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I'm having issues setting the state of any of the D Flip Flops, I have jumpers and a multimeter to work with. does the clock pulse have to more precise than touching the jumper to +5v?

74s374J
data sheet

AerodynamicBrick
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    Yes, you need a switch debounce circuit. See this answer https://electronics.stackexchange.com/a/18820/35022 – MarkU Apr 20 '17 at 00:36

2 Answers2

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The 74S374 is a bipolar TTL part. Bipolar TTL inputs source current, so must be pulled low by a very low value resistor, or more often a switch to ground, to guarnantee that the input will be seen as a Low. An open (unconnected) input will usually be seen as a logic High, but it is good practice to use a pull-up resistor to ensure that the input will be seen as High.

If you leave the clock input open, it will be seen as a High, and connecting it to Vcc will just make it Higher - it will never be Low, so no Low->High transition to clock the flip-flops.

Peter Bennett
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Maybe. The clock pulse has to go from a low level, < 0.8v, to a high level, > 2v, to strobe the flip flops. If the clock input was an open circuit before you hit it with 5v, you didn't have a low to high transition. You had a high to higher transition, which wouldn't do anything.

You'd be better off connecting the inputs to 5v through a resistor, then strobing by touching the jumper, also connected to the input, to ground.

Usually you will get many strobes (bouncing)by the jumper-touching. For what you seem to be trying to do it won't matter.