Parts List

You will need the following parts to try out this experiment:

Construction

In this project, you are really just connecting the resistor across the battery terminals and using the breadboard to hold the resistor in place, as shown in Figure 11-1.

Figure 11-1. Resistive heating experiment

Don’t attach the battery just yet, because as soon as you do, the resistor is going to get hot.

Experimenting

Connect the battery box, and use your thermometer to measure how quickly the temperature rises on the resistor. The resistor temperature will probably get up to about 170° F (75° C), so watch out—that’s hot enough to burn your finger!

The resistor is 100Ω and the voltage 6V, from which we can calculate the current as I = V / R = 6 / 100 = 0.06A. The power is the voltage multiplied by the current, which is 6 x 0.06 = 0.36W, or 360mW.

That’s a little more than the resistor’s maximum power rating of 250mW (1/4W), so the resistor may fail after a while.

Parts List

You are going to need the following parts to build this project:

You should get several 10Ω resistors, as these are likely to literally go up in smoke.

Hardware

By the time the Darlington transistor has taken its share, there will be about 3.5V across the resistor when the transistor turns on. This means a current of 3.5V / 10Ω = 350mA. This should not overload any USB device powering the project.

The heat power coming from the resistor will be I x V = 350mA x 3.5V = 1.225W. This is a lot more than the 1/4W that the resistor is rated at, but the resistor should survive long enough to burst the balloon. 

Figure 11-3 shows the breadboard layout for the project. 

The resistor should be firmly attached to the jumper wire pins (Figure 11-4), so that it does not come adrift when the balloon pops.

Figure 11-3. The breadboard layout for the balloon popper

Figure 11-4. Making a resistor lead

Software

There is no start button for this project apart from the Arduino’s Reset button. The Arduino sketch for this project starts as soon as it’s plugged in, so you should keep one of the jumper wires to the resistor disconnected until you are ready to do some popping. 

Go to /arduino/projects/pr_balloon_popper (which you’ll find in the place where you downloaded the book’s code):

const int popPin = 11;

const int minDelay = 3;  // Seconds 
const int maxDelay = 5;  // Seconds
const int onTime = 3;     // Seconds 

void setup() {
  pinMode(popPin, OUTPUT);
  randomSeed(analogRead(0));    
  long pause = random(minDelay, maxDelay+1);    
  delay(pause * 1000);              
  digitalWrite(popPin, HIGH);       
  delay(onTime * 1000);
  digitalWrite(popPin, LOW);
}

void loop() {
}

The constants minDelay and maxDelay specify the range of possible times after which the resistor will be powered up and the balloon might pop. 

The constant onTime specifies how long the resistor needs to be on. You may need a bit of trial and error to get this right. You don’t want the resistor to be hot for too long, or it will quickly burn out.

Random numbers in Arduino C are not truly random, but actually part of a long sequence. To ensure that you get a different delay each time your Arduino restarts, this line sets the position of that sequence based on the reading on analog pin A0. Since this pin is not connected to anything, the readings from it are more or less random.

The pause before the resistor gets switched on is decided by the random function that returns a random number between the two values supplied to it as parameters. 1 is added to the second parameter because the range is exclusive. The variable used is of type long because an int can only hold numbers up to 32,767, which would stop you specifying a range of possible delays beyond 32 seconds.

Delay for pause seconds.

Turn on the transistor and power up the resistor. Delay for onTime() then turn the power off again.

Using the Balloon Popper

While you are testing the project, you might want to add a bit of certainty as to when the balloon is going to pop, so change minDelay and maxDelay to perhaps 2 and 3, respectively.

Tape the resistor to the balloon and then plug the resistor leads onto the breadboard. Click the Reset button and wait for the bang. 

Boiling Water

As an example, let’s work out how well our little resistor would do at boiling some water.

Let’s assume that we want to boil a cup of water (about 250g of water). Let’s also assume that the water starts at room temperature (20°C) and will boil at 100°C; this means there is a temperature increase of 80°C.

