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| 74HCT74 NXP vs Texas |
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| David Hess:
--- Quote from: IDEngineer on December 28, 2018, 09:09:14 pm --- --- Quote from: David Hess on December 28, 2018, 08:36:49 pm ---Or use 74Cxx which is high voltage CMOS like the 4000 series but with 74xx pinouts and functions. --- End quote --- I had the exact same idea, but then I looked at specs and availability... the 74C74 is listed as "obsolete" from many manufacturers and the only one in stock (at DigiKey anyway) wants $7 each in low volumes. Also, the spec sheets I checked do not mention a Schmitt Trigger input on the clock, nor any other input. --- End quote --- Clock inputs often use Schmitt trigger operation to avoid the exact problem you had but like I said, this is not always reflected accurately in the datasheet and it varies between manufacturers for the same part. --- Quote ---There were other CMOS-based 74-series parts back in the day, so perhaps one of those is available and would work, but I'll leave such research as a worthwhile exercise for the OP. If he's got a $7 budget to solve this problem, my 25 cent recommendation and a couple of FET's would allow him to use 3.3 or 5V logic to control a 12V environment and still leave enough money for lunch. :) --- End quote --- Yea, availability and pricing will be a problem for such an old part and technology. I do not know offhand what is still considered current. It is too bad because high voltage 4000 and 74C series CMOS are very useful in mixed-signal applications which now have to be redesigned to use a lower voltage logic family and level shifting. |
| vk6zgo:
IC manufacturers are not always innocent when it comes to strange things happening when using supposedly identical devices only separated by a fair few years between manufacture. I had to work on a piece of equipment which used a mono stable IC in the 4000 series, where it was used to provide a quite long "on" time. Unfortunately, one of these devices became lost. "No worries, there are plenty in the Store!" When I tried to use the new one, it kept timing out early, so the equipment didn't work. The same result with all the new ones in the Store. It turns out that the manufacturer originally specified the mono for a very large range of "on" times, but some users had problems with them with short "on" times. The answer was to change the design, so that the original part number was good for fairly short durations, & introduce a new part number for people who want long durations. We had to specially order the "long duration" version, replace the originals with the new type in both the units using them, generate an internal Tech note, & change the original schematic.......Aaaarrrrgggghhhh! |
| Jan Audio:
So i build the new circuit, now i have something weird that wont happen with the NXP : If i touch with the scope or even a loose wire the output of the SN74HCT74N, all working good, soon as i release the wire, the thing stops working. In my circuit i attatched the output signal to the base of a transistor with a 470K resistor. I hope the SN74AHCT74N dont have that problem. Can someone explain ? thanks |
| IDEngineer:
--- Quote from: Jan Audio on January 02, 2019, 02:48:04 pm ---If i touch with the scope or even a loose wire the output of the SN74HCT74N, all working good, soon as i release the wire, the thing stops working. --- End quote --- Classic behavior of a very high impedance node. Try lowering that 470K value, paying attention to the output current capacity of the '74. |
| Jan Audio:
Thanks, i go read some datasheets. With a 10K pulldown resistor also did not work. |
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