Annoying problems getting started with logic circuits (SN74LS)

Question

I'm trying to get started making logic circuits but I'm not making much progress even with the most basic. I have a SN7408 which is an AND gate however it doesn't matter what I put over the inputs (1A and 1B) I get continuity from Vcc to 1Y which is rather irksome.

I have been playing around with a few USB power sources 5v from a 230v mains adapter. 5v from my macbook and 5v from a recently purchased phone powerbank.

I'm guessing I'm burning out the chips. How can I stop doing that?

Any help gratefully appreciated!

Cheers,

Andrew -

Could it be that you have just fried your part while "playing around"? — Eugene Sh., Dec 23 '16 at 20:44
Are you supplying +5 to pin 14? Assuming that the dark wire seen at the very bottom of your pictures is hooked up to pin 7 and tied to ground, then it looks like pin 14 is wired to a brown wire and up towards a switch block above (switch #1.) Do you supply +5 V in that fashion? — jonk, Dec 23 '16 at 20:50
[Where are your pulldowns?](http://electronics.stackexchange.com/questions/135464/can-a-transistors-emitter-and-collector-be-used-interchangeably) — Ignacio Vazquez-Abrams, Dec 23 '16 at 20:54
Can you tell us what you have done with the inputs?. Like wired them straight to 5v, straight to ground Or as it should be used a resistor to pull up and switch to pull down. — RoyC, Dec 23 '16 at 20:57
To start messing with logic it might be better to start with cmos logic, it is a bit bit more tolerant of supply and input voltages. — RoyC, Dec 23 '16 at 20:58
Bypass capacitor close to each IC, 0.1uF directly across the supply, right at the IC pins, is standard practice. Most ICs will not work right if the supply is unstable. — MarkU, Dec 23 '16 at 21:08
Just as a comment I'll say first: don't be discouraged! Whatever the problem, you'll be learning a lot about how many things can contribute to an unexpected result. But second, although a picture (photo in this case) may be worth 1000 words, a schematic is almost always better for questions like this. Granted, the photo lets us see partly what your circuit actually is, but its very hard to see the WHOLE circuit based on a partial photo of the proto board, and the devil is certainly in the details. — Randy, Dec 23 '16 at 22:03
To follow up what Randy said, for example, your photo doesn't show that you've actually connected your IC to power and ground. — The Photon, Dec 24 '16 at 00:29

score 6 · Answer 1 · answered Dec 23 '16 at 20:57

The inputs of bipolar TTL (74nn, 74LSnn) parts will appear as a High when not connected, and will source 1 mA or so (varies with specific family) when taken low, so they are normally used with a switch between the input pin and Ground to ensure that the input will really be recognized as Low when we want it Low. It is generally recommended to also use a 5 - 10K pull-up resistor from the input pin to Vcc, to ensure that the input will be seen as High when the switch is open.

CMOS parts (74HCnn, 74ACnn) have very high input impedances, and the inputs will float randomly if not connected. All inputs (even unused ones) MUST have pull-up or pull down resistors to ensure that they will be in a known state when not driven High or Low by other means.

score 3 · Accepted Answer · answered Dec 24 '16 at 01:09

To expand on Peter's explanation, your two inputs are currently either switched to +5V, or open. The problem here is that both +5 and "open" are seen as logic "one", and so the AND gate output will always be a "one". You can fix this by putting strong pulldown resistors (220 Ohm) on the inputs, but with TTL it is much better to use pull-up resistors and switch to ground. The pullups can be 1K to 10K or so. With CMOS logic the resistors can be much larger, but they are still necessary.

Annoying problems getting started with logic circuits (SN74LS)

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