Electronic – DIP switch replacement

SMD resistors are as cheap as dirt. Especially if you would buy full roll of let's say 10k ones (i.e. something widely-used, so that you could use in multiple projects). It's hard to overload pull-up resistors, as they can dissipate 0.125W usually.

There are also 4 and 8 resistors in 1 package, this will be even cheaper if you are on large scale.

You cannot use single resistor for multiple wires - as they will be shorted (or you will need diodes :-D )

You are right, you need something more than a simple single pole single throw switch to make a digital logic signal. Usually that something is a pullup resistor. The resistor goes between the power supply and the logic line, and the switch between ground and the logic line. When the switch is closed, it hard-shorts the line to ground. When the switch is open, the pullup resistor provides a weaker drive to power.

The tradeoff is that you want the pullup stiff enough (low enough resistance) so that it can overcome any current that other parts on the digital line might be leaking to ground. It has to be weak enough (high enough value), so that the current is limited to a reasonable value when the switch is closed.

Most CMOS logic gate inputs leak little enough that a few 100 kΩ to power is enough to guarantee the voltage will be interpreted as a logic high level. However, you don't want the line to pick up stray noise and of course you want some margin. If in doubt, use a 10 kΩ pullup.

This same concept works flipped around too. You can use a pulldown resistor and put the switch between power and the logic line. For various reasons, switches, whether mechanical or transistors, are usually connected to ground, and therefore pullups are more common than pulldowns.