# What is a pullup resistor and where is it used?

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#### scientificpakistan

##### Newbie level 5
What Is Pullup resistor?
Where Pullup resistor is used?

#### p.sivakuamr

##### Newbie level 3
Re: Pullup resistor

Hi
this is siva.pullup resistor is used to get the o/p to high level.Genarally pullup resistor is used external of the circuit.

#### vvvvv

##### Member level 1
Pullup resistor

Pullup resistor is connected to VCC

#### electronics_kumar

Re: Pullup resistor

then what is the difference between the pullup and pulldown resistor

#### IanP

Re: Pullup resistor

Pull Up, Pull Down
It is a bad idea to leave a controller input disconnected - just hanging in the air. At all times, it should be connected to either a logical 1 (+5 Volts) or a logical 0 (ground = 0 Volts). If you leave an input pin not connected ("floating"), it may read in as a 0, a 1, or just flap up and down in the breeze.
Simple electrical switches just open and close. If you hook a switch up to an input, and the switch is open, the input will float and can not be trusted. You have to find a way to make sure that you always present a solid 1 or 0.

A common way to impose a default input is with pull-up or pull-down resistors. These resistors can be used to shove a 0 ("pull-down" to ground) or a 1 ("pull up" to +5 volts) into input pins.

The 0 or 1 provided by the resistor can be safely overpowered by the switch when it is activated. You should use the resistor that does the opposite of what your switch will do when activated:

if activating the switch pulls the input low (to ground), use a pull-up resistor
if activating the switch pulls the input high (to +5 Volts), use a pull-down resistor
Regards,
IanP

#### Atzakas

##### Newbie level 6
Re: Pullup resistor

You can find a simple explanation here:

Code:
http://www.seattlerobotics.org/encoder/mar97/basics.html

#### sabu

##### Junior Member level 3
Pullup resistor

pull up resistor connected between supply terminal and o/p terminal of eny device ensures the o/p node eharges to a voltage from the supply,if the node is open circuited.
regards,
sabu

#### EEHardware

Pullup resistor

pull up=1; pull down=0

#### Miguel Gaspar

Pullup resistor

Pull-up resistor are used in transistor when one lead are uncompromised, like a colector when this lead is not conected, and to operate you conect a resistor between Vcc and this collector, such resistor is called pull-up.

Pull down is the oposite, that is for example a resistor conected betwen emisor an earth.

#### Mistico_Pereyra

##### Member level 3
Re: Pullup resistor

Miguel Gaspar said:
Pull-up resistor are used in transistor when one lead are uncompromised, like a colector when this lead is not conected, and to operate you conect a resistor between Vcc and this collector, such resistor is called pull-up.

Pull down is the oposite, that is for example a resistor conected betwen emisor an earth.

That's rigth, as a matter of fact, you call "Pull up" when the resisitior conects to VDD and you call "Pull down" when the resistor conects to GND!!

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