UV lights are an important tool with a ton of uses including florescent minerals and exposure of photo sensitive materials. Short wave UV lights can be rather costly. The most popular brand is Way Too Cool who makes a quality and durable light, but they can be a bit costly. I wanted to see if it is possible to make a cheaper alternative for one of these lights. After perusing eBay for a shortwave UV lamp, I came across an entire kit that included 2 shortwave lamps and a DC driver for around $25, which was exactly what I was looking for to make. All it needed was power and a case.
My initial instinct was to use a plastic project box, but I then remembered that we are not cavemen, we have technology. I drew up an extremely simple box design and laser cut it out of 1/8″ birch plywood.
When designing the bottom plate the lamps sit against, the reflector was a concern. Light is emitted around the entire circumference of the lamp, so I didn’t want the lamp simply recessed inside of a box, as a ton of light would be lost. I began playing with the idea of a complex curved reflector, but in the end I simply added a reflective Mylar material to the bottom panel, and have the sides extend downward to shield light from going sideways and allow the light to be set down.
I wanted the light to be as portable as possible, so the battery had to built in. The driver included with the 254nm lamps required 12v, and drew just over 3 watts at that voltage. My initial plan was to make a 3 or 4 cell lithium pack with 18650 cells, but this would require a BMS board. The issue with most of the cheap BMS systems for under $8 is that they aren’t reliable, and people report they don’t actually balance the batteries. I don’t want to take any risks on this, or spend more than $20 on a power supply for this light.
In the end I decided on using a single 18650 cell to power the driver. There is a cheap protection circuit to prevent over discharge of the battery and a DC-DC converter to step the batteries voltage to 12v. The charging is handled by a cheap USB charging module. Power to this module is provided from a dc barrel jack on the lid of a box. The voltage in from the jack runs through a 5v linear regulator so that voltages other than 5v can be used to charge the battery. This is not the most elegant or efficient solution in any way, shape, or form, however, it is cheap and gets the job done. If I was making more than one of these lights I would design a custom PCB to integrate all the electronics into a single board.
Finding a battery to suit this application was rather difficult. I was looking for a high capacity cell with low drain, but most of the cells out there are designed with super high C values for electronic cigarettes. I ended up choosing the LG MJ1. This cell has a capacity of 3500 mAh and a C value of just 10A. They can be found online for just under $4 per cell. A single cell should have enough power to run the light for about 3 hours. There is still plenty of room in the case, and it can easily hold 4-5 cells, but I’m not sure why you would need 15 hours of run time on a light like this.
The electronics were soldered together then heat shrunk to prevent shorts. Everything is held in place with hot glue. Again, if I was making more than one, a custom PCB screwed to the case would be a much better solution.
|LG MJ1 Battery||$4.00|
|Lithium Protection Circuit||$0.60|
|Lithium Charge Circuit||$0.50|
|DC-DC Step Up||$1.10|
My light was much cheaper than most shortwave light out there, although it is only 3 watts. If I was to make another, I would buy just the lamps and build the driver circuit myself. This would drive the cost down quite a bit as germicidal lamps a pretty cheap on eBay.
I’m super happy with the results. If were were to do this again, I would add screws to hold the case together rather than glue, a custom PCB, a UV pass filter, and higher wattage lamps.