The total energy needed to raise this 250g of water by 80°C is:

4.2 x 250 x 80 = 84,000 joules

In “Experiment: Resistor Heating”, your resistor was producing 0.36W or 0.36 joules per second. So, at that rate, it would take 84,000 / 0.36 = 233,333 seconds, which is roughly 64 hours!

For practical reasons described in the next section, it’s never actually going to get there.

How Peltier Elements Work

When a current passes through a junction between two different conductive materials, one side of the junction will get slightly hotter and the other slightly cooler. This is called the Peltier effect after the French physicist Jean Peltier who discovered it in 1834. It is also known as the thermoelectric effect.

The effect is relatively small, so to make it useful enough to do something like cool down a beverage for us, the effect needs to be multiplied. This is accomplished by placing a series of alternating junctions next to one another so that current can pass through each one in series, so that each contributes to the overall effect. Common, low-cost elements typically have around 12 junctions (Figure 11-6). On either side of the elements there is a base material that forms the bread of a junction sandwich.

Figure 11-5. A Peltier element

The different types of material used in the junctions are two types of semiconductor, like those used to make transistors and chips, but optimized for their thermoelectric effect. They are known as N or P type (negative and positive).

One very interesting feature of Peltier elements is that as well as being used to cool things, if you arrange for one side to be hotter than the other a small amount of electricity is generated. 

Practical Considerations

The main problem with Peltier elements is that the hot and cold sides are very close together and so the hot side will soon warm the cold side unless something is done to take the heat away as quickly as possible. 

The starting point for this is to use a heat sink, as shown in Figure 11-7.

Figure 11-6. Inside a Peltier element

In the cooling unit shown in Figure 11-7, the heat sink is a lump of aluminum with blades sticking out to increase the surface area and carry away the heat. The cold side of the unit is the block arrangement that is designed to protrude into the thermally insulated refrigerated compartment.

A heat sink on its own is not nearly as effective as a heat sink with a fan attached to it to take away the air that has been warmed by convection and replace it with new cooler air. In fact, some units have a fan on both sides of the Peltier element to help it work more efficiently (Figure 11-8).

Figure 11-7. Using a heat sink with a Peltier element

Figure 11-8. A Peltier refrigeration unit with dual fans

Parts List

You are going to need the following parts to build this project:

If you want to use a more powerful Peltier element than 4A, make sure that you also upsize your power supply to have a higher maximum current rating than the cooling unit. Allow at least an extra half amp for the fans and half an amp for luck.

Construction

If you unravel the wires of your cooling unit, you will find three pairs of wires: one pair for the Peltier element itself and one pair for each of the two fans. All of these require 12V from the power supply and the easiest way to accomplish this is to use a handy screw terminal to DC barrel socket adapter. Simply put all three red wires from the cooling unit into the screw terminal marked + and all three black wires into the screw terminal marked—as shown in Figure 11-10.

Figure 11-10. Wiring the beverage cooler project

You’ll need to cut off the top of the plastic container and cut a square aperture into the side so that the cooling part of the Peltier cooling unit sticks into the bottle (Figure 11-11). Make sure to choose a plastic container that is large enough to contain your favorite glass or bottle for cooling.

Figure 11-11. Adapting the plastic container

If the cool-side fan has screws that protrude, make a couple of holes in the plastic container for them too, so that the cooling unit is firmly attached to the plastic container. If you make sure that the hole for the cool-side of the Peltier unit is in the right place for the bottom of the Peltier cooler and plastic container to be level, then it won’t topple over. 

The great thing about using a plastic container that would otherwise be recycled is that if you make a cut in the wrong place, you can always just start over with a new container.

Using the Project

With everything assembled, plug in the power supply and both fans should start to whir. If you put your hand into the container, you should immediately feel the cold air coming from the small fan.

As we discovered with our calculations on trying to boil water, changing the temperature of even a relatively small amount of water takes a long time, so although this project will eventually cool down a warm drink, it is far better at keeping a drink cold that was cold to start with.

This project is also somewhat wasteful, as it uses up to 50W of electricity just to keep one drink cool. In Chapter 12, you can make this project a bit more efficient and add thermostatic control to it.