1 Hz

60 rpm



0.8 Hz

48 rpm



0.4 Hz

24 rpm




Figure 15: Four mirror surfaces per 360 degrees (left), five mirrors (middle), eight mirrors (right)



However, several issues have to be mentioned:


Issue 1:

Given an estimated optimal refresh rate of 4 Hz (four sweeps per second) of the original Skyliner™ toy, the rotation speed of a cylinder will be low, which would require a high-reduction gearbox for the motor. Of course a higher refresh/sweep rate would be better, but then the laser pulse time per dot has to be reduced remarkably. The question is how short the laser pulses can be, given the PIC and the transistors.


Issue 2:

If the beams point to the center of the rotating mirrors (more precisely: to the axis of the tube inside the mirrors), the laser diodes themselves will obstruct part of the projection. Therefore, the projection axis of the laser beams has to be displaced by a few millimeters to one side, away from the center of the rotating mirrors, so that the mirrors deflect the beams to the side, e.g., for about 90 degrees (see Figure 16).


Issue 3:

The biggest disadvantage of having eight or more reflecting surfaces is that the rotating mirror assembly gets bulky. (This issue will turn out to be the main reason to abandon the multiple surfaces mirror design.)


Figure 16: Eight-faced mirror with a linear laser array of 8 lasers, slightly displaced

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

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