0
\$\begingroup\$

I have these two descriptions of systems:

  1. A pump will start if the pressure and temperature of liquid in a tank reach a certain threshold. Pressure and temperature sensors output a HIGH signal if the threshold is reached which in turn, starts the pump by outputting a HIGH signal. Draw a truth table and circuit for this system.

  2. A light has two switches. The light will turn OFF or ON using either switch without considering the other switch's state. List the inputs and outputs of the circuit for the light. What logic gate would represent this circuit best. List the truth table for the system.

For #1, isn't that just going to be an AND gate? We all know the truth table for that.

But #2 is a little more difficult. I'm not exactly sure how to go about that one. I've looked through my table of gates and their truth tables, but none of them seem to be what I'm looking for.

Thanks for any help!

Truth table for #10 (for OP to fill out). [Transistor] A | B | Z ---+---+--- 0 | 0 | 0 0 | 1 | 1 1 | 0 | 1 1 | 1 | 1

EDIT: So it is just an OR gate? When no switches are on then the light is off; if either switch is on then the light is also on; when both are on then the light should be on. That's how I read it at least and it makes sense.

Thanks for you help transistor.

EDIT 2:

A | B | Z ---+---+--- 0 | 0 | 0 0 | 1 | 1 1 | 0 | 1 1 | 1 | 0

So it is an XOR.

\$\endgroup\$
8
  • 2
    \$\begingroup\$ I'd rather give you the satisfaction of figuring this out yourself. Edit your post to include your effort at the truth table. There are only two inputs with four possible states and one output with two possible states. There's no nice table function in the editor so put four spaces at the start of each line to force the markup into "code" which is monospaced. \$\endgroup\$ Commented Jan 31, 2018 at 20:32
  • \$\begingroup\$ #10 does not specify any polarity, so there are two possible gates, one the inverse of the other. Just assume (randomly) that when both switches output LOW, the output should be LOW, and construct the rest from there. You will then end up with one of the common logic gates. \$\endgroup\$ Commented Jan 31, 2018 at 20:41
  • 1
    \$\begingroup\$ When A is HIGH, and B is switched, the output must change! In other words, the output flips whenever you flip any switch. \$\endgroup\$ Commented Jan 31, 2018 at 20:47
  • \$\begingroup\$ @CL. Then what gate do I use? Or gates? I can't figure this out. \$\endgroup\$ Commented Jan 31, 2018 at 20:52
  • \$\begingroup\$ @Transistor I gave it a go, but I do not think it is correct. \$\endgroup\$ Commented Jan 31, 2018 at 20:52

2 Answers 2

Reset to default
4
\$\begingroup\$

Perhaps you live in a land of single-story houses and are not familiar with stairway light switches. The idea is that you can change the state of the light by switching either switch. Here's how they're wired.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Stairway lighting circuit. Note that the logic can be inverted if the wires are crossed over between the switches so there are two possible solutions.

Now back to your truth table ...

\$\endgroup\$
1
  • \$\begingroup\$ I made an edit above and it resembles an XOR. \$\endgroup\$ Commented Jan 31, 2018 at 21:04
0
\$\begingroup\$

To answer my own question, #10 can be done with an XOR (or EOR) gate.

\$\endgroup\$
1
  • 1
    \$\begingroup\$ Correct. You can accept this as being the correct answer even though you wrote it yourself. You can upvote mine if you consider it helped (but I'm not short so don't worry). \$\endgroup\$ Commented Jan 31, 2018 at 21:10