Tell me more ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.
up vote 0 down vote favorite

I am looking for a simple SR Latch. I'm new to electronics though and just doing a hobby project/teaching myself. Are SR Latches sold (I'm hoping to avoid building mine because I actually need at least 5) and if so where can I buy them (Radio Shack?) and what do I ask for? I link to a site with a part number would help too.

Per the comments. I have called Radio Shack and they said they didn't have them. The local big electronics store could only help me if I had a part number. I don't know if they actually can't help me or if I just don't know what to ask for. Is my best option just building them? I was kind of hoping I could just buy a small self contained item that was a SR Latch. If I were to wire them myself they would take up significantly more space.

share|improve this question
1  
This question will likely not be considered appropriate for the site, but the quick answer is, "74LS279" is a common part number for a quad SR-latch IC. And findchips.com is a site that lets you search multiple electronic components distributors for any part number you like. – The Photon Jan 28 '12 at 0:25
@ThePhoton: I agree with the assessment of the question as is, but with a few edits it could be a well-worded question. My first thought was, "Of course there's a quad RS latch available", but it took me quite a while to find one, and none of the 3 parts (LS279, CD4043/4044) are likely to be easily available except by mail order. – Jason S Jan 28 '12 at 0:38
1  
@JasonS, For me, TI's datasheet for LS279 came up in the top 5 Google results when I searched for "SR-latch". I had to look at a couple more links to remind myself that it only takes 2 NAND or NOR gates to construct an SR-latch. – The Photon Jan 28 '12 at 0:41
1  
@William: This and other stackexchange sites are more welcoming to questions that show an appearance of more effort on your part; if you said "I tried googling for SR latch but couldn't find a part number", and didn't say "link to a site with a part number would help too" (makes it sound like you're expecting us to do your work for you), I think your question would be better received. – Jason S Jan 28 '12 at 0:41
1  
@ThePhoton: When I google "RS latch" (which I was what I was taught it was called), I get the Wikipedia page on flip-flops as the first result, which nowhere mentions the 279, and a number of diagrams of the standard pedagogical NAND and NOR gate implementation. (If I recall correctly, latches and flip-flops can have metastability issues if you try to naively construct them from gates.) It took me several minutes of searching to find the logic gate numbers for RS latches, and that's for someone who's fairly familiar with manufacturers' and vendors' websites. – Jason S Jan 28 '12 at 0:45
show 8 more comments

closed as too localized by Kellenjb, Brian Carlton, Leon Heller, Kevin Vermeer Jan 30 '12 at 15:19

This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, see the FAQ.

3 Answers

up vote 4 down vote accepted

RS latches have several parts in the industry-standard 7400 and CD4000 series, all of which are quad packages in old logic families:

Another alternative, with two parts in a package rather than 4, but more available in modern logic families (read: lower-voltages) would be to use a 74xx74 dual D flip-flop as an R-S latch by grounding its data and clock inputs, and using the PRE and CLR inputs as S and R inputs, respectively. In this case, I'd start with a 74HC74.

share|improve this answer
Thanks for the information. I was hoping to be able to just buy a little latch. For some reason having to buy a quad seems not right and I need 5 anyway so I would have to get two of them. Arg. – William Jan 28 '12 at 2:54
@William, you can build a latch with transistors. Jason, could you expand on why I would pick 74LA279 over a CD4043? That is what makes this a serious answer instead of links to suppliers. – Kortuk♦ Jan 28 '12 at 14:28
I probably wouldn't pick either; LS279 is faster but CD40xx works with higher-voltage circuits, and neither work with supplies under 4.5V. I'd use the hc74 instead unless I were pressed for space and could get by with one of the others. – Jason S Jan 28 '12 at 20:49
@JasonS, I meant to say, why not take this as a chance to explain why you would pick each family and why they are separate chips. Make it a more thorough answer. – Kortuk♦ Jan 28 '12 at 22:03
up vote 0 down vote

you can make it using two NANAD or two NOR gates.You have no need two buy IV for S-R latch even.

share|improve this answer
1  
I know I could build one. But I need a minimum of 4 so if I was going to build one I would have to build 4 of them. This would take a lot more space than some pre-packaged version and I will probably only have a few square inches of space to put them in. – William Jan 28 '12 at 19:13
@William, space requirements are useful for your design. – Kortuk♦ Jan 28 '12 at 19:55
up vote -3 down vote

Digikey
Newark
Mouser
Future Electronics
Spark Fun

share|improve this answer
1  
I didn't "-1" you, but I agree with the sentiment. It seems like the OP is asking for more help in parts selection than in vendor selection. – Jason S Jan 28 '12 at 0:36
If you're going to include SparkFun, may as well include Jameco too: jameco.com . Historically they had awful prices, but checking them out today they beat out the big distributors on several hobbyist-friendly parts (74ls04 in DIP, 1n4004, etc.). These were their own featured parts so YMMV. – The Photon Jan 28 '12 at 16:39

Not the answer you're looking for? Browse other questions tagged or ask your own question.