T3W8: Soldering LEDs

This week we will solder the LED strip to the microcontroller that runs the wall clock. It's just three wires, but it will give us some experience with typical soldering tasks like tinning wires and soldering on PC boards.

Step 1: Cut and Strip The Wires

The LED strip has three wires: +5V power (red), DataIn (green), 0V ground (white). Normally ground would be black or green but this seems to be how WS2812 LEDs strips are wired. We will cut a short length of the three-conductor wire (maybe 5 inches) and strip maybe 1/8" of insulation from both ends of each of the three wires. Once you find a hole on the wire stripper that works well, keep using it for the other wires. When done, the wires should all be the same length and have the same amount of insulation stripped, so if there are problems, clip of the end of the wires and try again. Also if you strip off too much insulation, strip the same amount from the other wires and then clip the ends of the stripped wires shorter.

Step 2: Tin the strip contacts and one end of the wire

To make it easier to attach the wires to the LED strip, we will "tin" the ends, or put a coating of solder on the ends of the wires and on the bare copper contacts on the strip. Only tin one end of the wires, the other will go into the holes on the microcontroller board. To tin the wires, hold the wire with the third hand tool, heat the end of the wire with the iron, and then melt some solder onto the end of the wire. Don't leave a big blob, the tinned wire should have about the same diameter as before, just with solder holding the strands together.

To tin the contacts on the LED strip, it might be easier to put a blob of solder on the tip of the iron, and then touch the pad on the strip with the solder blob until some transfers to the strip and covers the exposed copper. Be careful not to "cook" the strip for too long with the iron, it will melt or blacken the plasticYou can add a little more solder to the pad so that it is raised above the strip, but don't leave a big ball of solder - if you do get a ball, try to wipe it off the strip with the solder iron tip.

Step 3: Attach the wire to the LED strip

Line up the tinned wire ends parallel to and on top of the tinned end of the LED strip. Make sure the red wire is lined up with the +5V label on the strip. Then remelt the solder to attach the wire to the strip - just a quick touch with the cleaned tip of the iron should do it. The strip should be flat on the soldering mat so you can press down on it with the iron, so you may have to put something on the strip to keep it flat on the table (it will want to curl).

Step 4: Attach the wire to the microcontroller board

Put the other end of the wires through the following holes in the microcontroller board: red to 5v, white to G, and green to D4. This is not the order of the wires in the bundle, you will have to swap the green and white wires, but all three holes should be next to each other on the board. Insert the wires from the side of the board with the USB connector and black IC so you can solder them on the other side with the metal rectangle. Bend the end of each wire around the board to hold it in place, and put tension on the wire so it doesn't fall out. Then flip the board over and solder the wire into the holes on the circuit board. Repeat for the other two wires, then trim the ends of the wire on the board with the wire clippers. Be very careful not to short out the wires with blobs of solder or the ends of the wires! This will give you a short from power to ground and will overload whatever you use to power the LEDs. Inspect both ends of the three-wire cable to make sure there are no shorts or stray wires before plugging in the board to a USB power source or PC.