Introduction: Scorpion Stinger - a High Power UV LED Flashlight

This instructable is about the construction of the ultimate scorpion hunting flashlight called the "Scorpion Stinger". It functions extremely well and knocks the socks off of the competition.  Check out the videos in the "Field Testing" section of Step #13!

Scorpion Stinger features:
  • Uses a re-purposed halogen flashlight that was acquired new for $10 which provides a plastic case with plenty of room inside for custom PCB's, a high discharge 3.7V 2200mA lithium battery, recharging circuitry, a trigger switch, a wall charger, and a car charger.  What a deal for $10.
  • The custom LED driver board sports 2 identical circuits that will boost a wide range of battery voltages (3V to 5 V)  to a variable voltage output of up to 18 volts at a constant current of 200mA each circuit.  The output current can be adjusted by a changing a single resistor.  This switched mode boost power supply also has a feature of over voltage protection should the LED's become disconnected during operation. This circuit is easily adaptable to support PWM dimming.

This instructable was submitted by the author, who is a member of  the Xerocraft Hackerspace in Tucson, Arizona , for additional consideration for the "Instructables Sponsorship Program".

In the desert southwest of Arizona,   scorpions are plentiful.  Because they can squeeze into tiny of cracks, sometimes you find them in your house, too.  For the good of all concerned, it is usually best to keep your property free of scorpions. Scorpion bites can be deadly to the very old, the very young, or people allergic to the scorpion venom.  In recent news, a lady was bitten by a nasty scorpion and left her with a $83,000 hospital bill, with nearly $40,000 of that bill for the anti-venom alone.

Recently, our home owners association received complaints of a large amount of scorpions in backyards along a particular street.   An exterminator found the source of the scorpions to be a large drainage ditch that was lined with large stones.  These large stones made for a perfect scorpion habitat.  Scorpions are notoriously difficult to kill using chemicals.  The exterminator said that because the scorpion population was in a drainage ditch that was considered a waterway, and he could not treat this area with chemicals or risk the wrath of the EPA for dumping chemicals into a waterway.  So, we were left to find another more environmentally sensitive solution.

Fortunately, scorpions are florescent and glow in the dark when illuminated with certain wavelengths of ultra violet light. It is really creepy to see, especially when they start to move.  Since chemicals are not an option, it was proposed that periodic nighttime patrols of the ditches with UV flashlights would allow for manual scorpion collection.  To test this idea, we went to our local drug store and purchased their largest UV flashlight that was marketed specifically for finding scorpions.   It was tiny and the low light output made if difficult to sweep large areas of drainage ditch. 

Clearly, a more manly  UV flashlight was needed.  We needed the Scorpion Stinger...

Step 1: UV LED Selection and Schematic

Since low construction cost was an important feature,  it was decided to use 10mm LED's sourced from Ebay. To create the UV light beam,   It was decided to use 20 pieces of a particular UV LED. At $1 each,  they are fairly affordable.  If you were to use an LED that only contained a single LED emitter, you would need 100 LED's to equal the power of the 10mm LED modules because each 10mm LED module contains 5 UV emitters.

These LED's claim to have the following specs: 
  • UV LEDs with a wavelength of 400nm to 410nm
  • 5 LED drivers per 10 mm LED module
  • 40,000mcd (0.5W) output each 10mm module
  • 3.6 Volt @ 100mA
  • A nice tight 45 degree beam pattern means external optics or focusers not required
The circuit was designed in KiCAD, an awesome free program.

My circuit discussion for the functional blocks can be found on the JPG annotation.

Step 2: LED Driver IC Schematic

Since each LED requires 100mA and 3.6 Volts,  it was decided to take the 20 LED's and put 5 LED's in series and have 4 strings of LED's.   However, experimentally the LED's were found to consume 100mA at about 3.1 Volts each.

Texas Instruments has some very nice LED driver chips out there. TI also has a really nice web based circuit design tool called WebBench.  I have attached the output of the WebBench tool that describes the operational electrical characteristics of the circuit.

The TI LM3410 was selected because it has thDuplicate circuite following features:
  • Constant current driver able to source 250mA each IC
  • Boost power supply ( 3.0V to 5.0V input yields up to 18 V output - perfect for batteries )
  • Integrated switching IC (reduced circuit board parts count)
  • Low parts count
  • Very high efficiency up to 87%
  • Unfortunately it only comes in surface mount
The circuit designed for the Scorpion Stinger contains 2 each LM3410 LED IC chips.  Each LM3410 will power 2 strings of 5 LED's in series.  This means that each LM3410 will source about 15 Volts at 200mA. This is well within the theoretical limit of the LM3410.

