How to select a Pull up or Pull down resistor value

If you don’t know what is pull up and pull down resistor then please click on the following the link.
Pull up and Pull down resistor

1)why we do not use a small value of pull up resistor / pull down resistor:

If we select a small value of pull up / pull down resistor  like 10Ohm or 100Ohm etc, then when the switch is pressed, a high amount of current will flow through circuit  and there is no flow of current through internal resistor (input impedance of microcontroller pin) as it has a high resistance path which is shown below. Problem with selecting a small value of the resistor is that it consumes more power whenever the user presses the switch. Actual problem comes when you are trying to operate microcontroller on battery supply, because of large current flows through the circuit when the switch is pressed and the battery will discharge soon so because of this we can’t select a value of resistor as small.



2)why we do not use a large value of pull up resistor / pull down resistor:

If we select a large value of pull up/ pull down resistor like 10M Ohm or 100M ohm etc, then when the switch is not pressed circuit will behave like voltage divider which is shown below. Ideally, pull resistor provides logic 1 when a switch is not pressed but as the value of pull resistor is larger than the input impedance of microcontroller input pin, it offers logic 0 to the microcontroller when the switch is not pressed which is shown below.





3)So to avoid the above two problems, the value of pull up resistor /pull down resistor should not be small or large. Normally it is considered as 1% of the input impedance of microcontroller pin. so normally the value of pull up/ pull down is 10k because the internal input impedance of microcontroller pin is normally in the range of  1M – 10M ohm.

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