CSCI 255 Lab

Lab 2 -- Introduction to Circuits



You will need the following equipment/components to create the circuits in today's lab:

Part 0: The Breadboard (background)

We'll use a breadboard to create electrical connections without soldering wires together. A breadboard is used to hold the components of your circuit, and connect them together. It’s got holes that you can push wires and components into and pull them out of.

breadboard      breadboard connections.

On each side of the board are two long rows of holes, with a blue or a red line next to each row. All the holes in each of these lines are connected together with a strip of metal in the back, as shown above. In the center are several short rows of holes separated by a central divider. The five holes in each row in the center are also connected allowing you to use these holes to connect components together.

The photo below illustrates how the holes are connected.

Part 1: Build a Power Supply

The circuits that you build in this lab and future labs will need a power supply. Follow the steps outlined below to construct a 3.3V power supply on your breadboard.

  1. Use your multimeter to identify 4 "good" (i.e., close to 1.5 V) AA batteries from the "lightly-used" battery box and place them in your battery holder. If you are unfamiliar with a multimeter, ask your instructor for help or check out the following link.
  2. Construct the portion of the power supply depicted below on your breadboard.

    power circuit

    Use the multimeter to verify that you have 3.3 V on the output leg of the regulator.


  3. Complete your power supply circuit by adding a "power-on light," and connecting both "+" rails to 3.3. Volts (the output of the regulator), and both "-" rails to ground.

    power-on LED

    Assure that your power-light is on.



Show your breadboard power supply to your instructor.  

Part 2: A Simple Switch

Now let's do something with all this power. We'll begin by building a simple circuit using a switch to control a LED. Create the circuit depicted below on your breadboard using a push-button switch.

switch circuit

Note: You can use your multimeter to determine which legs of the switch are connected when the switch is active (i.e., pushed).

Verify that the LED turns on when the switch is activated.


Show your switch circuit to your instructor.  

Part 3: A Transistor as a Switch

Transistors are the building blocks of digital electronic systems. As you will learn in future labs, any logic gate can be constructed from transistors and logic gates are the components of digital electronic devices. In this lab, we will use a transistor to replace the push-button switch in the circuit above. But first let's learn a little bit about transistors.


A transistor can act as a switch, just like the push-button switch above. But it's much more sensitive and versatile. The design of a transistor allows it to function as an amplifier or a switch. This is accomplished by using a small amount of current to control a gate on a much larger flow of current, much like turning a valve to control a supply of water.

Transistors are composed of three parts – a base, a collector, and an emitter. The base is the "gate" that controls the larger current flow between the collector and the emitter. The flow of current between the collector and emitter is controlled by the current sent to the base. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier. The same process can be used to create a switch but in this case a voltage threshold is applied to open the gate and a lesser voltage is considered "OFF."

Semiconductor materials make the transistor possible. These materials are constructed so that the effective internal resistance of a transistor varies depending on the power that you supply to the base.

We will be using 2 types of transistors, NPN and PNP. These are bipolar semiconductors with three connectors, base, collector, and emitter, as shown in the image below.


These 3 pins will always be identified on the manufacture's data sheet. The differences in functionality of the PNP and NPN type transistors result from different arrangements of the P-type (positively charged) and N-type (negatively charged) semiconductor materials that form the transistors. In a PNP transistor, the collector and emitter are P-type materials and the base is a N-type material; the reverse is true for an NPN transistor. Because of these differences you need to remember the following:

NPN Transistor basics

(Taken from Make: Electronics, by Charles Platt)
  1. NPN transistors are activated by positive voltage on the base relative to the emitter
  2. To start the follow of current from the collector to the emitter, apply a relatively positive voltage to the base.
  3. In the schematic symbol, the arrow points from the base to the emitter and shows the direction of the positive current. NPN
  4. The base must be more positive than the emitter to start the follow of current.
  5. The collector must be more positive than the emitter.

PNP Transistor basics

(Taken from Make: Electronics, by Charles Platt)
  1. PNP transistors are activated by negative voltage on the base relative to the emitter
  2. To start the follow of current from the emitter to the collector (Notice that this is the reverse of the NPN transistor.), apply a relatively negative voltage to the base.
  3. In the schematic symbol, the arrow points from the emitter to the base and shows the direction of the positive current.PNP
  4. The base must be more negative than the emitter to start the follow of current.
  5. The emitter must be more positive than the collector.

All Transistor basics

(Taken from Make: Electronics, by Charles Platt)
  1. Never apply a power supply directly across a transistor, instead insert a resistor. You can burn the transistor out with too much current
  2. Always protect the base of a transistor with a resistor just as you would protect an LED.

Now that you're familiar with transistors, let's use one. Replace the switch in your circuit from Part 2 with a NPN transistor. Your new circuit should be described by the schematic below. You may omit the push-button switch and connect/disconnect the resistor to power manually, if you prefer.

NPN circuit

Make sure that the LED turns on when you activate the push-button switch.


Show your circuit to your instructor.  

Part 4: A PNP Switch

Revise the circuit above to use PNP transistor as opposed to a NPN transistor. Remember the rules for using PNP transistors provided above.


Show your revised circuit to your instructor.