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John Bokma Pet scorpions
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Scorpion detection using UV LEDs

Glow in the dark | 20 comments

Note: UV light can seriously harm your eyes.

Some time after reading "A Powerful New Light Source for Ultraviolet Detection of Scorpions in the Field" I decided to build my own scorpion detection device using a bunch of UV LEDs instead of buying an UV flashlight. I ordered fifty 5mm UV LEDs (light emitting diodes) at the website of LSDiodes.

UV LED Specifications

UV LED scorpion detector circuit diagram

Since at the time of writing I have found 4 scorpions on my roof, I decided that I could simplify the design of the scorpion detector by using an universal adapter with an extension cable instead of batteries for power. I have plans to make a portable UV scorpion detector later on.

Circuit diagram of the scorpion detector using UV LEDs
Circuit diagram of the scorpion detector using UV LEDs

The maximum voltage of the adapter I bought is 12V, so with a typical forward voltage of 3.6 V, three LEDs can be connected in series. In order to limit the current to 20 mA, a series resistor of 56 Ohm (5%) was chosen, which is close to (12 - 3 x 3.6 V ) / 20 mA = 60 Ohm. It's more common to select the next higher value (68 Ohm in this case), but since the worst case current, 1.2 V / ( 56 Ohm - 5% x 56 Ohm ) = 23 mA, is well within range, I decided to use 56 Ohm.

Important note: do check the actual output of the adapter you are going to use under load. On the one I use the switch position says 12V, but the actual output is over 15V. I managed to kill quite some UV LEDs of my detector before I rememebered to check the actual voltage.

UV LED scorpion detector circuit board

I bought a small predrilled circuit board with some copper traces. I had to cut some traces and make new connections using parts of the cut off legs of the LEDs. I grouped the LEDs in 4 x 3 rectangles.

Bottom view of the scorpion detector circuit board
Bottom view of the scorpion detector circuit board

I soldered the resistors on the underside of the circuit board. I soldered first one group of 12 UV LEDs, and tested each row after I made the final connection of the row.

Top view of the scorpion detector circuit board
Top view of the scorpion detector circuit board

When I tested each row of 3 LEDs I made sure that I covered the LEDs first with a piece of thick paper since I didn't want to damage my eyes. After finishing the first group, I used the same way of soldering and testing on the 3 other groups. The final result has 48 UV LEDs.

UV LEDs burning behind a piece of thick paper
UV LEDs burning behind a piece of thick paper

With the finished ultraviolet scorpion detector I was able to make a living scorpion glow from a distance circa 5m (ca. 16 feet). Note that the UV LEDs get quite hot when burning, and the article I read recommends to use a fan to cool the UV LEDs.

Scorpions under UV light examples

I used a cheap webcam (around 16 USD) in combination with VirtualDub to make a few movies. Irfanview was used to create snap shots.

Male scorpion, species Centruroides flavopictus, glowing under UV light
Male scorpion, species Centruroides flavopictus, glowing under UV light

The actual glow color is more yellow / green. The effect can be best described as if the scorpion has been covered with glowing dust.

Close-up of the male scorpion's glowing stinger
Close-up of the male scorpion's glowing stinger

I have no idea how much damage the "black light" can do to a scorpion. As far as I can tell they notice the ultraviolet light and try to run away from it. When I use a normal flashlight they seem less annoyed.

UV LED Related

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