What is Needed to Start a PCBA UV Torch Project Here is a list of suggested key components needed for this small project:

• 395nm 5mm UV (ultraviolet) LED x 12
• 4V/500mAh SLA (sealed lead acid) battery x 1
• N/O (normally open) type momentary push button switch x1

UV light panel structure A UV LED PCBA panel - the head of a stain finder torch - can be fabricated on a small rectangular protoboard (so-called perfboard/veroboard). Below is the wiring diagram for the UV LED Panel.

Note that a common 5mm Chinese FS PCBA UV LED has a typical forward voltage (vf) of 3.4V and a typical forward current (If) of about 15mA. Check the actual specifications of the UV LEDs you will be using in your project and fine-tune your final solution accordingly. Since the UV LEDs are powered by 4V SLA batteries, a parallel combination is used here.

And, here's the UV LED panel part of my quick prototype:

After the light board is successfully built, test it with a 3V button lithium battery (such as CR2032) to make sure it is ok (see the picture below).

Set up the black light engine Next you can complete the wiring as shown in the picture below. Ordinary lenses and reflectors don't work well with UV LEDs, so this common white LED torch optic is not recommended here. Also, the supporting optics are not really important in this project - just go with the bare light PCB panel.

For push button switches you should use a type capable of switching a minimum of 500mA at 5VDC. Of course you could simply put a solid rocker/slide switch between the battery output and UV input in place of the momentary push button switch - meaning a (ny) switch capable of taking the current and voltage of the control board.

If everything looks good so far, you can give the bare metal a test run to see what the black light looks like. Recall that ultraviolet wavelengths are used to excite materials that fluoresce under inspection. The material being examined fluoresces by absorbing ultraviolet wavelengths and emitting light somewhere in the visible spectrum. It has been observed that many manufacturers of stain finder torches choose 395-400nm UV LEDs rather than the more expensive 365nm UV LEDs. I've found that in most cases, 395nm is as effective (maybe a bit better) than 365nm UV LEDs in terms of fluorescence.

If you want to make a fancy UV stain detector flashlight, some mechanical work is required, but luckily not much. The whole build can of course be built into a rectangular enclosure, for the enclosure you can use a 3D printed PCBA's enclosure, although a handmade box is another option (my sun is not up yet)!

For the first test, I placed a pair of green COB LED sticks (unpowered of course) on a black cutting mat and illuminated it with UV light from a distance of about 20cm. Below you can see the "fluorescent" results of an experiment run under my basic 395nm black light engine. Great, isn't it?

Unorthodox battery charger PCBA Now you need to think about power to keep the SLA battery fully charged. Building your own charger circuit for a 4V SLA battery is not as difficult as you might think. Here, it would be even better if you could build a (almost) universal USB-compatible charger circuit capable of receiving USB standard 5VDC power from any USB source and charging the battery. There are various ways to build this simple battery charger circuit without using a dedicated charge controller chip. Time to try it out with a tried-and-true idea. See the diagram below.

From a technical point of view, there really isn't any complex design math needed to fit it, but calling the basic SLA battery chemistry, the maximum charge current for a small (dual cell) 4V/500mAh SLA battery shouldn't be much higher than about 100mA. In addition, the highest charging voltage should be limited to 2.42V/cell, and then lowered to 2.20V/cell for float charging. I was just looking for a simple solution and it worked!

Some Mysterious Battery PCBA Chemistry The maximum charging voltage available with my simple battery charger setup that I made is about 4.7V (see picture above). I think the mysteries behind the 4V SLA battery chemistry confuse even many PCB design experts. According to my observation, the open-circuit terminal voltage of the battery is 4.2V (4.2~4.4) when fully charged, and 3.6V (3.6~3.7) when it is loaded. When the load (UV lamp board) is removed, the open circuit voltage bounces.

The introduced (single diode breadboard component) battery charging circuit is obviously not an appropriate answer, as it's a bit of an abuse of a sealed lead acid battery and would shorten lifespan. However, due to the simple circuit and low cost, the empiricism still chooses to charge the cheap 4V sealed lead-acid battery, especially in this project. Another interesting finding is that most of these batteries found in cheap gizmos use a charger circuit rated for 10-15% more than the battery. Oh, the puzzle has so many parts!

I'll come up with an improved charger design in a moment. In the meantime, you can learn how to charge a rechargeable battery without specifying a battery charger with this guide.

Understanding UV Ultraviolet light occurs between the visible spectrum and the X-ray spectrum. The UV wavelength range is specified from 10 nm to 400 nm (typically up to 430 nm). In general, UV light for LEDs can be divided into 3 general areas. These are classified as UV-A, UV-B and UV-C (see table below).

"Above" UVA LEDs are traditionally used in applications such as counterfeit detection/verification and forensics. The power output requirements for these applications are very low and the actual PCBA wavelengths used are in the 390nm – 420nm range.

Are there any safety risks with UV LEDs? It is best to limit exposure to UV emitters, as looking directly at UV LEDs that are highly directional and have a very narrow viewing angle can be harmful to the eyes. The "upper" UVA spectrum (wavelengths closest to the visible range and relatively lower in energy) is the safest UV of the three different spectrums, although high exposure has been linked to skin cancer in humans, as well as other potential problems such as accelerated skin aging.

If you work with or are exposed to UV light, be sure to wear UV protection, such as UV-safe glass, as the PCB components of these electronic devices can help absorb UV rays (visible purple and blue). Usually the filter glass is clear orange, but you can also find amber versions.

End of time In the end, it's worth remembering that this is just a design concept. The results of your project will depend on many factors, including the type of UV light source, battery pack, and optics (if used). Remember, the secret to many UV emitters is using the right UV reflector. Depending on the position of the light emitter and the geometry of the reflector, the reflected UV radiation is either widely distributed or distributed in bunches. Well, here's a deep dive into something that seems simple enough. I recommend spending a few hours checking that everything fits the picture and makes sense. As always, feel free to comment here with any questions.