A 1.6Mcd 2k lumen Fresnel lens spotlight
full CAD: Onshape
github: GitHub - ruckusmattster/laser_cut_spotlight
this project utilises 1/4 inch laser cut wood, a 200mm diameter fresnel lens, and a sft42r to create a handheld spotlight featuring: a calculated runtime of 3 hours, an arduino nano as a BMS, and a constant current buck driver.
short overview through photos:
Drilling mounting holes on a drill press using a laser cut jig to mark the holes
The next two photos show the LED mount, it uses 1/16 inch petg as a centering ring and two lasercut pieces of MDF secure the LED in place. The screws through the heatsink connect to a woodenn rectangle that is then bolted to the back face of the enclosure.
This is particularly unfriendly to work on as i didnt leave the builder much space for holding any of the nuts required to mount the heatsink - threading the holes would have helped but i couldnt locate the taps.
The mounting pressure is sufficient as you can see some squeeze out
These next two show the finished back panel - all electronics are mounted here - more details on this can be found below.
This is the first version of my design that i used for prototyping being glued together.
The next photos show the sanding and finishing stage of the final box. All inward facing parts were painted black and all outer faces were sanded and sealed with a water based finish - I am very happy with the achieved surface finish. I used the laser engraver to engrave the text on the front. The box is decently water repellent and i felt confident setting it down in the snow, its a very smooth finish thats pleasant to touch, though I was able to nick it with a box cutter quite easily.
This video is mostly irrelevant as I’m just cutting a test part, but its a really cool shot of the laser cutter (150W CO2 )
200mm Dia, 70mm focal length (a longer focal length would work too but you will likely end up wasting some light)
LUMINUS SFT-42R 6500K
Air Cooler (plenty overkill but i like the look)
CC CV Buck Boost Converter (would not reccomend - it has a max current of 8.5A - if you find a similar module that can handle up to 15A please let me know)
arduino nano
latching switch
JD1912 Automotive Relay
generic mosfet (controlled by the arduino to control the relay)
various resistors (for both a voltage divider and for controlling the mosfet + relay)
generic diode (for flyback current on the relay)
3x samsung 50e and suitable holder
thermal paste
1/4 inch MDF for entire body
1/16 inch petg (or any suitably thin material) as a centering ring
various screws
The battery pack is 3s1p and the arduino has its own 9V supply just in case.
This CC driver is fine, but i would like to run the sft42r closer to 13A, we are around 60% brightness right now. I considered a typical 20mm buck driver from convoy but i wasnt sure how it performs thermally. The current driver doesnt overheat but it does get pretty warm.
Huge shoutout and thank you to my friend from the Badger Solar Racing team - he guided me througb setting up the BMS and donated all the cells used, a cell holder, and a relay. He also helped me characterize a couple cells I had left at full charge for a while.
The schematic calls for using two MOSFETs, I didn’t have any suitable on hand at the time so swapped it out for a single automotive relay which was gifted to me. The code for the BMS was written by ChatGPT and reiewed by me and my friend.
Its a little finicky as you would need to have the exact same cooler, but thats what the “calculated offset” is for - making the hard to calculate adjustments.
Otherwise the electronics arent particularly specific, any way you decide to drive a LED will work.
I will update this section when im back and have access to my Opple and a luxmeter
Additionally, having all the electronics mounted to a single face of the light makes it very easy to service as everything can be taken off and tested without any obstructions.
Having the inside painted black has done a fair amount towards reducing artifacts, although i belive the majority of artifacts comes from the LED mount relying on two 1/8" pieces of wood. At the time i created these the kapton tape was out of stock, had it been available, I would rather have used sheet metal.
A side effect of the CPU cooler sticking out so far from the body of the light is that it can stand freely without a tripod horizontally or towards the sky. You can see this in the beamshots.
