Matthew Weinberger

Laser-cut Fresnel spotlight

A handheld 1.6 Mcd, 2,000-lumen spotlight with a 200 mm Fresnel lens, a parametric laser-cut body and an Arduino-supervised 3S battery pack. First place, Machine Made, BLF 13th Old Lumens contest.

2025Complete

The spotlight standing on its CPU cooler in a car park at night, beam pointed straight up into the sky.

The Whats-That-Over-There-Inator 5000

A handheld 1.6 Mcd, 2,000-lumen spotlight built around a Luminus SFT-42R and a 200 mm Fresnel lens, with a laser-cut MDF body and a Seeeduino Nano acting as a 3S battery management system.

1st place, Machine Made category – BudgetLightForum 13th Annual Old Lumens Contest (2025) Build thread: Ruckusmattster's 13th Old Lumens Entry – Machine Made

Beam into a hazy night sky Finished spotlight switched on

Specifications

Peak intensity1.6 Mcd (≈2.5 km ANSI FL1 throw)
Output~2,000 lumens
LEDLuminus SFT-42R, 6500 K, on a Convoy 5050 copper board
Optic200 mm diameter Fresnel lens, 70 mm focal length
Drive current8.5 A constant current (~60% of the LED's potential)
Battery3S1P Samsung 50E 21700
ProtectionSeeeduino Nano per-cell monitoring (3.00–4.25 V), switching the pack through an automotive relay
CoolingTower CPU air cooler
Body1/4" laser-cut MDF, 150 W CO₂ laser
CADFully parametric Onshape model

The 1.6 Mcd reading was taken before the LED was properly focused, so the focused value may be higher.

How it works

Optics

The SFT-42R sits at the focal point of the Fresnel lens, so the lens collimates it into a tight beam. Focus is extremely sensitive: moving the LED just 1.6 mm visibly changes the beam from collimated to converging-then-diverging. Laser-cut centering rings locate both the LED and the lens, which removes the guesswork from alignment during assembly.

Details and the thin-lens maths: docs/optics.md

Power path

  1. The Seeeduino Nano measures every cell voltage in the 3S pack through resistor dividers.
  2. When the power switch is on and every cell is between 3.00 V and 4.25 V, the Nano turns on a MOSFET.
  3. The MOSFET energises the coil of a JD1912 automotive relay (with a flyback diode).
  4. The relay connects the pack to a CC/CV buck-boost module set to 8.5 A with a 3.5 V ceiling, which drives the LED.

The Nano runs from its own separate 9 V battery.

Details: docs/electronics.md · Firmware: firmware/

Back panel with all electronics

Enclosure

  • Entire body cut from 1/4" MDF on a 150 W CO₂ laser, joined with finger joints.
  • Every dimension in the Onshape model is driven by variables, so you can rescale it for a different lens diameter, focal length, PCB thickness or material thickness.
  • All electronics mount to one back panel, so everything can be removed and tested with nothing in the way.
  • Interior faces are painted black to reduce beam artifacts. Exterior faces are sanded and sealed with a water-based finish, and the front text is laser-engraved.
  • The CPU cooler sticks out far enough that the light stands unsupported, either horizontally or pointed at the sky.

Build it yourself

  1. Copy the Onshape document and set the variables to match your lens, material and cooler.
  2. Use the calculated offset variable to handle the hard-to-calculate adjustments. The model assumes the same CPU cooler, so a different cooler needs this offset changed.
  3. Export the flat parts and laser cut them. A laser-cut drilling jig marks the heatsink mounting holes.
  4. Assemble the LED mount: a 1/16" PETG centering ring plus two laser-cut MDF pieces clamp the LED to the cooler with thermal paste. The heatsink screws go into a wooden block that bolts to the back panel.
  5. Wire the electronics on the back panel (see docs/electronics.md) and flash firmware/bms/bms.ino.
  6. Focus by adjusting the LED-to-lens spacing with shims. Check it in a dark room, ideally with fog or haze.

Full parts list: BOM.md

The electronics aren't specific to this build. Any way you choose to drive the LED will work.

Drilling the cooler with a laser-cut jig LED clamped to the cooler SFT-42R on its Convoy 5050 board

Painting interior faces black Finished enclosure with engraved front

Lessons learned and future improvements

  • Driver current. The buck-boost module tops out at 8.5 A, which leaves the SFT-42R at about 60% brightness. The target is around 13 A, with a module rated for 15 A.
  • LED mount. Two 1/8" MDF pieces are probably the main source of beam artifacts. Sheet metal would be better.
  • Assembly access. There's very little room to hold the nuts that mount the heatsink. Tapped holes in the heatsink would fix this.
  • Focus adjustment. Shimming works, but you need very thin shims to get it exact. A planned alternative: a spring-tensioned screw through a threaded hole in the heatsink sets the position, then screws through slots perpendicular to the lens lock it in place.
  • Optical losses. Next step is measuring lens losses cheaply by heating an insulated metal target and comparing thermocouple readings with and without the lens.

Beamshots

Beam on a tall building Light standing on its cooler, beam straight up Beam skimming the ground

Credits

  • A friend from the Badger Solar Racing team guided the BMS design, donated the cells, cell holder and relay, and helped characterise the cells.
  • The BMS code was drafted with ChatGPT and reviewed by both of us.
  • Thanks to the BudgetLightForum community for feedback in the build thread.