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Digital Logic Signals
9 questions By Tony R. Kuphaldt
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Question 1 of 9
A rheostat (variable resistor) and a switch are both examples of electric components exhibiting different degrees of conductivity:
Which of these devices would be considered discrete and which would be considered continuous in terms of their electrical conductivity? What do each of these words mean, and how might they apply to variables in electric circuits other than conductivity?
Reveal answerA continuous quantity is one that may be smoothly varied from one extreme value to another, while a discrete quantity is one that can only assume a finite (limited) number of distinct states. Here, the rheostat exhibits a continuously adjustable electrical continuity while the switch is discrete because it can only be conducting or non-conducting.
Follow-up question: what is the difference between a continuous voltage versus a discrete voltage?
Notes:The purpose of this question is to get students thinking in terms of “digital” quantities, which by their very nature are non-continuous. Since most electronics curricula focus on continuous quantities before discrete, it is good to have students reflect on the inherent simplicity of discrete circuitry and components after having studied continuous (analog) circuitry.
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Question 2 of 9
Digital logic circuitry makes use of discrete voltage levels: each “logic gate” sub-circuit inputs and outputs voltages that are either considered “high” or “low”. Define what both of these terms means in a digital logic circuit powered by 5 volts DC.
Reveal answer“High” = (nearly) 5 volts between the gate input/output and ground.
“Low” = (nearly) 0 volts between the gate input/output and ground.
Notes:This is a very simple concept, but worthwhile to cover in its own question just to be sure no students misunderstand when the concept is later applied.
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Question 3 of 9
Determine the logic levels (either “high” or “low”) at each of the test points in this circuit with the toggle switch in the open position, as well as the status of the transistor and LED:
- VTP1 = (high or low?)
- VTP2 = (high or low?)
- VTP3 = (high or low?)
- VTP4 = (high or low?)
- Transistor = (on or off?)
- LED = (on or off?)
Reveal answer- VTP1 = high
- VTP2 = high
- VTP3 = low
- VTP4 = high
- Transistor = off
- LED = off
Follow-up question: show how you would calculate reasonable values for the two resistors in this circuit.
Notes:This question applies the concepts of “high” and “low” voltage signals to a simple transistor circuit, reviewing transistor operation in the process.
