UV Stain Finder Flashlight – ElectroSchematics.com

Ultraviolet (UV) stain finder flashlights come in all shapes and sizes. They are useful in a variety of ways. Black lights can help you spot urine stains, fake currency, or even deadly scorpions. Today you can buy inexpensive commercial UV stain spotter flashlights, but in this post, you can find some thoughts on making your own stain finder flashlight at home using cheap and easily available components. Even with a homemade black light, you can see fluorescence, the radiating glow emitted by certain substances (and some beats) when exposed to ultraviolet light. Let’s get started!

What you’ll need at the start

Here’s the list of proposed key components required for this little 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

Ultraviolet light panel construction

The ultraviolet LED panel – the light head of the stain finder flashlight – can be fabricated on a small piece of rectangular prototyping board (the so called perfboard/veroboard). Following is the wiring diagram of the ultraviolet LED panel.

Note that the typical forward voltage (vf) of a generic 5mm Chinese UV LED is 3.4V and the typical forward current (If) is roughly 15mA. Check the actual specs of the UV LED you are going to use in your project and fine-tune the final scheme accordingly. Since the UV LEDs are powered by a 4V SLA battery, parallel combination method is employed here.

And, this is the UV LED panel part of my quick prototype:

After successful construction of the light panel, test it with a 3V lithium coin cell (CR2032 for example) to ensure that it’s okay (see below).

Set up the black light engine

Next you can complete the wiring as shown in the below schematic. Ordinary lens & reflector won’t work well with ultraviolet LEDs, so such a common white LED torch optics is not recommended here. And, that supporting optics is not very significant in this project – simply go along with your bare light panel.

For the push-on switch you should use a type that can switch a current of minimum 500mA at 5VDC. Of course, you could simply place a robust rocker/slide switch in place of the momentary button switch – which means a(ny) switch able to withstand the current and the voltage – between the output of the battery and the input of the ultraviolet light panel.

If everything is in order so far, you can take a test run of the bare model to see the effect of black light. Recall that ultraviolet wavelengths are used to excite a material that fluoresces under inspection. The material under inspection fluoresces by absorbing ultraviolet wavelengths and emitting light somewhere in the visible spectrum. It’s observed that many manufacturers of stain finder flashlights opt for 395-400nm UV LEDs, rather than more expensive 365nm UV LEDs. I’ve found that in most instances, 395nm does equal (perhaps a little better) job at fluorescing than 365nm UV LEDs.

If you want to make an exquisite ultraviolet stain finder flashlight, some mechanical work is called for but fortunately not much. The entire build can of course be built into a rectangular enclosure, and for the enclosure you can use a 3D printed one, although a handmade box is an alternative (my sun isn’t up yet)!

For the first test, I put a pair of green COB LED sticks (of course unpowered) on a black cutting mat and shined the UV light at it from circa 20cm distance. Below you can see the ‘fluorescing’ result of that experiment carried out under my basic 395nm black light engine. Fantastic, isn’t it?

The unorthodox battery charger

Now you need to think about power provision to keep the SLA battery charged up. Building your own charger circuit for a 4V SLA battery is not quite as hard as you may think. Here, it’s much better if you can build an (almost) universal USB compatible charger circuit, able to receive USB standard 5VDC supply from any USB power source and charge the battery. There are different ways in which such a simple battery charger circuit can be built without using a dedicated charge-controller chip. It’s time to try it out with an empirically proven idea. See the schematic below.

From a technical point of view there really isn’t any complex design math required to rig this up, but calling up the basic SLA battery chemistry, maximum charging current for the little (two-cell) 4V/500mAh SLA battery should be no higher than about 100mA. Besides, the higher charging voltage should be limited to 2.42V/cell, and then should be reduced to 2.20V/cell for float charging. I simply looked for a hands-down solution, and it worked!

A little cryptic battery chemistry

The maximum charging voltage available from my simple battery charger setup is around 4.7V (see above figure). I think the mysteries behind the 4V SLA battery chemistry confuse even the experts. It’s observed by me that the open circuit terminal voltage of the battery is 4.2V (4.2 to 4.4) when fully charged and 3.6V (3.6 to 3.7) under load. After the load (ultraviolet light panel) is removed the open circuit voltage bounces back.

The introduced (one-diode) battery charging circuitry is obviously not an apt answer as it is somewhat abusive of sealed lead acid batteries and can shorten service life. However, because of the simplicity of the circuit and low cost, the empiric idea is still opted for charging the cheap 4V sealed lead acid battery, especially in this project. Another interesting observation is that most batteries of this type found in cheap gizmos use a charger circuit with a voltage rating 10–15% higher than the battery. Oh, there are so many pieces to the puzzle!

I’ll come up with an improved charger design after a while. In the meantime, you may go through this guide to learn how to charge a rechargeable battery without a designated battery charger https://batteryuniversity.com/learn/article/charging_with_a_power_supply.

Understanding Ultraviolet

Ultraviolet light occurs between the visible and x-ray spectrums. The Ultraviolet wavelength range is specified as 10 nm to 400 nm (often up to 430nm). Commonly, UV light for LEDs can be broken down into 3 general areas. These are classified as UV-A, UV-B and UV-C (see table below).

The ‘upper’ UVA LEDs have been traditionally used in applications such as counterfeit detection/validation and forensics. The power output requirements for these applications are very low and the actual wavelengths used are in the 390nm – 420nm range.

Do UV LEDs pose any safety risks?

It is best to limit exposure to ultraviolet emitters as looking directly into a UV LED that’s highly directional with a very narrow viewing angle can be harmful to the eyes. The ‘upper’ UVA spectrum (which is closest in wavelength to the visible range with relatively low energy) is the safest of the three various spectra of UV light, although high exposure has been linked to skin cancer in humans as well as other potential issues such as accelerating skin aging.

If you are working regularly with ultraviolet light, then it is necessary you wear one UV safety glass for protection as it can help in absorbing harmful ultraviolet light (visible violet and blue). Usually the filter glass is clear orange, but you can find amber versions, too.

The temporal end

Last, it’s worth keeping in mind this will be just an idea for design. The result of your project is dependent on a lot of factors, including the type of ultraviolet light source, battery pack, and the optics if used. Remember, the secret of many UV emitters lies in the use of appropriate UV reflectors. Depending on light emitter position and reflector geometry, the reflected ultraviolet radiation is either broadly distributed or bundled. Well, that’s a rather in-depth look into something which may appear quite straightforward. I suggest taking a few hours and checking everything’s conceived and makes sense. As ever, any questions, feel free to comment here.

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