# Questions about signal and logic circuit

I just learned about basic logic and logic circuit from a Discrete Math book and I have a few questions:

• What exactly is a signal in Engineering? From Wikipedia, a signal is defined as a time-varying quantity, but I don't really understand what that means. Does the definition mean that anything that can change in a circuit is a signal?

• From my understanding of what signal is, in a switch circuit, the input signals are the states of the component switches and the output signal is the state of the whole circuit itself while in a logic circuit, the input signals are the voltage at points that are not behind any gates(and also behind a switch? I'm still not sure about this) and the output signal is the voltage at the point behind all logic gates, with respect to the Ground. Is my understanding correct?

• In a circuit that contains both switches and logic gates (not like just a few switches followed immediately by logic gates but maybe a bunch of switches connected in series and parallel and then is connected with a combinational logic circuit), what exactly are the input signals and output signal? I mean, is it possible for the input signals and output signals to be of different types?

• Everything depends upon the context. A power supply isn't called a "signal" when it is being used as a power supply. But a power supply turning on can signal something else, when it observes the power supply output for some other reason. And a power supply has signals within it which are needed to maintain regulation. It's in part about what you are working on at the time and at what level you are working. Don't look for bright-line definitions here.
– jonk
Apr 10, 2021 at 6:55
• "a signal is defined as a time-varying quantity" What is it about this definition that you do not understand ? Pressure of fluid in a tube, potential of a point in a circuit, photons passing pass a point can all be thought of a signals.
– AJN
Apr 10, 2021 at 6:58
• #ncc291203, I would recommend to follow these newbie friendly tutorials: (1) Digital Logic Gates - Electronics Tutorials electronics-tutorials.ws/logic/logic_1.html. This chapter starts witht the following: A Digital Logic Gate is an electronic circuit which makes logical decisions based on the combination of digital signals present on its inputs, ... (2) You might also like to buy some basic LEDs, button switches, basic logic gates (AND, OR, NOT etc), breadboard, jumper wires etc to start your practical work in the electronics hobbyist journey. Happy learning. Cheers. Apr 10, 2021 at 7:01
• Signal need not even be a time varying quantity. A space varying quantity like an image or magnetism of particles on a magnetic tape also can be thought of as signals and operations that can be done on time varying signals can be made to operate on space varying signals too.
– AJN
Apr 10, 2021 at 7:01

What exactly is a signal in Engineering? ... Does the definition mean that anything that can change in a circuit is a signal?

I don't have a dictionary definition but I think you can consider a signal as any modulation of an energy source that conveys information rather than power. This covers such cases as transmission of information, digital or analogue, by electrical, optical, audio, fluid pressure, mechanical coupling, etc.

... in a switch circuit, the input signals are the states of the component switches ...

I'd say the signals are the voltages provided by the switches.

... and the output signal is the state of the whole circuit itself ...

Again, it's the voltages output by the whole circuit, but you've got the right idea.

... while in a logic circuit, the input signals are the voltage at points that are not behind any gates(and also behind a switch? I'm still not sure about this) and the output signal is the voltage at the point behind all logic gates, with respect to the Ground.

Don't get so hung up on this. The input signals have to come from somewhere and may be as a result of some previous logic, simple switches or even (in contradiction to my "definition") from a permanently high or low logic level.

A signal doesn't necessarily have to be referenced to ground. Some signals are differential such as RS485 and USB where a pair of wires are used and swap polarity with each other to provided the signal.

In a circuit that contains both switches and logic gates (not like just a few switches followed immediately by logic gates but maybe a bunch of switches connected in series and parallel and then is connected with a combinational logic circuit), what exactly are the input signals and output signal?

Electrically there is no difference so it really just depends where you define the boundary of your system.

I mean, is it possible for the input signals and output signals to be of different types?

Sure. For example, a logic-level signal could be converted to a higher voltage or to an optical signal for fibre-optic transmission.

... but when 2 switches are connected in series if the first switch is open then no matter what state the second switch is, the voltage between it and the GND is always 0. simulate this circuit – Schematic created using CircuitLab

Figure 1. Switch logic feeding transistor logic.

For a mechanical switch circuit to feed a transistor logic circuit it is important that the logic input is not left floating where it might assume a logic 1 or 0 or indeterminate value. For this reason the logic would require a pull-down resistor so that if one of the switches is open that a definite 0 is received on the buffer input. When SW1 and SW2 are both closed a logic 1 is forced on the input despite the pull-down resistor. (That's the error in your reasoning. CMOS input impedance is so high that it doesn't pull the input to zero. The input voltage is floating and could be any value.)

That's probably why I think the input signals of a switch circuit are the state of the switches rather than something "voltage-related".

The switch status results in a voltage. The transistor logic only knows about the signal voltage and knows nothing about the switches.

• Can you explain more on this part: "I'd say the signals are the voltages provided by the switches". I "sort of" understand this in the case 2 switches are connected in parallel, but when 2 switches are connected in series if the first switch is open then no matter what state the second switch is, the voltage between it and the GND is always 0. That's probably why I think the input signals of a switch circuit are the state of the switches rather than something "voltage-related". Apr 10, 2021 at 8:46
• See the update. Apr 10, 2021 at 9:00
• Thanks for the update, but the real question I want to ask is what do you mean by the voltage provided by the switch? I don't think it's the voltage at the end side of the switch (due to my above reasoning). Maybe it's the voltage at the end of a switch in a (hypothetical) circuit containing only a battery and that switch? (I'm sorry if I sound like a skeptic here, but I just want to clarify things) Apr 10, 2021 at 19:52
• OK, then justify your statement, "... if the first switch is open then no matter what state the second switch is, the voltage between it and the GND is always 0." It's not 0. It's undefined. It's floating until you connect it to something referenced to ground (such as Figure 1, R1). Apr 10, 2021 at 19:55
• I understand that the pull-down resistor is required. What I trying to say is, suppose the voltage provided by a switch is the voltage between the endpoint of that switch and the GND. In case the first switch is open, the voltage of that switch is 0V, so no matter what the state of the second switch is, its voltage is always 0V, so the input-output table of that circuit will only have 3 rows. Therefore, the voltage provided by the switch is not the voltage at the endpoint of that switch. That's why what I'm trying to ask is what do you mean by the voltage provided by the switch Apr 10, 2021 at 20:11

a Discrete Math book

So a signal is what isn't noise. It's the same in engineering.

It doesn't matter if we talk about voltages, currents, magnetic orientations, a hammer ringing a bell, or light where there should be no light. All what's needed is that it must be an intentional(!) change(!), and it must be distinguishable(!) from the unintentional(!) changes of the same quality(!) but different quantity(!) that are always there.