Step 3: Case and Battery Selection

THANK YOU HARBOR FREIGHT TOOLS!  I scored this awesome 1 Million candlepower spot light that served as the case/battery/charger for my project for just $10 on sale from Harbor Freight tools item#69286
  1. Disassemble light and recycle the metal reflector, light bulb, and glass cover.
  2. Remove the "guts" and save for later.
For only $10, this light contains:
  • A pistol grip case
  • A high discharge 3.7 volt Lithium battery
  • Charging circuitry
  • AC wall charger
  • Car charger
  • Trigger switch
  • Wrist lanyard
  • Rubber lens bumper
  • Lots of wide open real estate for custom PCBs or a second battery
I have attached some dis-assembly  photographs of this flashlight.

Step 4: PCB Artwork

The PCB Artwork was created using KiCAD - an awesome free program.  Here is the artwork for both the LED PCB and the LED Driver PCB. A PDF is provided for all layers that will keep the artwork to scale so you may reproduce it yourself.  This is a two sided circuit board because of heat dissipation considerations.
  • Note the oversize LED copper pads for heat dissipation.
  • Note that all parts except the 2 driver IC's are through hole. 
  • Note that the LM3410 IC has a thermal pad on the body of the chip itself, which is critical in getting the heat away from the chip.  You might want to install a few heat vias around the LM3410 to conduct heat from the bottom layer up to the top layer (they are not shown in the artwork).

Step 5: PCB Toner Transfer

The circuit board was created using the PCB toner transfer method.  I used PCB FAB IN A BOX.  In addition to some awesome toner transfer paper, they give you a film that coats the toner to correct for any porosity in the toner so you get great results.  They claim you can do 8 mil traces. 
  1. Print your PCB artwork on the toner transfer paper
  2. Tape toner transfer paper to PCB copper clad
  3. Run through hot lamination machine
  4. Soak off the toner transfer paper in water
  5. Cover PCB with green TRF (toner reactive foil)
  6. Put PCB back through hot lamination machine to bond green TRF to toner
  7. Peel off green TRF plastic
  8. Trim PCB down to final dimensions

Step 6: Etch PCB

Now we will remove the unwanted copper to make our PCB.  I use the acid etch HCl/H2O2 method, but these chemicals are really strong.  Most people use FeCl, but I don't have the patience for it, and it is messy.  Some people say that HCl/H2O2 is more environmentally friendly to dispose of, especially when it is loaded up copper.
  1. Wear the proper protective gear.  These chemicals are really nasty. I also use a respirator.
  2. Etch circuit board in etchant
  3. Stop etching process in a bowl of water
  4. Take the round LED PCB down to the final dimension.  I used a wet tile saw.   Don't dry sand or file circuit board material because the fiberglass dust from the PCB will get into your lungs and that is bad for your health
  5. Drill holes in PCBs
  6. Remove green TRF off of PCB with acetone

Step 7: Populate Custom LED and LED Driver PCBs

Now it's time to fab the PCBs.  I acquired most of my parts from Digikey.
  1. Populate the circuit boards
  2. Test
LED PCB Construction Notes: 
  • If you do not have plated through hole boards (home brew PCB will not), you must solder the top side of each LED in addition to soldering to bottom side. 
  • Start soldering top and bottom sides of those LED's in the middle and work your way out.  This will allow the top side of the circuit board to dissipate the LED heat, too.
  • I did not trim the legs of the LED's to allow them dissipate additional heat.
LED Driver PCB Construction Notes:
  • Use your favorite cable/connector to join the LED board to the LED driver board.  Make sure the wire is thick enough to support 200mA.
  • The wire that connects the LED Driver board to the battery needs to be a heavy gauge.  It will carry somewhere  between 3 to 5 amps at times.
  • Each TI LM3410 IC will only source about 200mA.  Don't try to push them past that with this design.
  • If the LED's start to flicker you either have a low battery condition or a heat problem with the LM3410 IC's.

