FmC's Hand Made DIY 2015 comp entry; "WW2 Searchlight". Finished!

That mirror looks great, but too long a focal length, & a little too large. Also, I don't think they are going to ship for 'free' somehow :)

There's quite a few places selling it, & the cheapest I could get one for is around $145.

Maybe for a long focal length like that, you could use a microwave dish. It seems the main problem would be getting it smooth and shiny. A simple way might be to cover it with aluminum foil.

I spent a little time yesterday making a parabolic cut-out. I'll take it with me to the shops today, on the chance that I find a bowl or something with the same contour.

And a question for the driver gurus;

If I were to piggyback 3 or 4 FET's on the same board, will the 13a have a strong enough drive signal to operate them all?

With a nice high gate voltage a good FET can run up to 100A.

Feel free to chime in over here...

Little bit of progress yesterday in post #1.

Actually, It may have been a step backwards.... :~

& a small step forward today in post #2.

Sorry to hear about the set back, but it's great to see you have a sense of humor about it with "Jack" and all.

A little more progress today in post#3.

At least the write-up is already excellent, great reading, that will impress the judges as well :-) , I hope that the mirror will work out in the end. Keep going!!!

Light engine is looking great and so it your write up and pics. :)

Jacks a good looking assistant but he appears to goof of as much as my two assistants. I bet if the three of them got together there'd be a bunch of trouble. Its good to see some progress on the light.

That shallow, long focal length Edmond Scientific reflector looks like it should be used in a retro configuration. The light sources they used in WWII search lights emitted in all directions.

What about using your template to make a clay negative and forming the plaster over that

Yep, been there, done that yesterday.

See end of post #1 for latest update.

While I was researching making parabolas, I came across the term 'spinning' a few times.

As it turns out, a liquid that is spun at a constant speed in a container forms a parabola on it's surface. The faster the rotation, the shorter the focal point.

This is what I'll be trying next, by using a low viscosity epoxy, or plaster mix.

The key seems to be keeping a consistent speed, whilst the solution sets.

This is going to produce some serous lumens. :slight_smile:

Others of my group did that when I was in Germany, to use as a light gathering mirror. I think we used polyester. It should start as a liquid and not change volume too much when it hardens. I don’t recall how we silvered the surface. Evaporating aluminum on it in a vacuum probably gives the vest shine, but without a vacuum one could stick aluminum foil to the surface.

The 1964 book “Direct Use of the Sun’s Energy” by Farrington Daniels has text about making parabolas and a photo-plate showing how they made a concrete mold to form plastic parabolic mirrors. It’s out of print but cheap if you can find it. Paid $2 for my copy.

Lore from fellow radio nuts has it that the best satellite dish to use as an antenna reflector is the old “Dish 500” model; newer ones have an offset designed in for the receive head location. Parabolic reflector principles are the same for light or radio, same stuff different wavelengths.

Don’t think outside of the box, throw the box away!

Phil

Thanks Phil.

A common satellite dish was one of my first thoughts, but for the (yet to be divulged) body, the focal length is too long. I need the focal length to be no more than ~25cm.

I started piecing out a jig to use for spinning up the reflector tonight - hopefully I'll be at the stage where I'm making a test run with it on the weekend.

I bet you will get it sooner or later. Lots of great ideas come from trial and error. Been there done that. Keep at it. You will get there yet.

Bump for today's updates at the end of post #1.