CSCI 255 — Breadboards, transistors and gates

In this lab you or going to build a few circuits on a breadboard. For most of you this will be a new experience. Work slowly and carefully.

A presentation

Let’s start with a Breadboarding: A Presentation written by Charles A. Schuler for his book Electronics Principles & Applications.

Works of Art

Here’s a couple of breadboards built for a paper we published about the CSCI 255 labs a few years ago.
 

Stuff you will need

• A breadboard: The small ones are big enough, but the larger ones provide a bit more elbow room
• A battery pack with 4 AA batteries
• A resetable thermal fuse
• Two tactile switches
• Three LED’s
• Six 2N3904 transistors
• Six 1 kΩ resistors (Brown-Black-Red)
• Eight 10 kΩ resistors (Brown-Black-Orange)
• Wire

Danger, Danger

It is important that you follow lab directions carefully. If you don’t, you may create short circuits which might make components hot enough to burn your fingers. The smell of burning plastic has permeated the air in previous CSCI 255 labs.

You should also be careful about cutting wires. When making the cut, be sure that the part of the wire that you aren’t holding is pointing toward the table and not toward anyone’s eyes. We’d actually prefer that, when appropriate, you just use the cut pieces of wire left over from previous years. There’s a lot to choose from.

Voltage

Usually you’d use voltage regulators in a computer hardware course; but, since precise voltage levels are not needed for this lab, so we’re going to make do with 4 AA batteries in a pack.

If you really want to know how to build a voltage regulator, look at the Fall 2013 lab on breadboards or make your way through the LM2937-3.3 datasheet.

Using the fuse

We want you to connect power to the breadboard through a resettable thermal fuse. If you short your circuit, the fuse will heat up and break the circuit. At that point, you should fix your short (probably after talking with the instructor) and try again. By then, the fuse should be cool down enough to pass current.

Start by filling your battery holder with 4 AA batteries and turn off the switch on the battery holder. (If you don’t have a switch on your holder, you should probably leave one of the batteries out for a while.) Connect the battery holder to your breadboard on the left end of the board by following these steps.

• Connect the ground lead of the battery holder to the blue bus strip on the lower edge of the breadboard
• Connect the 6 v lead of the battery holder to one end of the resetable thermal fuse
• Connect the other end of the resetable thermal fuse to the upper red bus strip

Wire the voltage input end of the circuit. Be sure that the part with the fuse looks like the following picture.
    

Connecting the rails

The bus strips on the lower and upper edges are not connected. If you have one of the larger breadboards, it is likely that there are four pairs of unconnected bus strips along the edges of your breadboard. You need to connect these strips. Be sure to use black wire for ground and red wire for the supply voltage.

Power-on LED

Far too often have circuit designers have invested a lot of time “debugging” a circuit that just isn’t receiving any power. We’re going to add a power-on light so that you will know that your circuit is getting the electrons it needs.

Your light will be the light-emitting diode (LED). LED’s can be tricky. They are two things you need to know before using them. First, the LED is a diode, a one-way channel for current. You must orient it correctly in your circuit for it to light up. The anode must be connected to the higher voltage; and the cathode, to the lower voltage. If the leads of the LED have not been cut, the cathode lead will be the shorter. Often, other students (and instructors) trim the leads so that they fit closer to the breadboard. In that case, you must look at the LED from above. The cathode side will either be flattened or notched.

Second, the LED must be used with a current limiting resistor.

You can find many tutorials on LED circuits and many LED calculators on the internet, but just stick with 1 kΩ resistors today.

Complete your power supply circuit by adding a “power-on light” and connecting both “+” bus strips to the output of the regulator, and both “-“ rails to ground on the right end of the board. Be sure that your power light is on.

When you complete this task, your circuit should look something like the picture below:

Input from and output to humans

You need to be able to communicate with your breadboard. You will give inputs to your breadboard by pressing switches. The breadboard will turn on lights in response to your switch presses.

Input — The switch

This part of the lab may be far more complicated than you expect.

We are going to use small tactile switches for our presses. The switches we use will also be single pole single throw (SPST), which can be either open or closed; and momentary, which return to their usual state as soon are you release the button; and normally open, which are closed only when you are actively pressing the switch. (It is possible that some normally closed switches have infiltrated our supplies. Be on the lookout for this.)

The switches we supposed to stick in the holes of the breadboard:

Unfortunately many of these switches do not fit well in the breadboard and will pop out occasionally. If this happens, just press them hard enough to push them back in.

The wiring within these switches can be confusing. An SPST switch really needs only two connections, but these switches have four. This is because these tactile switches contains two pairs of wires and an internal switch that can connect the two pairs. Here’s a dimensional drawing from SparkFun that illustrates, in the bottom right corner, the connections.

The hard part is remembering that the always-connected pairs run along the long sides.

To avoid floating inputs to electronic circuits, pulldown (on left) or pullup (on right) resistors are used to make sure the switch output, labeled D, is always tied to either ground or the supply voltage.

Because many embedded systems provide internal pullups, we’ll use pullups in this lab with normally open switches. This means that the output of the switch is connected to ground when the switch is pushed.

Near the left end of the circuit build two switches in the pulldown configuration. Here is how one of them looks. The yellow wire marks the output of the switch.

