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when i turn on the circuit, the AND (74LS08) gate keeps sending signal to the 4 outputs, even without signal from the inputs. I may have broken it because i applied 7,5 volts before correcting the circuit voltage.

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At best the part is marginal. To clarify (and to check your IC), if your inputs are all 0 your output should be 0, if all inputs are 1 your output should be 1. If this is not the case your gate is toast.

My only worry is that you mention "without signal from the inputs". The output is not a tri-state device. The output will always be either 0 or 1 (and never off - a third state). Leaving your inputs floating by not connecting a signal to them (and expecting that the output is off) is incorrect.

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  • \$\begingroup\$ i'm studying, as you can see, i don't really know how to estipulate 0 and 1 signals, if, for example, the switch pressed and creat a current of 5 volts, will it be enough to 'tell' the IC that it's a 1\High? \$\endgroup\$ – Levi Trevisan Mar 12 '13 at 21:14
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TTL inputs are high when unconnected, therefore you should see a high output if both inputs on a 74LS08 gate are floating. The output should go low if you ground either or both of the inputs.

It isn't a good idea to leave logic inputs floating, even with TTL logic, as it can make them susceptible to noise.

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  • \$\begingroup\$ heyyy! thanks man, i didn't knew that the switches must be grounded, it's working now :] \$\endgroup\$ – Levi Trevisan Mar 12 '13 at 21:35

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