pull-up resistor

thatsme

I am confused about the concept of a pull-UP resistor.
With resistance, voltage will always be decreased???? right?
Can someone please point me to a resource that explains this? thanks.

I am on p 79 of What's a Microcontroller (Fig 3-9), and I just cannot understand this concept.
A little confusing also is that when using the pullDOWN resistor it pulls voltage to -0-. There should still be some voltage....is it more like "not enough to matter"?

I tried eliminating the pull UP resistor....when the button is pushed the stamp resets. Maybe this means that the voltage was low, causing the reset. So it really needed a pullUP resistor apparently.

I have just discovered the stamps. Great fun! I have wanted to learn something about electronics for years, so this is an excellent way. At one time I programmed a lot in BASIC so the programming part is easy.

Best regards
Mark

Mike Green

Say you have a switch connecting a Stamp I/O pin to ground when it's closed. What's the voltage on the I/O pin when the switch is open? The answer is "I don't know". The I/O pin isn't connected really to anything at that point. As a result, the voltage on the I/O pin will be influenced by static charges in the vicinity, by induced 60Hz hum from power cords and wiring in the walls, and by other sources of electrical and magnetic noise nearby.

A pullup resistor provides a known voltage to an I/O pin. Since the input circuit draws very little current (microAmps), the resistor can be high in value (10K to 100K typically) and connects the I/O pin to the supply voltage of the chip (+5V in this case). When the switch is actually closed, the high value limits the current through the switch and the switch can very easily bring the voltage of the I/O pin very very close to ground.

You can reverse the roles of the switch and pullup to make a switch and pulldown resistor with the switch connecting the I/O pin to the supply voltage and the resistor providing a known zero voltage to the I/O pin when the switch is open. It's often easier to connect the switch to ground, so most circuits of this sort use a pullup resistor.

thatsme

Thanks. That will help me digest this. (Tho it will take a little more thought to reach karma on it !)

I sure am glad I discovered these products. They are great!
Thanks to you and other great educators on here too.

m