You can try to test your switch by connecting it to an LED, but the 10 kΩ resistor may restrict the current too much for you to see the LED is on. However, you can use the multimeter to measure voltage.

Output — The LED

We will use a transistor, LED, and a couple of resistors to make an LED driver to allow a small current source to control the LED. LED drivers with transistors look a lot like inverters. In fact, if you remove the LED, you pretty much have an inverter. And, if you add a transistor to the power-on LED circuit you built early, you pretty much have an LED driver.

But first let’s learn a little bit about the 2N3904 transistors we’ll be using this lab. Start with a very quick (no reading allowed) look at the 2N3904 data sheet. The 2N3904, like all bipolar junction transistors, has three external connections: a base, a collector, and an emitter. The design of a transistor allows it to function as an amplifier or a switch. A little bit of current flowing into the base can control a much larger flow of current between the emitter and collector. This is similar to your faucet, where a valve (base) controls the flow from the water supply (collector) to the drain (emitter).

The standard schematic drawing of a generic NPN transistor is shown below on the left. The 2N3904 is is made in a TO-92 package. On the right is a picture that illustrate how its three physical pins are allocated the collector, base, and emitter functions.
    
Before using a transistor be sure to look up its datasheet. The location of the base, emitter, and collector can vary.

To understand a little more about NPN transistors without plowing through pages of textbooks, try out some of Paul Falstad’s excellent Java Applet animations.

Start by opening the NPN animation an another browser tab. Note the following about the NPN:

• The collector should be at a higher voltage than the emitter. This is why the emitter is fixed at 0 V (ground) and the collector voltage cannot be decreased below 0 V.
• The transistor “turns off” as you lower the base voltage.

Back to the lab

Now that you’re familiar with transistors, let’s use one.

• The emitter of your transistor should be connected directly to ground.
• The collector of your transistor should be connected to a 1 kΩ resistor.
• The other end of the 1 kΩ resistor should be connected to the cathode of the LED.
• The anode of the LED should be connected to the power supply.

Finally, the base of the transistor should be connected to a 10 kΩ resistor. The other end of the 10 kΩ resistor is the input to your circuit.

Your new circuit should follow the schematic shown below. (OK, the LED and current limiting resistor are switched, but that’s not a problem.)
    

Near the right end of the circuit build two LED drivers and use the switches on the left end to turn the LED’s off and on. Your circuit should look a bit like the following, but it should have twice as much stuff.

Perhaps you are wondering what is the point of having a transistor-controlled LED driver. Why add the transistor when we are already able to turn on the LED? There are two reasons. First, any digital circuit can turn a transistor on and off. Second, transistors can control devices, such as motors, that require for more current than a digital circuit can produce.

Building some inverters

Your next task is to build four RTL (resistor-transistor logic) inverters in the middle of your breadboard. The RTL inverter will be implemented with a couple of resistors and a single transistor as shown below. It looks a lot like an LED driver without the LED. The input to the inverter is connected to the transistor though a base resistor, Rbas. There is a collector resistor, Rcol, that connects the high voltage to the collector of the transistors.

Use a 1 KΩ resistor for Rcol and a 10 KΩ resistor for Rbas. There’s nothing magic about these choices. We just happen to have a lot of them. (In the drawing above the triangle at the top is a connection to the power supply and the triangle at the bottom is a connection to ground.)

We’re not going to give you any more pictures for this lab. You should be able to follow the schematic. However, we do suggest you put the transistor and collector resistor on the bottom half of your breadboard. Then connect the base resistor and a wire to the transistor’s collector on the top half of your breadboard as shown in the following fuzzy picture

This way it will be easy to find the input and output of the inverter.

The remainder of the lab will be accomplished solely by connected the inputs and outputs of your inverters with wire. No additional transistors or resistors are required.

The NOR Gate

Today, most Boolean functions are implemented with NAND gates. Unfortunately, it is difficult to build a reliable NAND gate NPN transistors. So, we are going to build a NOR. This is not at all a problem when building larger circuits. Any Boolean function can be implemented with two levels of NOR gates using a product of sums, similar to how a sum of products can be used in a two-level NAND implementation.

Here’s an RTL circuit for a two-input NOR.

If you are thinking that an NOR looks like two NOTs sharing a single pull-up transistor Rcol, you are on the right track.
+ ~=
But it’s even better than that because two 1K Ω resistors in parallel make a 500 Ω resistor and a 500 Ω resistor is good enough for our NOR. (This means you don’t even have to remove the extra 1K Ω resistor to build your NOR.)

By the way, the Apollo Guidance Computer used about 2,800 RTL chips, each containing two three-input NOR gates, for all the manned flights to the moon. If you click the image below, you will see that our NOR differs little from NASA’s.

Now create your own version of this 1965 classic with a single NOR from two NOT’s by adding one wire!

An OR Gate

NOT a NOR to build an OR using the following Boolean identity.

• x OR y = NOT(x NOR y)

An AND Gate

Use your two NOR’s and two NOT’s to advantage with De Morgan’s law to build an AND gate.

• x AND y = (NOT x) NOR (NOT y)

The SR flip-flop

For our grand finale, build a SR flip-flop by connecting your two NOR gates as shown below. You should already have one NOR on your breadboard. Built a second one and then connect the wires as shown.