Pre-lab: Pots, Soldering, Teensy Installation

The questions below are due on Friday September 13, 2024; 05:00:00 PM.
 
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In the week 2 lab, we'll be building a really cool circuit. This prelab consists of three parts:

  1. Some exposure to a new kind of component we'll use in the week 2 lab.
  2. Some tutorial information about soldering.
  3. Double-checking that your Arduino install has all the necessary pieces.

1) Potentiometers

In the week 2 lab, we'll be working with a new kind of circuit component called a potentiometer (or "pot"), so we'll start a first exposure to potentiometers; we'll do a little theory here so that by the time we're dealing with these components in lab, we'll be ready (we'll also see potentiometers later in this pset as well).

A potentiometer is a three-terminal device whose electrical properties depend on the angle of a mechanical knob. We can think of a potentiometer as being made up of two resistors, whose resistances sum to some value R_P, and whose resistances vary with the angle of the knob, which we will quantify using a number \alpha: one resistor has resistance \alpha R_P, and the other has resistance (1-\alpha)R_P. Schematically, we can represent pots in the following two ways:

The quantity \alpha corresponds to the angle of the knob, normalized to be in the range [0, 1]. \alpha=0 corresponds to the knob being turned all the way in one direction, and \alpha increases as the angle increases, until reaching \alpha=1 when the knob is turned all the way in the other direction.

As the angle of the knob increases, the resistance between the bottom and middle terminals increases and the resistance between the middle and top terminal decreases. These changes in resistance occur such that the sum of the top and bottom resistors is constant.

The illustrations below show how these resistors are actually connected inside the potentiometer:

 

 

The way this is implemented is with a single band of resistive material (total resistance R_P) and a "wiper" that moves with the knob. As the angle varies between from \alpha=0 to \alpha=1 and the wiper arm moves, the resistance between the middle terminal and each of the two outside terminals will change. The internals look something like the drawing below:

 

 

Over the course of the next few labs, we'll see some ways that we can make use of this kind of device, which will involve building a circuit like the one shown below:

 

For the voltage divider configuration above, what is the value of v_{out} we expect (in Volts) when \alpha=0? When \alpha=1? When \alpha=0.5?

Enter your answers below as Python expressions. Your expressions can involve variables V_I, R_P and/or alpha, which represent the variables V_I, R_P, and \alpha, respectively.

When \alpha=0, v_{out} =~
When \alpha=1, v_{out} =~
When \alpha=0.5, v_{out} =~

What is the general relationship between v_{out}, V_I, and \alpha? Enter your answer as a Python expression. Your expression can involve variables V_I, R_P and/or alpha, which represent the variables V_I, R_P, and \alpha, respectively.

v_o =~

2) Soldering Intro

This lab is going to be our first exposure to soldering (in American English, this word is usually pronounced like "SOD-der-ring" (IPA: /ˈsɒdərɪŋ/); the L is silent). Soldering involves joining two pieces of metal together using solder ("SOD-der"). We'll use a soldering iron like the following to accomplish this:

Note that there is a little stand (on the left) to hold the iron (bottom right), and it connects up via a cable to a little base station (in the middle) where you can turn it on and off or adjust the temperature. The grey pad is the thing you should be holding on to. The metal tip of the iron GETS REALLY HOT (it needs to in order to melt the solder), so be really careful not to touch it.

2.1) Soldering 101

Creating a solder joint involves a few steps:

  • Heat both the part (the pin sticking through) and the pad (the metal contact on the PCB) for 2-3 seconds using the iron, making sure you're in contact with both.
  • Feed a small amount of solder onto the joint. It should "flow" smoothly onto the lead and make a shape like a little Hershey's Kiss.
  • Continue heating for another second or so, then remove the iron.
  • Let the joint cool before moving things around.

The whole process looks like the following (in real time):

2.2) Common Problems

Adafruit also has a good guide to soldering, and in particular a really nice page showing several common problems and how to fix them. The following picture from that guide is a summary of some common issues:

Again, we're looking for a nice pyramid- or Hershey-kiss-shaped joint.

2.3) Safety

Important Safety Guidelines

Soldering is not edge-of-your-seat daredevil-type stuff, but there are lots of things that can be dangerous here. Be careful, pay attention to what you're doing, and take your time; and we should all have a fun safe time.

Some things to pay attention to:

  • Wear safety goggles at all times when soldering and when clipping wires. Getting solder or flux or small pieces of wire in your eyeballs is not fun and can cause serious damage.

  • The tip of the iron gets really hot. For the love of all that is holy, don't touch it. There's a perfectly-good handle you should use instead. Minor burns can result if you touch the hot part even for a short while, and if you remain in contact with it, those burns can become serious. Seek immediate medical attention if a burn breaks the skin. The parts you're soldering will also stay hot for a while, so be careful when touching them.

  • If you want to, use the little vent fans we have around. The rosin in the solder vaporizes at low temperatures and can cause (usually-mild) eye and respiratory irritation if you're exposed to it for a long time. This is not likely to bother most people especially since we'll only be soldering for a short while; but if you have asthma or are at all concerned about possible respiratory irritation, feel free to use a vent fan; we should have enough to go around.

  • Be really careful with the iron around power cords. If the iron melts through plastic on a power cord, it can cause electric shock.

Did you read and understand the safety warnings above?

2.4) Complete Build

You don't have to watch this if you don't want to, but if you do want to, here is a video showing the complete process of constructing this week's lab project. The video comes in three flavors:

3) Software Installation

It's also worth double-checking that you finished the Teensy installation from last week. Some of you, in an effort to fix a missing Arduino.h, may have undone one of the necessary steps for this week's lab with respect to Teensy installation.

To check, open up Arduino, choose "Teensy 4.0" from the Tools -> Board menu, and then after slecting that, you should see "XInput" as an option under "Serial Type." If you don't see that option, you should go back and re-do step 6 "Add XInput support for Teensy" from the Arduino install instructions from the last p-set.