Digital Logic Signals

If we need to produce a discrete logic signal (“high” or “low”) from a mechanical switch, the most direct way of doing so is to use a single-pole, double-throw switch (SPDT) like this:

In the “High” position, the switch directly connects the signal line to V, ensuring a high logic state; in the “Low” position, the switch directly connects the signal line to ground, ensuring a low logic state. What could be simpler?

However, often a SPDT switch is not feasible and we must use a SPST (single-pole, single-throw) switch instead:

A problem often arises with such configurations because in the open position there is neither a connection to V nor ground. In other words, these two SPST configurations produce the exact same indeterminate logic state (“floating”) when their respective switches are open. To remedy this, resistors are often added to such circuits:

Explain what functions the pulldown and pull up resistors serve, and also why they are referred to by those names.

A common mistake made by students new to digital circuits is to misplace the pull up or pulldown resistors in schematic diagrams, and also in the circuits they build. Study the following schematics and determine whether the resistor in each one is a properly-placed pull up or pulldown resistor, or if it is improperly placed:

A logic probe is a very useful tool for working with digital logic circuits. It indicates “high” and “low” logic states by means of LED’s, giving visual indication only if the voltage levels are appropriate for each state.

Here is a schematic diagram for a logic probe built using comparators. Each comparator has a threshold adjustment potentiometer, so that it may be set to indicate its respective logic state only if the signal voltage is well within the range stated by the logic manufacturer:

Explain how this circuit functions.