To make the light source even less obvious, a UV LED can be placed in a small black tube (I use heat shrink tubing) which serves as a "snoot" to hide the light source. The entire assembly is taped to a length of black solid copper electrical wire which serves as a stake to push into the ground.

superbrightleds.com sells a 380nm 20mW UV LED for $0.67. The part number is RL5-UV0230-380.

Use Ohm's Law to calculate the voltage drop resistor to use. You need to know the voltage and current consumption of the LED. In this particular case, the LED uses 3.5V at 30ma.

Ohm's Law is I = V / R, where I=current in Amps, V=Volts, R=resistance in Ohms.

Since I have a LOT of partially-depleted 9-volt batteries laying around, the appropriate resistor for me to use is 184 ohms. (9-3.5)/.03=183.333. However, this is not a common resistor value. A 330 ohm and 470 ohm resistor when placed in parallel equal 193.88 ohms, which is close enough. Resistors come in different watt values. Watts = Volts X Amps, 9 x .03 = .27W. Use 1/2 watt resistors if available, however since I am using two resistors in parallel I can get away with 1/4 watt resistors.

If you want to be precise, three resistors in parallel, 2.2K, 2.2K and 220 yield exactly 183.333 ohms and the LED will be at full brightness.

Viewed from the back:

Keep in mind these photos were taken with slow shutter and large aperture settings. I could not see the grass with my naked eye.

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