Step 8: Custom Paint

The plastic shell of the Scorpion Stinger was painted a really neat color of "glow in the dark".
  1. Prime the plastic outer shell with a white plastic primer
  2. Spray the plastic shell with whatever color you want, in this case glow in the dark paint available from Walmart or Ace hardware
  3. Overcoat with Clear Enamel spray paint to provide a smooth and tough finish

Step 9: Final Assembly Part 1

  1. Install LED Driver PCB into the plastic housing
  2. Secure the LED PCB with a screw into the plastic housing.  There is a hole provided in the PCB for this purpose that is aligned with a spot in the plastic housing
  3. Put the battery, charging circuit and trigger switch back into the plastic housing
  4. Attach battery to the LED Driver PCB. Cover the terminals, you don't want this to short out!
  5. Reassemble the plastic housing

Step 10: Final Assembly Part 2

  1. Install LED PCB into hole where reflector once fit
  2. Secure LED board from wobbling in the hole by using a few dabs of silicone adhesive or hot melt glue
  3. Reinstall black plastic retaining ring
  4. Reinstall black rubber bumper

Step 11: Applying Custom Graphics

The graphics on the plastic case were done by using "Waterslide Decal" paper.  It is available on the internet or on ebay.  I used the stuff for inkjet printers. I don't know why people don't talk about this stuff more often.  It is a great way to put custom graphics on your projects.
  1. Design Graphics on your PC (or Mac)
  2. Print graphics using inkjet printer onto waterslide decal paper
  3. Cut decals to final dimensions
  4. Spray decals with a spray  lacquer to seal in the ink. Do this in several coats with about 1 minute between coats.
  5. Soak decals in water for 30 seconds to 1 minute
  6. Slide decal off of paper and place onto plastic flashlight body
  7. With a towel wipe all water and trapped air-bubbles under decal
  8. Let decals dry for a few hours
  9. Overcoat decals with a layer of spray varnish (or spray enamel, although I have not tried spray enamel yet)

Step 12: Performance and Comparison Testing

Don't settle for any old small and weak drug store UV flashlight, you need the Scorpion Stinger.

The performance of the Scorpion Stinger was outstanding.  It produced lots of good quality UV light, which the "Drug Store" UV light did not do. During 30 continuous minutes of  field testing.  the Scorpion Stinger behaved very well without any heat related issues. 

A comparison test between the "Drug Store" UV flashlight and the Scorpion Stinger is very stark.

The Scorpion Stinger blows away the competition!

Step 13: Field Testing

We went hunting for scorpions with the Scorpion Stinger.  Wow, this makes hunting easy. 

With the Scorpion Stinger, the scorpions now fluoresce so much it is like they have a light bulb inside them.

These are a few photos after searching with the Scorpion Stinger.  The bright green dots in the photos are the scorpions fluorescing in the UV light of the Scorpion Stinger.

Scorpion Touch and Run

Searching for Scorpions

Scorpion florescence demonstration

Step 14: Finished Product Views - Scorpion Stinger High Power UV LED Flashlight

Here are views of the Scorpion Stinger with custom graphics. 

Since it is painted with glow in the dark paint, it really looks cool in the dark.  The custom scorpion decal actually has the scorpion glowing in the dark because of the paint underneath.

Step 15: Future Development Work

There were some technical challenges that had to be overcome on this project. Most were heat related because of the LED lights or the extreme heat generated by the boost power supply.

In all cases,  I felt that the use of a ceramic aluminum sub straight, instead of the standard FR4 fiberglass would have helped a great deal with the heat issue.  However, this material is quite difficult to cut and work with.  For this, a laser cutter would have been very handy, especially with custom shaped round circuit boards. The direct toner transfer method is good to make PCB at home, however the fine pitch of surface does pose a challenge.  At one point when fixing a solder bridge on a SMD component,  my soldering lifted a track.  This track had to be replaced with a single strand of copper wire.

Boosting 3.0 Volts up to 15.5 Volts is no trivial task.  This means that the supply current must be about 6-7 times the total LED current after you account for a 70% power supply conversion efficiency.  The LED driver circuit is usually about 87% efficient, however the battery current draw was 1 Amp higher than I expected.  Additional research is necessary, but "as-is",  the circuit works fine with the battery at the current efficiency.

In the future, a CREE style UV LED Scorpion Stinger flashlight  is planned.  However, surface mount CREE LED's generate a tremendous amount of heat in such a tiny space that these must be done with circuit board material that has an aluminum ceramic to carry the intense heat away from the module with additional heatsinking on the opposite side of the PCB. 

While using a boost switched mode power supply  IC with an integrated switch did reduce the parts count significantly, it was more difficult to deal with the heating issues of the switch.  While the circuit for the LM3410 IC behaves properly for the current configuration, there is no excess capacity in the design. 

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