3. Production Methods for Electronics

If you are going to make a circuit that you want to use for more than a few minutes, you are going to want to make it robust and reliable. You want the electrons to consistently go where you intend for them to go, and for them not to go where you don’t want them to.

If current doesn’t flow where you want it to, that is an open circuit. This happens when wire connections break.

If current flows where you don’t want it to, that is a short circuit. This happens when insulation fails, allowing metal to metal contact in a place where you don’t want it.

Make your connections strong, and your insulation good, and you will spend more time enjoying your work, and less time tracking down faulty connections.

Using a wire stripper

Bare wires can conduct electricity on contact. Since we are trying to get the electrons to follow a particular path in the circuit, we want contacts only where we choose them to be made. A contact in an unplanned location allows electrons to skip past part of the circuit. We call that a short circuit, or short.

We use insulated wire to keep the electrons following the circuit plan. We must remove the insulation to make metal-to-metal contact where we choose.

Here’s how to properly use a wire stripper.

  1. Know the gauge of the wire you are using. You can usually find it on the spool, listed as AWG (American Wire Gauge)
  2. Choose the correct notch for that wire gauge. Read the labels on your wire stripper. 
  3. Position the wire stripper perpendicular to the wire.
  4. Close the stripper to sever the insulation.
  5. Open the stripper slightly to avoid contact between the knife edges and the metal wire.
  6. Pull the insulation off, maintaining the perpendicular orientation of the stripper to the wire.
  7. Trim the bare part of the wire to the length that you want.
Good wire stripping technique. Tool is perpendicular to the wire.

Good wire stripping technique. Tool is perpendicular to the wire.

If you slant the tool, the cutting edge will dig into the wire, and may cut it clean through. Don’t slant the wire stripper.

Bad wire stripping technique. Don't slant the tool.

Bad wire stripping technique. Don’t slant the tool.

Once you have stripped the wires, you can twist them together to make electrical contact. You can then solder the twisted parts to make a permanent connection. If you want a connection that is easy to change, you might want to use a Breadboard.

Breadboards

This is a breadboard. Engineers use them to make rough drafts of electrical circuits, because they are easy to rewire.

[[Photo of the front of a breadboard]]

What’s going on inside a breadboard? If you were to peel the tape off the back of a breadboard, you would be able to see where the wires go.

The back of a breadboard, with the insulating cover peeled off.

The back of a breadboard, with the insulating cover peeled off.

As you can see,  any position in either – row connects all the way across, and any position in either + row connects all the way across.

If you want the top – row to connect to the bottom – row, you must add a wire. Likewise for the top and bottom + rows. 

In the numbered columns, 1-30, abcde are all connected, and fghij are all connected, but they are not connected across the midline.

You can push 24  or 26 gauge single strand wire into any of the holes to connect to that row or column. Header pins, and the pins of many components, fit neatly into the holes as well.

Stranded wire will not work well in a breadboard.

Making wire splices

We frequently need to solder the ends of two (or more) wires together. We twist them together, solder the joint, and then cover it with heat shrink tubing to prevent it from shorting to something else. Here’s the twisting part, from before.

An in-line wire splice.

An in-line wire splice.

Soldering

Many electronics components are connected with solder. Solder is a metal that melts at relatively low temperatures to make very high electrical conductivity connections between two wires. It’s a little bit like a conductive glue, and, in fact, there are epoxy-type glues that do conduct electricity. If you want to solder something, you need some solder that melts at a lower temperature than your wires do. There are lots of solders available, and, currently, there are some good solders on the market that do not have lead in them. Lead is toxic, so if you can avoid it, do so.

I personally like the American Iron and Metal Lead-Free Solder #SAC305 GlowCore 2.5% 0.020” DIA. It works just as well for me as the tin-lead solder alloys I have used for the last 30 years, and the flux in it does not require cleaning after the soldering is done.

Flux is a chemical that breaks down the surface tension of the solder so it can flow onto the wires to make good contact. Like water sticking to a surface, when solder flows smoothly onto a surface, we say that the solder is wetting to the surface.

To make a good solder joint:

  1. Put on a pair of safety glasses. A droplet of molten metal in the eye is painful and can be avoided.
  2. Set the soldering iron to 650 ºF = 340 ºC. Much hotter and the flux will burn away before it can affect the surface tension of the solder. Much cooler, and the material you are soldering may not get hot enough to form a good joint.
  3. Dampen the sponge with water. It should be damp, not soaked.
  4. Make the joint you wish to solder, in this case by twisting together a pair of bare wires.
  5. Position the joint you wish to solder so that you can see it well and reach it easily, and so that it stays still while you are working on it.
  6. Start the fume-catcher, and position it as close to the joint as possible. The fumes from the burning flux and molten metal are not good for your lungs. Avoid breathing them.
  7. Wipe the tip of the iron on the sponge to clean it off. It should look silver colored, not charred or smoked.
  8. Bring the tip of the iron close to the joint.
  9. Touch the solder to the tip of the iron, to melt just a little solder to form a fresh surface on the soldering iron tip.

