Rooting around in the fascinating stuff at the bottom of a draw labelled 'Engineering - Junk Miscellaneous'. Delving amongst the delightful...

Hysteresis mystery solved, probably

mark-6.1-crop.jpgI think I have got to the bottom of the 74HC74 hysteresis mystery

The one that meant my LED-controlling bistable switch was not switching properly.

It looks very much like some brands of 74HC74 have hysteresis on the clock, and others do not.

I found an old RCA part, whose data sheet specifically mentions hysteresis, and the circuit worked like a charm immediately.

At this point I feel the need to say that the part I found was actually a TTL threshold CD74HCT74, so it was not quite a change-one-thing-at-a-time operation.

But the slow negative slope that was causing so much trouble with the other 74HC74s is even slower as it passes through the TTL threshold.

Anyway, all the above was with a nice lab PSU and to get nice clean operation in the real application, which has a noisy negative rail, I made a few more changes compared with the previous circuit.

mark-6.1.jpgThe capacitor is now returned to the quieter +VE rail to help prevent noise getting out of the box on the push button leads – which is why the switch is conneced to the positive in the first place.

There is a 100R resistor to stop the capacitor discharge current gradually eating away the push button contacts.

And there is an RC filter to stop the -VE rail noise upsetting the CLK threshold voltage and messing up the switching again.

What do you think?


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I will keep it that way for as long as possible.


Tags: blog, clock, data sheet, push button, time operation

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