First of all I am a beginner.
I am practicing with the 74LS32N (OR Gate) IC , I noticed that it behaves like an AND gate instead of an OR Gate.
Only when both the inputs are 1 , the output is 1 , (Led will light up)
What have I done wrong here ?
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\$\begingroup\$ Check power supply connections and bypass caps. Since it is an LS TTL amily chip, the 10k pull-down resistor makes little sense. It would be fine with CMOS inputs, but with LS TTL, the buttons should pull low and the resistor should pull high and have lower value like 4.7k. LS chips also can't source out a lot of current to drive the LED, but it can sink in a lot more. \$\endgroup\$– JustmeCommented Jun 27, 2021 at 7:46
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\$\begingroup\$ Have you tried a different 7432? \$\endgroup\$– HandyHowieCommented Jun 27, 2021 at 10:57
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\$\begingroup\$ @Justme thanks for the advice , I changed the resistance but the result is the same \$\endgroup\$– Kandy boyCommented Jun 28, 2021 at 3:52
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\$\begingroup\$ @HandyHowie , yes \$\endgroup\$– Kandy boyCommented Jun 28, 2021 at 3:52
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\$\begingroup\$ Can you add a photo of the constructed circuit? \$\endgroup\$– HandyHowieCommented Jun 28, 2021 at 5:45
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1 Answer
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74LS TTL needs more than 10K for a pulldown. If you measure the input voltage without the buttons pressed, it is probably around 4V. A 1k pulldown would be more successful.
The reason is that the LS TTL input sources current - something like 400uA. Ohms law tells us V = I x R. 400uA x 10000 = 4V.
Try with 1k. 400uA x 1000 = 0.4V.
Or use 74HC/HCT chips.
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1\$\begingroup\$ how does this explain it acting like an and? \$\endgroup\$ Commented Jun 27, 2021 at 10:27
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1\$\begingroup\$ @Jasen - good question. I dare say there are other factors involved. Fix the obvious defects and see where that leads. \$\endgroup\$– KartmanCommented Jun 27, 2021 at 13:00
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\$\begingroup\$ @Kartman I tried with 1k and 10k both, I got no new result \$\endgroup\$ Commented Jun 28, 2021 at 3:51