UV Flashlight

A UV source for the bench — checking gels, spotting contamination, finding what a spray bottle missed. Three high-power emitters on star PCBs, a constant-current driver, a switch, and a lump of aluminium to dump the heat into. There is no microcontroller in it and nothing to flash.

The electronics are finished; the printed body isn’t

There are no STLs in this folder yet, so what follows is the parts list, the circuit and the two things that will bite you. That is enough to build the light on a bit of aluminium bar today. The body follows when the CAD is worth publishing.

If what you actually want is a UV sensor — an LTR390 reporting UV index into Home Assistant — that is a separate project with a complete ESPHome config.

365 nm, or don’t bother

This is the only decision on the page that you cannot fix later, because it is baked into the emitter you solder down.

Band	Wavelength	What it’s for
UV-A “blacklight”	395–405 nm	Resin curing, pet stains, party props. Sold as UV, barely is
UV-A	365 nm	Fluorescent dyes, gel work, contamination checks
UV-B / UV-C	Below 320 nm	Sterilisation, and a different set of hazards. Not this project

395 nm emitters are cheaper, and to your eye they look far brighter — which is exactly the problem. What you are seeing is visible violet, and that violet floods the thing you are trying to look at. A 365 nm emitter of the same wattage looks disappointing right up until you point it at a gel, at which point the gel is the only lit thing in the room. Buy 365 nm and accept that it looks dim.

Parts

Some links below are Amazon affiliate links. As an Amazon Associate I earn from qualifying purchases — at no extra cost to you. It helps keep these guides free. Outside the US, they should send you to your own Amazon store; if one doesn't, searching the part number there will find it.

Chanzon 3 W COB UV emitter, 10-pack — read the caution below: these are the 395 nm ones. They are the emitter this was prototyped on and they are excellent for curing resin and for props. For gel work, buy the same physical part at 365 nm instead. Either way the numbers on the listing are what the rest of this page is built on: 3.0–3.2 V, 400–500 mA per chip.
20 mm star PCBs, 25-pack — the aluminium-core discs the bare chips solder onto. The emitter is useless without one; there is nowhere else for the heat to go.
CC/CV step-down module — 6–40 V in, 1.2–36 V out, with a current trimmer as well as a voltage one. Ships as a 2-pack. This is a driver, not a power supply: you set the current and it holds it as the LEDs warm up and their forward voltage sags.
12 V 4 A supply for the bench version, plus 5.5 × 2.5 mm panel jacks. That brick’s plug is 2.5 mm ID, and it will sit loose and intermittent in the 2.1 mm jacks that come in most assortments — check the number before you order, because the two sizes look identical in a photo.
Aluminium heatsink bar, 100 × 25 × 10 mm, 5-pack — the stars bond to this. Cut one to length and it is also the spine the printed body wraps around.
Silicone thermal adhesive — this is what bonds star to bar. Not hot glue, not epoxy, and not thermal tape, which is fine for a MOSFET and marginal under a watt and a half of LED.
A switch. Mini momentary buttons, 30-pack at 1 A rating, or a latching rocker if you want it to stay on without being held. The momentary is the better choice, for a reason worth reading before you decide: see Making it handheld.
24 AWG silicone hookup wire — silicone, because the strands nearest the emitter live somewhere warm and PVC insulation shrinks back off a joint you cannot see.
PETG filament for the body when the files land. Not PLA — see Heat.
M3 heat-set inserts and an M3 screw assortment for closing the body onto the heatsink bar.

The emitter linked above is 395 nm, not 365 nm

That is the substitute, and it is a real difference rather than a shade of purple. A 395 nm chip puts most of its output where your eye can see it, so a gel under it sits in a violet wash. Everything else on this page — the star, the driver, the current, the thermal path, the arithmetic — is identical for a 365 nm chip of the same 3 W package, so build it either way and swap the emitter when you order the right one.

If you only want a torch, buy a torch

Building this makes sense when you want a specific form factor, a fixed bench head, or three emitters’ worth of output. If you want a 365 nm light in your pocket by Thursday, the DARKBEAM 365 nm is cheap, USB-C rechargeable and focusable, and it is worth owning either way as the reference you check a build against. The BORUIT V10 is the same idea with red, green and blue inspection modes in the same body.

Wiring

Three emitters in series is the arithmetic that makes a 12 V brick work. 3.1 V × 3 puts the string at about 9.3 V, which leaves the buck converter the couple of volts of headroom it needs and lands the whole light at roughly 4 W. Four in series is 12.4 V and will not regulate off 12 V at all.

