• 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

Transistor: A small electronic device used in a circuit as an amplifier or switch.
Symbol for Circuit
Diagrams npn transistor

•Considered by some to be the greatest invention of the 20 th 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

The small base current controls the larger collector current.

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.

The small base current controls the larger collector current.

Small load
(input)
Large load
(output)

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.
DC
Large
Current
Load
A small current amount switching on a larger current
1 K
5.1 K
Small Current
Load

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.

1 KΩ
10 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?

1 KΩ
10 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?

1 KΩ
10 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?

Build the circuit as shown using 6 DC volts to test using actual components using
Snap Circuits
® spring sockets.
1 KΩ
10 KΩ
On Pg. 7 use the following to fill in the blanks:
• Low or high
• Insulator or conductor
• Closed (on) or Open (off)
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).

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.

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