1. Pull-Up & Pull-Down Resistors
The How and Why
Instructor: Richard T. Vannoy II

2. Pull-Up & Pull-Down Resistors
Why are they used/needed?
What are they?
What’s the difference between pull-up & pull-down?
How do they work?
What size resistor should I use?

3. How NOT to connect!! Switch closed = Good Ground
Switch open = FLOATING!!

4. Pull-Up Resistor Switch open = Good 5 volts
Switch closed = Good Ground
I = V / R
= 5V / 10K
= 0.005A
= 5mA

5. Pull-Down Resistor

7. What Value for R1?? R1 = 1/10th of R2 R1 Same as R2 10 Times R2 10 KΩ
Switch Open
Volts
4.54 V
2.5 V
0.45 V
Switch
Closed
0 V

8. THREE GREAT LINKS
Very Basic Circuits; The Pull-up Resistor is an excellent article with illustrations.
SparkFun article 'Pull-Up Resistors’ goes deeper and shows you how to determine the size of pull-up resistor to use.
Arduino article 'Digital Pins’ gives a good explanation why 'floating' pins cause problems, and shows how to access the built-in 20K pull-up resistors in the Arduino series of boards.

9. The End
Any questions?