From	To	Notes
Barrel jack +	Switch
Switch	Driver IN+	Switch the driver’s input, never its output
Barrel jack −	Driver IN−
Driver OUT+	Star 1 +
Star 1 −	Star 2 +	Series
Star 2 −	Star 3 +	Series
Star 3 −	Driver OUT−	Closes the loop

The switch goes on the input side because opening the output of a constant-current supply drives its loop straight to the voltage limit, and the current overshoots when you reconnect. Switch the 12 V in and the driver starts clean every press.

Setting the drive current

Do this before the LEDs are anywhere near it. A CC/CV module arrives with both trimmers wherever the factory left them, and one wrong turn kills three emitters.

Turn both trimmers fully anticlockwise and feed the module 12 V with nothing on the output.
Short the output through your meter on its 10 A current range. Wind the voltage trimmer up until current starts to flow, then set the current trimmer to 0.45 A — mid-range of the emitter’s rated 400–500 mA.
Remove the short. With the output open, set the voltage trimmer to about 10 V. That is above the string’s 9.3 V and well below the 12 V input, so an accidental open circuit cannot spike.
Connect the string. It should settle around 9.3 V at 0.45 A, and the module’s CC indicator should be lit — if it is in CV instead, the current trimmer is set higher than the string will draw.
Feel the bar after a minute. Warm is correct. Too hot to keep a finger on means the thermal path isn’t finished, and you should turn it off rather than find out how it ends.

Do not run a UV emitter straight off a battery through a resistor and call it done. Forward voltage falls as the die heats, current climbs, the die gets hotter, and the failure mode is that the light quietly loses half its output over a few hours and never gets it back. That is what the constant-current module is for, and it is the reason it is not optional here.

Heat

UV LEDs are far less efficient than white ones. Most of what you put in comes straight back out as heat, and unlike a white LED they do not fail loudly — they just dim, permanently, and you have no way of knowing by how much.

So: emitter onto a star, star bonded to aluminium with thermal adhesive, and aluminium in contact with air. Three stars at 450 mA is about four watts into a 100 mm bar, which the bar handles on its own without a fan.

The printed body is a shell around that bar, not a heatsink, and it should touch the aluminium as little as it can get away with. Print it in PETG. PLA goes soft around 60 °C, which a 4 W head reaches without trying, and PLA is also the filament that yellows and embrittles under exactly the wavelength this thing emits. A UV light with a PLA body is a slow experiment in photodegradation conducted on itself.

The filter is the upgrade that matters

A ZWB2 / UG-1 filter is a piece of glass that passes UV and blocks the visible violet riding along with it. Without one you get a bright purple flood and washed-out fluorescence; with one the beam looks almost dark and whatever fluoresces is the only lit thing in front of you.

Nothing in this build needs it and every part of this build is better with it. ZWB2 filter glass is sold in 20 mm and 25 mm rounds that sit neatly over a star, and it is also the reason a cheap emitter becomes usable: these chips leak a broad shoulder of visible light, and the filter takes it away.

Making it handheld

The bench version is the one the parts above build. To carry it, the constraint is the driver — it needs 6 V minimum on its input, so a single 18650 at 3.7 V will not run it no matter what else you do.

Two cells in series is the answer. Use Samsung INR18650-35E cells in snap-in holders, wire the holders in series for 7.4 V nominal, and drop the string to two emitters — 6.2 V instead of 9.3 V, which is what leaves the buck any headroom at all off a pack that starts at 8.4 V.

You get roughly the top half of the pack that way. Below about 7.2 V the driver runs out of headroom, falls out of constant current, and the light starts to dim. That is a readable low-battery warning rather than a fault, and it is the price of not carrying a boost stage as well.

Charge them out of the light on a four-bay cell charger rather than building a balance charger into a torch. Two removable cells and no BMS is a simpler and safer object than a sealed 2S pack you cannot inspect.

Use the momentary switch, not the rocker. A deadman switch on a UV source means it cannot be left on, face-up, on a bench, pointed at whoever walks past next. That is worth more than the convenience of a latch.

365 nm UV damages eyes and skin and does it without hurting at the time. Wear UV-blocking eye protection — those are tanning-bed goggles, so they cover the eyes and nothing around them. Clear polycarbonate safety glasses block this band too and cover more of your face. Tinted plastic carrying no UV rating blocks nothing, whatever it looks like, and fashion sunglasses are worse than none because the dark lens opens your pupil. Never point it at anyone, never look into the emitter to check whether it is on, and remember that cheap chips leak wavelengths shorter than the one on the label.

Files & downloads

The source files for this project aren't in the repo yet — there's nothing to download beyond the notes. They land in projects/uv-flashlight/ when they do, and this section fills itself in.

Every file for this project on GitHub →