Transistors

Transistors

• Transistors• Parts of the Transistor• The First Transistor• Transistors as Amplifiers• Transistors as Switches• Night Light Circuit

TransistorsTransistor: A small electronic device used in a circuit as an amplifier or switch.

Symbol for Circuit Diagrams

npn transistor

The First Transistor

Replica

•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.

Parts of the Transistor

BA

SE

EMIT

TER

CO

LLEC

TOR

The small base current controls the larger collector current.

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.

How it WorksThe small base current controls the larger collector current.

BA

SE

EMIT

TER

CO

LLEC

TOR

Transistors as Amplifiers

Large load (output)

Small load (input)

Transistors as Switches

DC

1 K

5.1 K

Large Current

Load

Small Current Load

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

Night Light CircuitChallenge: 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.

Night Light Circuit

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?

1 KΩ

10 KΩ

Night Light Circuit

In brightness the photoresistor’s resistance is low.

Does voltage reach the base of the transistor?

Will voltage be able to reach the light?

1 KΩ

10 KΩ

Night Light Circuit

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?

1 KΩ

10 KΩ

Night Light CircuitBuild the circuit as shown using 6 DC volts to test using actual components using Snap Circuits® spring sockets.

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).

1 KΩ

10 KΩOn Pg. 7 use the following to fill in the blanks:• Low or high• Insulator or conductor• Closed (on) or Open (off)

Night Light CircuitCreate 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.

Image Resources

Microsoft, Inc. (2009). Clip Art. Retrieved January 27, 2009, from http://office.microsoft.com/en-us/clipart/default.aspx