The DIP Switch Challenge

How many LEDs can you individually switch on a 170-socket breadboard?

LEDs must be current limited
LEDs must be individually controlled with a DIP switch
LED brightness must be uniform in all states
Breadboard sockets must be used for all connections
You can use as many of these components as you would like: LEDs, wires, resistors, DIP switches
You can only use one breadboard and one battery pack

If your boss were paying you money for this work, she would also expect you to do these things

Provide a schematic drawing of your design
Provide a circuit layout drawing for your design
Provide a description of the feature set of your design
Provide a Bill of Materials and BOM cost for your design

For this challenge, you will take on the role of an electrical engineer.

Electrical engineers love to save money. They design products that minimize part count and cost, but still provide the required features.
Electrical engineers hate it when things break. They design products that will work as expected for a very long time.
Electrical engineers use precise language to describe their work. They are proud of their designs, and they use precise language so that there is no confusion when working with other people or in using their designs.
Another language electrical engineers use is that of schematic drawings. These drawings use symbols to specify how electrical components are connected to each other. An electrical engineer who only speaks English and another engineer who only speaks Spanish can still discuss a circuit because they can draw schematic diagrams to communicate with each other.

The Circuit

Here is a diagram of the switched LED circuit. When looking at a schematic diagram, one thing to keep in mind is that lines represent connections between components. We can connect components by hooking them directly to each other, or by connecting them with a wire, so when we go to build the circuit, what we end up with on the circuit board may look a bit different from what we see in the schematic diagram, but it is still the same circuit.

In the middle of this circuit, there are three components, a switch, an LED and a resistor.

The symbol for the switch is near the top of the circuit. It looks like a line segment that is part of draw bridge between two small circles. The switch that we will use is like a miniature light switch. You can think of a switch as a wire that is designed to be easily connected and disconnected. When the switch is disconnected, as it appears in the schematic symbol, it is called an "open" switch. This is because there is an opening between the pieces of wire that the switch can connect. Similarly, when the pieces of wire are connected, the switch is called a "closed" switch.

A little further down in the circuit there is a symbol that includes a triangle. This is the LED or Light Emitting Diode. LEDs are components that produce light when electricity flows through them, and they are becoming very common today, because they are one of the most efficient ways of creating light. The triangle in this symbol shows the direction in which electricity flows through the LED (think of the triangle as the tip of an arrow head). The small arrows to the side are a hint that the LED produces light.

At the bottom of the stack of three components is a resistor, which looks like a section of zig-zagging lines. A resistor limits the flow of electricity. There are many types of resistors, and one of the most most important characteristics of a resistor is its "resistance" or "ohm rating", which specifies how difficult it is for electricity to flow through the resistor. We will use 100 ohm resistors so that about 12 milliamps of electrical current will flow through the circuit. If we used a higher resistance, less current would flow, and the LED would not be as bright. If we used a very low resistance, too much electricity may flow through the circuit, and the LED could burn out.

There are two more symbols in the schematic diagram, but they don't exactly represent components. At the top of the diagram there is the symbol "V+" which is sitting on top of some lines that form a T. This is the power source for our circuit, and you can think of it as where the circuit connects to the positive terminal of a battery.

At the bottom of the diagram there are a series of horizontal lines that become shorter. This is the "ground point" for our circuit, and we will connect this to the other end of the battery.

So we've met all the parts of the circuit now, but we haven't discussed how the circuit works. Electrical circuits are based on the flow of tiny subatomic particles called electrons. It takes special equipment to see and measure the flow of electrons, so in order to understand how a circuit works it is common to build a mental model where water takes the place of electrons.

We all can imagine what it is like to ride down a water slide, and one way to think of a circuit is to imagine water running down hill. If you also think of how we can pour water over a water wheel, then it makes sense that we can get electricity to do work for use as it flows through electrical components. For example, electricity flowing through an LED can cause the LED to light up.

Here is a table that compares the electrical components in our diagram to plumbing components.

Using the flow of water as a model for the flow of electricity turns out to be a very natural and useful way to think about how electric circuits work. Here is a more detailed description of the circuit again, this time with plenty of references to water.

The circuit starts at V+, which is the positive terminal of the battery. This is like the water at the top of a water tower, ready to flow down into our circuit.

A wire connects the battery to the beginning of the circuit. This is like water running through a pipe to a house that is hooked up to the water tower.

A switch is at the beginning of the circuit, and it can start and stop the flow of electrical current. This is like a valve that we can turn off and on to shut off the water or let it flow.

Next is an LED, which lights up when electricity flows through it. This is like a tiny water wheel, which is fun to watch spin. Well, kind of like that.

Next is a resistor, which limits the flow of electricity. This is like a pipe constriction, where a big pipe leads into a smaller pipe, limiting the flow rate of water.

Finally there is the ground point, which is like water finishing its trip through a house and going down the drain to the sewer.

