Transistors Magic of Electrons © 2011 Project Lead The Way, Inc.

Transistors Transistors Parts of the Transistor The First Transistor PLTW Gateway Unit 6 – Lesson 6.2 – Electronics Transistors Transistors Parts of the Transistor The First Transistor Transistors as Amplifiers Transistors as Switches Night Light Circuit This presentation is intended to be used with Activity 6.2.6 Transistors

Symbol for Circuit Diagrams Transistors PLTW Gateway Unit 6 – Lesson 6.2 – Electronics Transistors Transistor: A small electronic device used in a circuit as an amplifier or switch. Symbol for Circuit Diagrams As you are learning, Transistors are SEMICONDUCTOR devices with three small leads (legs). A very small current or voltage at one lead (leg) can control a much larger current flowing through the other two leads. This means transistors can be used as AMPLIFIERS and SWITCHES. npn transistor

Transistors PLTW Gateway Unit 6 – Lesson 6.2 – Electronics The First Transistor Considered by some to be the greatest invention of the 20th century Invented at Bell Laboratories in 1947 It is a crucial component in almost all modern electronic devices Engineers are working toward making them smaller so that more can be placed on computer chips. Many today are already smaller than a human hair. Replica

Parts of the Transistor Transistors PLTW Gateway Unit 6 – Lesson 6.2 – Electronics Parts of the Transistor The small base current controls the larger collector current. BASE EMITTER COLLECTOR Add a second junction to a PN junction diode and you get a 3-layer silicon sandwich. The sandwich can be either npn or pnp. Either way, the middle layer acts like a gate that controls the current moving through the three layers. The three layers of a transistor are the COLLECTOR, BASE, and the EMITTER. The collector leg must always be connected towards positive power. The emitter leg must always be connected towards negative power. The base is very thin and has fewer doping atoms than the emitter and collector. Therefore, a very small emitter-base current will cause a much larger emitter-collector current to flow.

Transistors PLTW Gateway Unit 6 – Lesson 6.2 – Electronics How It Works When there is no current to the base, no current can flow between the collector and the emitter, and the transistor is off. A small current to the base enables the transistor to conduct current from the collector to the emitter, and allows a larger current to flow through the transistor. Components connected to the collector can use this larger current. Note to the Instructor: The diagrams on this slide seem to indicate that there is a mechanical “switch-like” movement of a part in the transistor. This is simply a visual analogy to help students understand what is happening – that current applied to the base is allowing current to flow from the collector to the emitter. There are no moving parts in a transistor.

Transistors PLTW Gateway Unit 6 – Lesson 6.2 – Electronics How it Works The small base current controls the larger collector current. BASE EMITTER COLLECTOR

Transistors as Amplifiers PLTW Gateway Unit 6 – Lesson 6.2 – Electronics Transistors as Amplifiers Small load (input) Large load (output)

Transistors as Switches PLTW Gateway Unit 6 – Lesson 6.2 – Electronics Transistors as Switches Draw the schematic diagram for the circuit as shown using 6 DC volts. When the push switch is closed, you should notice that the LED in the circuit connected to the base is very dim. But the other LED that is controlled is much brighter. A small current amount switching on a larger current Large Current Load Small Current Load Students may be able to build the following circuit without viewing the physical image.

Transistors PLTW Gateway Unit 6 – Lesson 6.2 – Electronics Night Light Circuit Challenge: Create a system with a light that comes on when it is dark, similar to a street light. In the following circuit, will the LED be bright (on) at day or night? The LED will be bright during the day because brightness causes the photoresistor’s resistance to decrease. When the resistance of the circuit decreases, more current can flow to the LED.

Transistors PLTW Gateway Unit 6 – Lesson 6.2 – Electronics Night Light Circuit 1 KΩ Current will always take the path of least resistance. Does current reach the base of the transistor? Will current be able to reach the light? 10 KΩ The resistors in the circuit are added to protect the LED and the base of the transistor, and make a small contribution to the resistance in the circuit. Current cannot reach the base of the transistor in order to cause current to flow through the transistor to activate the LED because it is easier (less resistance) for the current to take the path directly back to the power source. If there is no current to the base of the transistor, the LED will not light.

Transistors PLTW Gateway Unit 6 – Lesson 6.2 – Electronics Night Light Circuit 1 KΩ In brightness the photoresistor’s resistance is low. Does voltage reach the base of the transistor? Will voltage be able to reach the light? 10 KΩ Current does not reach the base of the transistor, because it is still easier for the current to go through the photocell and back to the power supply than it is for the current to flow through the resistor. If the base of the transistor isn’t activated, current will not flow to the LED.

Transistors PLTW Gateway Unit 6 – Lesson 6.2 – Electronics Night Light Circuit 1 KΩ In darkness the photoresistor’s resistance is high. Will the base circuit of the transistor be activated? Will current be able to reach the light? 10 KΩ Once the resistance of the photoresistor becomes high enough (in light), it will bypass that path, go through the base, switch on the transistor’s path, and light the LED.

Transistors PLTW Gateway Unit 6 – Lesson 6.2 – Electronics Night Light Circuit Build the circuit as shown using 6 DC volts to test using actual components using Snap Circuits® spring sockets. 1 KΩ On Pg. 7 use the following to fill in the blanks: Low or high Insulator or conductor Closed (on) or Open (off) 10 KΩ If the circuit does not work as expected, troubleshoot using the flow chart provided in Activity 6.2.6 Transistors (omit the suggestion to check solder connections).

Night Light Circuit Create a permanent night light circuit. Transistors PLTW Gateway Unit 6 – Lesson 6.2 – Electronics Night Light Circuit Create a permanent night light circuit. Once you have tested all components to ensure that they are working properly, solder them to a permanent board as shown by your instructor. Your instructor will demonstrate effective soldering techniques and safety. If the circuit does not work as expected, troubleshoot using the flow chart provided in Activity 4.2.6 Transistors.

Transistors PLTW Gateway Unit 6 – Lesson 6.2 – Electronics Image Resources Microsoft, Inc. (2009). Clip Art. Retrieved January 27, 2009, from http://office.microsoft.com/en-us/clipart/default.aspx