During the design phase, a lot of my coworkers asked if i would make it a bat signal, after being badgered about this for almost two years now, I relented. One face has a 1/4" tall hole for inserting negatives. the lens can be defocused by adding a extra spacer. Combined, this creates a larger spot with a decently sharp image cut out from out (in theory).
Looks nice, was it inspired by the bat signal?
Theres actually a slot on one face for inserting shadow shapes - then a spacer can be installed to increase the spot size and hopefully create a bat-signal-esque effect 
i will hopefully demo this after winter break
I’d like to see that
almost fully finished updating the writeup. ill add some videos of the case, measurements, and more beamshots when i go back to uni.
in the meantime are there any more details you are all interested in?
Awesome build mate.
Very neat build and great documentation Ruckusmattster!!!
And thanks for the laser cutting video, very cool. 
Thank you, I’m excited to use this laser cutter more, it’s absolutely brilliant how fast I can iterate designs and have large accurate parts.
I’m thinking of entirely redoing my entry from last year in a much smaller form factor using the laser cutter.
Just need to find a way to get 340VDC (roughly) from 120VAC ideally without a big transformer
Really neat build! Where did you get the Fresnel lens and what are the search terms to get the right kind? I experimented with Fresnels but the one I had just converged the light again instead of collimating it.
Under “parts used” you can see the exact one i used - let me know if this link doesnt work - i recall people having issues when the link ends in “.us” but I’m not sure how to get a regular link.
this is the link:
https://www.aliexpress.us/item/3256807718403632.html
Are you familiar with the thin lens equation?
1/f = 1/u + 1/v
f = focal length – u= source to lens distance – v = lens to smallest point of the beam
in the case that we use this lens on direct sunlight (the easiest way to get a collimated source) then the equation looks like this:
1/f = 1/∞ + 1/v
in this case u → ∞ and 1/∞ = 0. as our source is very far away/collimated all rays of light are coming in parralel to the lens and being bent to a point
we want the opposite of this, we want to take our LED - which we’ll call a point source - and have it be collimated so that v is ∞. then our equation will come out to 1/f = 1/u + 1/∞ which is the same as f=u.
in the case that the LED is correctly positioned at the focal length of the lens it must create a collimated beam. Are you sure that your LED is correctly positioned?
it will be easy to see this effect in a dark room with a fog machine - as you move the LED from the focal length away the lens you will see the smallest point of the beam moving towards the lens as f is constant.
do you have any photos of the lens or a link to where you bought it?
Thanks for the tip. I ordered a 28 mm FL lens, but eyeballing it around the 28 mm range didn’t collimate the light. Maybe they sent me the wrong one.
The thin lens equation makes sense. My current thing to figure out is how conservation of etendue affects flashlight design.
do you have a link to the lens you got?
was there anything else in the path other than the lens?
Nothing else. Lens is here 1 Stück 50 55 60 70 80 90 100 mm runde Fresnel-Linse optische PMMA-Kondensator-konzentrische Linsen mit Gewinde für LED-Lichtprojektor VR - AliExpress 1420
D 90 F 25 mm
I just looked again at your build and it is brilliant… “What’s-that-over-there-inator”
I ought to learn to use the laser cutter in our lab. I could 3D print the parts but 3D printing flat parts is not great.
You can share the Onshape read-only, then everyone who wants to edit it can make a copy for their own.
Need to run the calcs on whether SFT-42R or SFT-90 gives more throw.
Once you learn the laser cutter and the basic laser cutter related tools in onshape
Life will change
That lens looks good to me, I’ll try update the post with my onshape in about 17hrs
Hmm. Maybe I’ll give my original lens another try. Perhaps the positioning needs to be more precise than I imagined to focus properly (I just eyeballed it last time).
there is a 1.6mm difference is spacing between these two photos
curious. so fine adjustment is definitely necessary!
I will try and design some (screw-based?) fine focusing mechanism.
have you got an estimate of how much of the light is reflected off the inner surface of the lens instead of going through?
I bet this would be an amazing application for a wavien collar.
