Make a signal flashlight that lets you talk to fireflies using a green LED.

Supplies

If you're new to soldering, this is a good second project. If you can solder two wires together, then you're ready to go!

I had a class group assemble these (after I'd done some pre-assembly to save time), and a group of novice solderers assembled their flashlights with ease.

(I needed a classroom-sized set of these, so I ordered enough parts to make 20 flashlights. If you only want to make one flashlight, you might choose different suppliers that will sell you individual parts.)

Here's what you'll need:

Electrical Components:

A green LED. I used Adafruit part number 298, because it emits light at a wavelength of 565 nanometers, which is outstandingly-close to the colors emitted by most North American fireflies! (This LED takes 2.2 to 2.5 VDC at 20 mA.)
Battery holder for two AA-size batteries. I used Digikey part BC12AAW. (Make sure that your battery holder holds the two batteries end-to-end, or "inline," so that it will fit into the flashlight's handle!)
Holder for LED. I used Adafruit part number 2175. (This is intended to fit into an 8mm hole, but will fit in a 5/16-inch hole.)
Panel-mount, momentary push-button switch (normally-open). I used these from DIYHz, but others should work just fine!
Panel-mount, round rocker switch. I used these from Apiele, which already had wires attached. (They're intended to fit into a 20mm hole, but snap nicely into 3/4-inch plastic pipes!)
33-ohm resistor. (If you use a slightly-different LED, you may need a slightly-different resistor.)
Two AA-sized batteries.
Some thin wire. I used 24-gauge wire for my project. Each flashlight needs about 6 inches of wire in all.

General Supplies:

3/4-inch plastic pipe. You'll need a 6-inch length for the "handle," and an additional short (about 1/4-inch long) piece.
1-inch-wide metal washer. If you're lucky, your hardware store will have a 1-inch-wide washer that already has a 5/16-inch hole. If not, you'll need to find a washer with a smaller hole, then drill a 5/16-inch hole in it.
Elbow joint for 3/4-inch plastic pipe.
Heat-shrink insulation tubing. (Alternatively, you can use electrical tape.)
Super glue. (Hot-melt glue will also work, but it might not stand up to as much wear-and-tear once kids get their hands on the flashlight!)

Tools:

Wire cutters and strippers.
Drill and 5/16-inch bit.
Small, needle-nosed pliers. (I needed two pairs to install the push-button switch.)
Soldering iron and solder.
Scissors.
Heat gun (if you're using heat-shrink insulation).

Step 1: Make the Lamp Assembly

Prepare the washer. If your 1-inch-wide washer doesn't have a 5/16-inch hole in it, then use a drill to widen the hole. (5/16-inch is a good fit for the 8mm LED holder.) If you'd like, you can paint the washer black, to help your LED stand out.
Place the LED into the LED holder.
Snap the LED holder into the washer.
Solder the resistor to the LED's negative terminal -- that's the short wire. (Depending on the length of your resistor and LED, you may want to trim the wires so that the whole LED-and-resistor assembly will be less than 2 inches / 5 cm long.)
Solder a short length of black wire to the resistor. (About an inch or two should be enough.)
Solder a short length of red wire to the LED's positive terminal -- that's the longer of its two "legs."

(When I did this as a classroom project, I made the lamp assembly for each flashlight beforehand. With that out of the way, the group was able to assemble the rest of the flashlight in a one-hour session!)

Step 2: Prepare the Push-Button Switch

Solder two short lengths of black wire to the terminals of the push-button switch. (The color helped me to keep things organized, but it isn't essential.) Use either heat-shrink tubing or electrical tape to insulate the connections. (You don't want any unwanted connections, and those wires will end up bending around quite a bit!)

Step 3: Attach the Push-button to the Plastic Pipe "Elbow"

Drill a 5/16-inch hole in the elbow, to give you a place to attach the push-button switch. (The plastic pipe elbows that I found had a convenient flat spot from the manufacturing process. It's the perfect spot for attaching the pushbutton!)
Insert the push-button into the elbow, and tighten it down with the included nut. (I found that it takes two sets of pliers for this fiddly step: one to hold the switch in place, and another to tighten the nut. It's a great opportunity for teamwork with a friend!)

Step 4: Attach the Lamp Assembly

Cut a small ring from a length of the 3/4-inch plastic pipe. I found that a length of about 1/4-inch worked well, but the exact size isn't important. (A plastic-pipe cutting tool is a great asset here, since it doesn't create sawdust!)
Press the ring into one end of the elbow.
Slide the lamp assembly into the elbow, so that the washer sits up against the ring. When you're satisfied with the fit, glue to washer to the ring. (I found that super glue was much more kid-resistant that hot-melt glue.)
Solder one of the black wires from the switch to the black (negative) wire from the lamp assembly. Insulate the connection with either heat-shrink tubing or electrical tape.

The "head" of the flash light now has two loose wires emerging from it: a red wire from the lamp assembly, and a black wire from the switch. Ready for the next step!

Step 5: Attach the Rocker Switch to the Handle

Cut a 6-inch length of the plastic pipe to serve as the flashlight's handle.
Press the rocker switch into one end of the pipe. (A switch for a 20mm hole fits remarkably well in a 3/4-inch plastic pipe!) The wires should extend from the open end of the handle.

The rocker switch that I used already had wires attached to it (which were, confusingly, red and black). If your switch doesn't have wires, you'll need to attach about 6 inches of wire to each of its terminals.

Step 6: Add the Battery Pack to the Handle

Put two AA batteries in the battery holder. (Rechargeable or single-use batteries will both work.)
Slide the battery holder into the handle so that the all the wires stick out of the open end of the handle.
Connect the negative (black) wire from the battery holder to one of the wires from the switch; solder and insulate the connection to make it permanent.

The switches I used came with one red and one black wire attached, which can lead to some confusion. I needed to attach the negative (black) wire from the battery holder to one (red) wire from the switch. Afterwards, I had a much-more clear situation where only one red and one black wire emerged from the handle: a red wire from the battery's positive terminal, and a black wire that lead through the switch to the battery's negative terminal.

In the end, there should be two loose wires emerging from the handle: one red, one black.

When in doubt, double-check the wiring diagram!

Step 7: Connect the "Head" and "Handle"

Solder the one black wire from the handle to the one black wire from the lamp assembly. Insulate the connection, either with heat-shrink tubing or electrical tape.
Solder the one red wire from the handle to the one red handle from the lamp assembly, and insulate the connection.
Press the handle and "head" sections together. (It may take a little coaxing to get all of the wires to fit inside. Thank goodness you insulated all of your connections!)

Your firefly-signalling flashlight is fully assembled!

Step 8: Talk to Fireflies

When your flashlight is just being stored (or is bouncing around inside a backpack), you can switch the rocker switch "off" to prevent any unwanted flashing or battery draining.

When you're ready to take your flashlight out on a firefly-searching hike, switch the rocker switch "on."

Try to repeat the flashing patterns of the fireflies you meet, and see if any come for a visit (like the male Photinus firefly in the photo above)! It's an amazing way to communicate with your blinking neighbors!

For some great information about using this flashlight to talk to fireflies, check out:

The excellent guidebook "Fireflies, Glow-worms, and Lightning Bugs" by Lynn Frierson Faust.
The Massachusetts Audubon society's Firefly Watch program has a great guide to firefly-flashing patterns.

[Special thanks to my colleague Janean Kazimir for her advice on talking to fireflies, even though she says "firefly" and I say "lightning bug."]