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In a lab for an "Introduction to Electric Circuits" course, I built a simple Half-Adder circuit using an 74LS08 IC & 74LS86 IC. However, I'm experiencing issues with the output.

My input voltage supplied to Pin 14 on the 74LS86 IC was +5V and I verified that the output voltage (Pin 3 on the 74LS86) was +3.4V. The results from this IC were what I had expected; however, the output of the AND gate (Pin 3 on the 74LS08) was LOW when the two inputs were HIGH and was HIGH when the inputs were LOW. Below is the Truth Table for the 74LSO8 IC.

A   B   Y
0   0   1
0   1   0
1   0   0
1   1   0

And here is the Circuit Diagram

enter image description here

What would cause this behavior? As I understand, when the inputs are LOW the output should be LOW and when the inputs are HIGH the output should be HIGH.

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  • \$\begingroup\$ You've got the AND gate function wrong! Look it up. \$\endgroup\$
    – Leon Heller
    Jun 25, 2013 at 19:07
  • \$\begingroup\$ @LeonHeller I did misunderstand the output when both inputs are LOW, but the functionality isn't as expected when the inputs are HIGH. For instance, when the inputs are HIGH the output is LOW and when the inputs are LOW the output is HIGH. What would explain this? \$\endgroup\$
    – Jonathan
    Jun 25, 2013 at 19:23
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    \$\begingroup\$ Can you make a photo of your actual circuit? We might see something in the construction that would explain what you're seeing. \$\endgroup\$
    – The Photon
    Jun 25, 2013 at 19:24
  • \$\begingroup\$ I second ThePhoton's idea. \$\endgroup\$
    – Josh
    Jun 25, 2013 at 19:36
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    \$\begingroup\$ Adding real info regarding your particular circuit may help. As mentioned, photos with some labels would be useful in helping us help you. \$\endgroup\$
    – Jay Greco
    Jun 25, 2013 at 19:36

2 Answers 2

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You have misunderstood how AND gates work. Here is a very nice list of truth tables for various digital logic circuits. AND gates are only high when both inputs are high. You are thinking of an XNOR gate which produces a high output whenever both inputs are the same. The AND truth table is:

1in 2in out
0   0   0
0   1   0
1   0   0
1   1   1

and the xor is:

1in 2in out
0   0   0
0   1   1
1   0   1
1   1   0

Another thing you can do is check to make sure the AND gate is working as it should. Disconnect the XOR gate and connect the two inputs directly to the AND inputs and make sure the outputs match the truth table.

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  • \$\begingroup\$ I had forgotten my simple AND logic for the case in which the inputs are LOW, but the output is LOW when the inputs are HIGH. What would cause this? \$\endgroup\$
    – Jonathan
    Jun 25, 2013 at 19:26
  • \$\begingroup\$ Are you completely sure you have the circuit properly connected? Also, are you completely sure of the parts you are using? If the output is as you say, it sounds like you may be using a NAND gate instead of an AND gate. What are the outputs of the AND gate when the inputs are different? If the output is high when the inputs are different, you have a NAND gate. \$\endgroup\$
    – Josh
    Jun 25, 2013 at 19:29
  • \$\begingroup\$ It functions properly for all cases except when the inputs are HIGH. When the inputs are HIGH, the output is LOW. \$\endgroup\$
    – Jonathan
    Jun 25, 2013 at 19:31
  • \$\begingroup\$ @JonathanWhitaker Check my edited answer. \$\endgroup\$
    – Josh
    Jun 25, 2013 at 19:33
  • \$\begingroup\$ I have edited my original post, take a look at the Truth Table for the outputs that I am receiving. Let me know what you think. \$\endgroup\$
    – Jonathan
    Jun 29, 2013 at 21:18
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You misunderstand the AND gate. It is high only when both inputs are high. Not also when they are both low, as you said. And if one is high and one is low, the output is also low.

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  • \$\begingroup\$ Ok, so aside from being HIGH when the inputs are LOW I should expect to see that the output is HIGH when both inputs are HIGH. That being said, the main issue is that when the inputs are both HIGH the output is LOW. Likewise, when the inputs are LOW, the output is HIGH. \$\endgroup\$
    – Jonathan
    Jun 25, 2013 at 19:22

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