1. Pull-up & Pull-down Resistors
Often we want to connect a digital input line to
our microcontroller. Typically this might be to
allow us to monitor the on-off state of a switch.
Eg.
5V
Microcontroller
switch
0V
(gnd or )
At first glance this seems fine.
When the switch is closed, the pin on our
microcontroller is tied to 0 volt, ie. low.
In contrast when the switch is open we would
want the pin to be 5 volts, or high.

2. The input pin would tend to “float” high. This
however isn’t a true input signal, it is a very
weak input and can readily switch from high to
low through the slightest of electrical
interference in any of the wiring.

3. A simple solution might appear to involve
simply connecting the other end of the switch
to our 5 volt supply
5V
Microcontroller
switch
0V
(gnd or )
This will give us a 5 volt (high) signal on the
input pin when the switch is open. When the
switch is closed however we will get a short
between supply and ground => zero
resistance => infinite current - this is not good
news.

4. The problem can be remedied by simply
putting a resistor into the circuit. This is the
pull-up resistor.
5V
10 kΩ
Microcontroller
switch
0V
(gnd or )
When the switch is open, the input to the
microcontroller is high. There is no direct
connection to the 5v rail, however because the
input impedence to the microcontroller is high,
very little of the 5v is dropped over the pull up
resistor.
When the switch is closed current flows down
through the resistor and through the closed
switch to ground. The input pin is tied to
ground and so will read low.
This gives us what we want.

5. A variation on this is the pull-down resistor.
This ties the input pin to ground rather than the
supply voltage.
ie.
5V
switch
Microcontroller
10 kΩ
0V
(gnd or )