With the fresnels in hand, a pair of plywood box enclosures were built...one for the receiver and one for the transmitter. Eventually they would be coupled together so that both would be pointing at the same distant spot.
The next step was to use John's design to mount the receiver and transmitter modules so they could be locked into position once aligned properly. His system used the 1/4" split shaft locking mechanism removed from an old Allen-Bradley
potentiometer to hold a short length of rod fastened to the module's case.
This allowed the shaft to be moved forward and
backward for focus while the slot in the mounting plate allowed for vertical
centering. The plate mounting mechanism itself allowed for lateral centering.
This system allowed for the locking of the receiver's photodiode at the exact
focal point of the fresnel lens.The same scheme was employed for the
transmitter's LED as well, since accurate focusing was critical there also.
In order to focus as much of the LED's light onto the
primary fresnel lens, a small inexpensive (secondary) collimating lens was
required. This assured that the fresnel was properly illuminated out to its
edges and no further. Any light spilling over the edges of the fresnel would
just be wasted.
Our particular fresnel had an effective aperture of 260mm
and a focal length of 200mm, producing an F-number (f/D ratio) of .76.... Clint suggested that our collimating lens should have an F-number of ~ 1 - 1.2 and be a PMN
(Positive MeNiscus) type and that we hedge our bets by trying lenses above and
below that value. Ideally the collimator should be at least 25mm in diameter
for ease of mounting and, when perfectly illuminating the fresnel, be as close to
the LED as possible, if not touching it. Just placing a less than ideal secondary too close to the LED would end up over-illuminating the fresnel, while having it too far away would under-illuminate it.
Accordingly, four small glass
collimating lens of various F-numbers were purchased from Surplus Shed at around $4 each. Each lens
was then mounted on a drilled-out piece of PCB material using 'JB Weld'.
Once cleaned-up, the lens board was then positioned directly over the center of the LED on a machine-screw carriage mount. The carriage allowed the lens to be locked into position once it
was correctly positioned. All four lenses were tested to see which one would
correctly illuminate the fresnel while still being as close to the LED as possible.
The eventual winning secondary lens was #L10016 (.9 f/D) which allowed for
a sharp and fully-illuminated fresnel while being just a few millimeters above
The next step was to adjust the entire LED and secondary
carriage for the sharpest focus on a distant flat surface. This was done over a
distance of about 200' and was a fairly fine adjustment.
Once done, it was
actually possible to see the two fine wires connecting to the LED die on the
distant projected image.
With the final focusing taken
care of, the tone modulator and MOSFET LED driver were installed. This used an
IRF540 switching FET, driven by the digital tone signal to control the current
through the LED.
All we could do now was patiently wait for a nice
clear evening to put the system to work.