This is an exercise that I set up as part of learning electronics. Feel free to comment.
Pullup and pulldown resistors are used in digital circuits to keep induced voltages away from floating pins (i.e. pins that are not part of a circuit, e.g. when a switch is the open state), forcing them to have voltage within ranges that can be determined by the load as ON or OFF respectively. In this circuit both the left and right side of the battery uses a pullup and pulldown resistor (100 ohm) respectively for this purpose.
A floating pin without a pull-up/down resistor is not connected to any voltage and will instead act as an antenna and pick up energy from devices in the nearby environment. This in turn may induce a voltage on the floating pin. This is avoided by fixating the voltage on the floating pin with a relatively high resistor in series with the desired voltage.
When the switch is turned on only a relatively small current will flow through the pull-up/down circuit. This is a small cost in exchange for having the pin always set to a discrete state.
Left side: When the switch is OFF the pull-up resistor pulls up the far side of the main load (represented by the left 10 ohm resistor) to 5V. When the switch is ON the main load receives its required current while only a small current flows through the pull-up resistor.
Right side: The right side works according to the same principle, only fixing the far side of the main load to 0V when the switch is OFF.