This little bit of molten solder on the iron tip is there to fill the gaps in the contact between the iron and the joint. It is not the solder that will fill the joint. We will add that from the other side of the joint.

You cannot successfully “paint” solder onto a joint by putting some on the iron and then applying the iron with the solder on it to the joint. This method does not work because solder flows towards heat. The solder will flow towards the iron, which it is already on, and not onto the wires.

To make a good solder joint, apply the iron to one side of the joint, and the solder to the other, so that the solder must flow through the joint to get to the iron.

The action of liquid flowing towards heat is called viscothermal pumping, and that is the mechanism by which solder flows. If you keep that mechanism in mind while you are soldering, your solder joints will be more likely to be good ones.

  1. Heat the joint for a second or so with the iron before applying solder.
  2. Apply the solder to the other side of the joint from the iron, so that the solder must flow through the joint to get to the iron. Feed the wire in as it melts, to fill the joint.
  3. When the joint is fully silver-colored from the solder, stop feeding the solder wire.
  4. Hold the iron on the joint for a count of three, to make sure that the whole joint has been heated above the melting point of the solder.
  5. Withdraw the iron from the joint.
  6. Wipe the tip of the iron on the sponge, to clean it off.
  7. Set the iron back in its holder, so you don’t burn yourself or the table with it.
  8. Wash your hands when you are done soldering. You don’t want to rub that stuff in your eyes.
Apply the iron to the joint, then apply the solder.

Apply the iron to the joint, then apply the solder.

Feed the solder in to fill the joint.

Feed the solder in to fill the joint.

Heat the joint after you withdraw the solder, to flow the solder evenly into the joint.

Heat the joint after you withdraw the solder, to flow the solder evenly into the joint.

A twisted in-line splice, soldered. Trim the ends off so they don’t poke through the insulation.

A twisted in-line splice, soldered. Trim the ends off so they don’t poke through the insulation.

Using heat-shrink tubing

Where we have made a solder joint, the wires are necessarily bare. We use heat shrink tubing to cover them to prevent accidental electrical contact, and to protect the joint from bending, which can cause the wire to break. The tube reinforces the joint, relieving the metal from the strain of bending. We call that sort of joint protection strain relief. Strain relief is very important in electrical work, because broken joints cause circuits to fail. We put a lot of work into getting our circuits to function, we do not want them to fail.

  1. Select a diameter of heat-shrink tube that will just fit over the joint.
  2. Cut a length of tube so that the tube will extend past the joint on both ends, by about twice the diameter of the tube on each end.
  3. Slide it into position centered on the joint.
  4. Shrink it with a heat gun, or, in a pinch, the flame from a cigarette lighter.
A soldered splice joint encased in clear heat shrink tubing.

A soldered splice joint encased in clear heat shrink tubing.

Build your own test probe.

Wouldn’t it be nice to have your own test probe? You can use it to see if there is voltage present on an Arduino output pin, which we will need to do in the next lesson.

Let’s make one! 

Here’s the schematic of the circuit:

Test probe schematic.

Test probe schematic.

Gather the LED, resistor, and two pieces of wire, one with black insulation and one with red insulation. We usually reserve red wires for +5V and black wires for Ground, so we will follow that convention here. The electrons don’t care what color the insulation is. We use color coding to remind us of what our intentions were when we designed the circuit.

Twist the wires together a the correct places, following the schematic above and the hookup photograph below. Solder the connections.

Test probe, partially soldered.

Test probe, partially soldered.

Apply three pieces of heat shrink tubing.

  1. Put one piece on the positive leg of the circuit, completely covering the resistor and all the bare wire from the LED down to the red insulation.
  2. Put a second piece on the ground leg of the circuit, completely covering the bare wire, from the LED down to the black insulation.
  3. Shrink both of the tube segments that you just installed.
  4. Add a large diameter piece of tube to cover from the bottom part of the LED down to the red and black insulation. This tube is only for strain-relief. It keeps the legs of the circuit together so they don’t bend at the entry point into the LED body. If they bent there, they would break off, and then you would need to rebuild the circuit.
Test probe, complete with heat shrink tubing.

Test probe, complete with heat shrink tubing.

Connect the black lead to the negative side of your 4.5V battery, or to Ground on your Arduino. Connect the red lead to the positive side of the 4.5V battery, or to +5V on the Arduino. It should light up. If it does not, try flipping the polarity, that is, connect the black wire to +4.5V, and the red wire to the negative side of the battery. If it works then, cut off the wires, and replace them so that black goes to negative and red goes to positive. This will save you lots of confusion later, so it is well worth the effort.

©Paul Mirel 2015: For educational purposes only, you may freely reproduce this information, provided you cite this page as the source. Commercial uses are prohibited.

Skills

Posted on

2016-09-01

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