Substituting LS for 74 series? - Page 1
Forum section: electronics / projects

author: iXod
author_rank: Frequent Contributor
timestamp: January 29, 2017, 10:05:12 pm
content: In an existing antique circuit using 74121 one-shot, if the only repair part I can get locally is an 'LS121, any reason to not use this as a substitute for the 74121?Thanks.

author: james_s
author_rank: Super Contributor
timestamp: January 30, 2017, 02:59:08 am
content: Most of the time this will work. I don't recall all the specifics but LS is "Low power Schottky", it seems to be the most common of all the 74xx series.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 30, 2017, 06:48:12 am
content: Art of Electronics, 3rd ed., p462 and thereabouts discusses monostable timers. Unfortunately they don't discuss the '121, or the 7400 series '123. The related information may be useful, at least.Offhand, I might be concerned that the 7400 version can handle larger capacitances for the same discharge delay time. Which might need a smaller C and larger R to keep the same time.They were never particularly accurate anyway, and different brands have different timing and stability...Tim

author: David Hess
author_rank: Super Contributor
timestamp: January 30, 2017, 03:20:44 pm
content: Except for output drive current differences, low power schottky TTL can almost always replace standard TTL and I think that is the case here. The datasheets for the TI 74121 and National 74LS121 look identical and even have the same 0.7 time constant. As far as I can tell, TI never made a 74LS121.Update: I just noticed that the National 74LS121 datasheet I found is mislabeled and is actually for the National 74121 so of course it is identical to the TI 74121. I have found no 74LS121 datasheets. TI made a 74L121 which has about half of the drive capability but of course it draws about half of the power and is half of the maximum speed but otherwise looks like a direct replacement.

author: rstofer
author_rank: Super Contributor
timestamp: January 30, 2017, 04:23:49 pm
content: Why not use the real deal?http://www.jameco.com/z/74121-Major-Brands-MONOSTABLE-MULTIVIBRATOR-DIP-14_49322.html

author: drussell
author_rank: Super Contributor
timestamp: January 30, 2017, 06:49:24 pm
content: That totally depends on what it is driving, fan-out on outputs, etc. Remember that while the LS series don't suck nearly much input current as plain 74 series, they also have limited output source/sink capability.The data sheets tell you all of this. You need to look at the circuit to see what it is driving on each output and see if the LS121 has enough oomph. If not, you could change some of the chips it is driving to LS also but then you need to check everything else downstream of those, etc... or just get a standard 74121 and be done with it.Digikey stocks them.Edit: Actually, I don't even remember seeing anLS121 anywhere, ever. I have several 74121s in my logic chip drawer but no 74LS121s and it I don't see a data sheet for an LS anywhere in any of my data books or anywhere online.Are yousurethe local one you have available is really LS?

author: Zero999
author_rank: Super Contributor
timestamp: January 30, 2017, 08:00:05 pm
content: Quote from: rstofer on January 30, 2017, 04:23:49 pmCost? Higher power dissipation then the 74LS version?Personally I'd use the HC or HCT versions. If a much longer delay is required, then I'd use a counter+oscillator or MCU.

author: drussell
author_rank: Super Contributor
timestamp: January 30, 2017, 09:52:18 pm
content: Quote from: Hero999 on January 30, 2017, 08:00:05 pmUh... I thought he was repairing something thatalready useda 74121...Also, where exactly have you found said 74HC121? Or an LS121 for that matter?I don't believe such an animal exists, though I would be happy to be proven wrong with a photo of one in the wild!

author: Zero999
author_rank: Super Contributor
timestamp: January 30, 2017, 10:53:09 pm
content: Quote from: drussell on January 30, 2017, 09:52:18 pmThere are some 74LS121 ICs floating around in the lab where I work.You have a point about the 74HC121 though. I've never seen one of those. I've got plenty of 74HC123s, which I believe can be configured as a non-retriggerable monostable, but that's no good if you're reparing something.

author: drussell
author_rank: Super Contributor
timestamp: January 31, 2017, 07:11:51 am
content: Quote from: Hero999 on January 30, 2017, 10:53:09 pmUnlike unicorns, I believe that a 74LS121 might well truly exist, however, I don't recall ever actually seeing one in person and, after looking back in my stash, I'm now sure theydidn'texist back when I used an abundance of 74xx series logic in projects in the 1980s....I would still like to see an example of one! (and the datasheet for it, etc...)There are several 74xx series ICs that did not have LS equivalents back then (and usually for a good reason) and I would be fascinated to see the history of newer versions if they actually managed to make some of those circuits function to some kind of reasonable spec as other family variants...

author: David Hess
author_rank: Super Contributor
timestamp: January 31, 2017, 07:43:12 am
content: Quote from: drussell on January 31, 2017, 07:11:51 amI have occasionally found datasheets for TTL alternatives to standard TTL which were either so rare as to be unavailable or in at least one case never released.LS seems to be the most commonly available for me except for specific parts that never made it into LS but except for high drive parts, I do not know why they did not. I wish everything could have eventually have made to to FAST (fast advanced schottky) or AS (advanced schottky) TTL, especially counters like the 7490 and 7493 and power hungry function.

author: james_s
author_rank: Super Contributor
timestamp: January 31, 2017, 08:14:24 am
content: The remarkable thing is how many of the 74 series ICs are still manufactured. I suppose it's inevitable that most of them will go away at some point, but you can still buy brand new parts for many of them, in DIP packages no less. I doubt there are many new products made that use them anymore.