Proper soldering technique is actually quite easy, but many folks just don’t get a good explanation or a shoulder to watch over. It's this construction phase, not the electronics theory, that’s the biggest barrier for newcomers. Their project falls apart, they’re discouraged and quit.
Using the recipe and materials previously shown, we’ll demonstrate how to best construct a set of battery-powered glowing eyes — two LEDs — suitable for a Halloween jack-o’-lantern or a mask.
Plug in and switch on your soldering iron, it needs a few minutes to fully heat up. While waiting, gather your parts: battery pack, wire, resistors, LEDs and heat-shrink tubing (you’ll also need a lighter, matches or heat gun for the tubing). If you have the kind of iron stand with a sponge, take a moment to wet and wring it out (in a bathroom or utility sink — don’t do this over a kitchen sink full of dishes).
I’m using red LEDs, hence the 150 Ohm resistor value from the Recipe page (other colors might need different resistors). Luckily I had some 150 Ohm resistors in my parts collection. If not, I’d just step up a slightly higher value…180 or 220 Ohm…close enough, it’ll still work if that’s all you can find.
Clip the “+” leg down to about 1/2 inch long, on both LEDs. Do the same on the resistors, clipping one end to the same length. Resistors don’t have a + or – side, either end will do.
The iron is probably ready by now.
Wipe both sides of the tip against the damp sponge to clean off any old sooty residue. Or if you have the brass “Brillo Pad” style sponge, just stab it with the iron a couple times.
Add just a small amount of solder to the tip of the iron…
…this is called tinning the iron.
That dab of solder on the tip is not for joining parts. It helps with heat transfer. Same principle as licking your finger to test the wind (don’t try this right now, you’ve been handling solder).
The idea is to transfer heat to the parts, then apply more solder to make the connection.
Touch the “tinned” iron to one side of the connection and wait about a second. After heating, you can then add more solder from the other side, waiting another second for it to spread out. Don’t melt solder on the tip and then wipe it on the parts…that makes a really poor connection!
Notice how the molten solder flows all throughout the connection via capillary action. No blobs, it’s smooth and shiny. That’s a good solder joint.
Don’t blow. Let the solder cool at its own pace. Blowing makes it brittle.
This is the essence of a strong bond. Don’t just tack the parts together; solder alone isn’t good for load-bearing. Twisting + solder makes the toughest connections…NASA requires it! Costumes are hard on electronics…pulling, flexing, perspiration, cleaning. This is why we go to these extra lengths.
Solder smoke does not contain “lead fumes.” Lead boils at thousands of degrees, many times hotter than our soldering iron. The fumes are the flux boiling off…tree sap, basically. Still, that’s no reason to breathe it…smoke is smelly and irritating. If you start doing lots more soldering, invest in a desktop fume extractor, a small filter fan that pulls away the smoke. For infrequent soldering, just keeping your face away from the smoke is usually sufficient.
As you pull the iron away, a little solder comes with it. This provides tinning for the next connection, and so forth. Every few connections, or once a minute or so, clean the tip on the sponge and re-tin it. Otherwise the flux all boils away, the solder gets gummy and doesn’t flow well.
Repeat the soldering for the second LED.
After soldering and cooling off, trim the other leg of the resistors and the LEDs to about 1/2 inch.
Cut two wires about 6 inches long…use different colors if you have them. Strip about 1/2" of insulation from both ends.
Take one of the wires (red, if you’re color-coding) and do the same twist-and-solder procedure to the free end of the resistor.
Then repeat with the other wire on the other leg.
Do this with just one of the LEDs.
Cut a couple pieces of heat-shrink tubing, a bit longer than the exposed wiring parts.
Slide these down the wires, butting them right up against the LED.
Heat with a match or lighter (or heat gun if you have one), turning the parts over to get all sides. Done right, the tubing will hug the wires.
A very common novice mistake is holding the flame too close, charring the plastic. You just want to catch the heat rising off the flame.
Cut two more pieces of wire, about 12 inches long this time, and strip both ends.
Clip the connector off the battery pack (if it has one) and strip about 1/2 inch of insulation from the wires. These will usually be stranded wire…twist it a bit to keep it from fraying.
Using the first LED for reference, cut two more pieces of heat-shrink tubing to the same lengths as before.
Now pair up the wires from the first LED side-by-side with the newly-cut ones, and slide the heat-shrink over these (the longer piece goes on the wire with the resistor). This time the heat-shrink goes over the wires before soldering. Push it down a ways, so it’s not too close to the stripped wire ends.
Twist the wire pairs together, twist these pairs to the corresponding legs of the LED, then solder the connections (clean and re-tin your solder tip first, it’s been a minute).
These three-way inline splices are tricky. If you can master this, electronics is your oyster.
After both connections are soldered and cooled, slide the heat-shrink tubes to butt against the LED, and apply heat as before.
Another common heat-shrink error is forgetting to put the tube on the wire before soldering. I’ve been soldering since dinosaurs roamed the Earth and still make this mistake.
Cut a couple more pieces of heat-shrink tubing, about 3/4" to 1" long. Slide these over one pair of wires (either the battery pack wires or the wires leading to the LEDs).
Twist the wires, solder the connections, and shrink the tubing as before.
One more heat shrink tip: keep the un-shrunk tube away from the connections while soldering; the hot wires will shrink the tubes prematurely. This stuff is just fraught with peril, isn’t it? But it’s so much nicer than electrical tape!
Notice most of our connections (except the 3-way) are inline splices…the wires join in a straight line.
A pigtail splice, wires side-by-side, isn’t recommended for wearable projects…it’s weaker if wires get pulled. Pigtails work fine for house wiring because nothing moves.
Et voila! Lights!
If everything works, you can switch off the iron, put away your tools and solder and go wash your hands.
If nothing lights up, check the most obvious: are the batteries installed the correct way in the holder? Are the batteries fresh? Do they work in another device?
If your solder didn’t flow well into connections, this is a cold solder joint. Try wiggling the wires. If the LEDs flicker, that’s probably a cold joint. Clip it off and try again. This is why having a few spare parts is a good idea.
Keep in mind that LEDs are “one way” — a specific side must connect to + vs –. If only one LED is lighting, the other might be backwards. Try clipping the wires, swap and re-solder them. If neither lights, both might be reversed, in which case you’d swap the wires from the battery pack.
Occasionally an LED is damaged by heat from soldering. This can happen if you linger too long. Solder connections should be made quickly.
If nothing seems to work right, sometimes test-building with alligator clips first can be insightful.
I put mine inside this metal dragon. When installing the LED eyes somewhere, be careful not to bend too sharply close to the LEDs, or you’ll break off the legs.
With a fresh set of AAA batteries, this is probably good for at least a full 24 hours (probably a whole year’s worth of convention outings), or a bit shorter as additional LEDs are added. AA batteries have about three times the capacity, but are somewhat larger and heavier, making them less comfortable for wearable projects.
The Adafruit Guide to Excellent Soldering discusses soldering tools and techniques in greater detail, while Collin’s Lab shows video of these methods in action. Both show circuit board soldering (rather than wire-to-wire as we’re doing here), but the essential concepts still apply.