But wait! If the positive battery terminal is like the top of a water tower, how do the electrons / water get there in the first place? In a water tower there is probably a pump (run by electricity!) that pumps water to the top of the tower. In a battery, there are chemicals in the battery that have a reaction that you can think of as boosting electrons from the ground terminal back up to the positive terminal.

Note that we call our picture a "circuit" diagram, and the word "circuit" may remind us of a "circle", or running a round trip. In our circuit, electrons make a round trip from the positive battery terminal, through the part of the circuit where they do work, end up all tired out at ground, and then get re-energized and boosted back to the positive terminal.

Another interesting characteristic of this circuit is that you can arrange the switch, the LED and the resistor in any order, and the circuit will still work the same way. This is a "series" circuit, which means that electricity is flowing through a series of components, and these circuits can often be rearranged.

The Parts Bin

We will have a parts bin full of components we can use to build our circuits. The next section describes these components.

The description of each component includes the cost of the component so that you can determine the cost of your complete circuit design. Remember that engineers like to save money, so it is good to consider alternative designs that might lower the overall cost.

Breadboard

The breadboard gives you a place to connect the components in your circuit. The grid of sockets in a breadboard is connected internally so that you can connect two components by pushing their leads in to sockets that are next to each other. Since we can not see inside the breadboard, it's very helpful to know exactly which sockets are connected internally, so here is a diagram.

The breadboard in this challenge has 170 sockets that are connected in groups of 5. The groups are shown in the grey rectangles in the diagram. The sockets are also known as tie-points, because they tie components together. When you press a component lead into a socket, it will be connected to all the other sockets in the group, so any component leads that are pushed into the same socket group are connected to each other.

If you need to connect more than 5 component leads, you can use a wire to connect more socket groups. The socket groups connected by these "jumper wires" will all be connected.

You can also pull the components out of the board and rearrange them. The way you physically arrange components on a circuit board is known as the "circuit layout".

The name breadboard is confusing, because this is more of a rearrangeable-circuit-connecting-board. It is called a breadboard because some of the first people that experimented with building circuits would attach components to a wooden breadboard. Early components were larger than they are today, so a wooden breadboard was a good tool for building experimental circuits.

The cost of a set of 5 breadboards is $2.43.

Battery Pack

The battery pack uses two batteries and provides approximately 3 Volts to the circuit when using alkaline batteries. V+ is available at the end of the red wire, and Ground is available at the end of the black wire. You can plug these wires directly into a socket in the breadboard in order to connect them to your circuit.

The cost of a set of 10 battery holders is $3.96.

Yolanda in the product planning group says that this product will be sold "batteries not included", so you don't need to include the cost of batteries in the overall cost.

Wires

Wires connect parts of the circuit to one another. We have solid wires, which are stiff, and jumper wires, which are more flexible. Remember that the connections in the breadboard can act like wires to connect parts of the circuit together.

140 solid 22 gauge wires

65 flexible jumper wires

$2.83

$1.88

DIP Switches

The DIP switch has a funny name that comes from the way the switch can be built in to a circuit board. DIP stands for "Dual Inline Package", which means that it has two rows of metal pins for making connections. This works out nicely with the breadboard, because we can push the DIP switch into the connections in the breadboard, and the pins and the sockets line up. Unfortunately, the pins on the DIP switches are a little short for the breadboards, and they tend to pop out of the breadboard when you are not looking. When you are showing off your circuit, you can rest your finger on the DIP switch so that it makes a good connection. This is called "fudging" and it is an important skill.

The tiny switches in the DIP switch can be switched one way to the open state so that there is no connection between the pin on one side of the switch and the pin on the other side. Sliding the switch the other way to the area marked "ON" will close the connection and connect the pins.

We have DIP switches that include 2, 4 and 8 positions. Try to think of the best way to use DIP switches in your design so that you can switch as many LEDs as possible. If you put too many switches in your design, there will not be room for the other components and their connections to complete the circuit.

LEDs

Light Emitting Diodes emit light when electricity passes through them. We have red, green and yellow LEDs. One lead on the LED is longer than the other, and this connects to the anode, or positive side of the LED. The other lead which is shorter connects to the LED cathode and leads to ground. Note as well that the there is a flat spot in the round LED case next to the shorter lead.

100 3 millimeter red LEDs

100 5 millimeter green LEDs

100 5 millimeter yellow LEDs

$1.34

$2.27

Resistors

As described earlier, resistors are electronic components that allow electricity to flow through themselves, but provide a certain amount of resistance.

The cost of a set of 100 100 ohm resistors is $0.99.

Bonus Points

Draw a schematic drawing for your circuit.
Calculate how much your circuit costs to build.

Questions to Think About

Do you think someone would buy your product if they saw it in a store?
Did the circuit that you built end up looking like a rat's nest? What could you do to give it a neater appearance?

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