"I've been staring at 555 timers all day, and this has been the most helpful description of how they work. Thank you!" - Comment from below. Same for me :)
It really is a pleasure when you're trying to learn something and you're in new territory and you find someone that can explain something in a clear and illuminating manner. Thank you.
AMAZING. This is the kind of step-by-step top-notch shit my University should be giving me for what I pay. This is real teaching, understanding, learning. Thank you very much, I must integrate every detail into my understanding of the topic or else I understand nothing at all, and you went through every piece of this circuit. Exactly what I was looking for. Thank you again I just felt a wave of relief because I know I can now complete this lab and I have learned something.
The other 555 timers make one or two good points before they lose me, so I watch many to figure this chip out. Skinny's video is the most understandable out there, step by step I learned more from his than any other. Keep up the good work!
I am at the second year of the university of applied science to piraeus at GREECE and i am study electronic engineer .Your video for 555 informed me very very well for the 555 !!!.....
This is way easier to understand in 'ordinary English' compared to any of the 'best tutorials' which assume people already know various electronic terms. Thanks
Phenomenal. The way you stepped through the logical process made it make perfect sense. I went to trade school for electronics and no one could adequately walk the class through just what made the ol triple nickel tick, but I understand it now. You've earned a new subscriber.
thanks Skinny R&D! Having finished a tech school back in 1980, the 555 timer has been around for a while. Your explanation has been so-o-o-o much better than trying to decipher the data sheet!
Man, you're a great teacher. Loved it. Now I know the inner working, the general current flow and most importantly the connections. The formula is perfect and thanks for the last tip on making Rb a potentiometer to vary the output frequency. More from you will be very appreciated at any time.
Thank you! This is my first attempt at trying to understand the 555 chip. Because I'm not formally trained and/or schooled in this field, ( I've had an inexplicable fascination with microelectronics and the physics behind it for over 55 years... I just can't explain why...), I'm not fully understanding the tutorial. I must reiterate that this is because of my ignorance in this field and not due to any failure on your part to explain this chip. I'm sure that those who have a far better grasp of this topic will find this tutorial 100% understandable. I noted that my first point of confusion was when you kind of glanced by the Capacitor for pin 5 (around 5':30" on the video). I think I understood you to say that this pin should ALWAYS have a cap connected to it, (I believe that's the negative leg), but I don't see why it needs to be there if it's not going to close this, or any other circuit in this type of A-Standard mode 'clocking-timer' example. The second point is around the 8':50" spot in the video. You mentioned that when the Flip Flop = 0, then the Output Stage voltage will be high. That voltage would be approximately equal to 9v minus 1.7v, (or otherwise, 7.3 volts?). I'm not clear as to where you got the 1.7 volt figure. Is that some sort of formula constant? (Kind of like the 63.2 figure when measuring a sine wave from an oscilloscope grid?). And finally, the third point where you reference the Frequency to equal 1.44 / (Ra + 2Rb) x C. Where did the 1.44 figure come from? I understand this video was posted some time ago and don't really expect a reply, but I'm just taking a shot at it and maybe you'll find my comment at some point in time. Thanks again for a great tutorial and a very important topic to me. Rich
Awesome video, explained it far better than the supposed "best 555 timer on youtube" video... thanks for your staggered approach and giving the learner chance to take in the information. You would make an excellent lecturer.
Thanks so much man. Just started my electrical engineering degree and they threw a 555 at us in the third week. You made it very clear compared to many other tutorials I've seen.
At first thanks for this good video and i want to add notes : 1. we can control the time of on and off of the led by :Ton =.693*(Ra+Rb)*c , Toff=.693*Rb*c 2. the output of flip-flop is connected to the pin 3 not on Transistor and the complement of the output of f-f is connected to the transistor. 3.Trigger pin is responsible to make set not rest , and the Threshold is responsible for rest not for set . :)
that was great, I built several 555 projects and kept searching on youtube to get a explanation on on how it works and all I would get is either people just hooking it up and saying "this is how it works " or some engineer with a way over complicated explanation of theory and formula tables. Thanks for making it simple, now I get it to the point I can even modify in a different circuit and drive a mosfet with it because I "get it"...
This explanation of 555 circuit is amazing. Congratulations .The other one on youtube who does perfectly explaining the mechanics of the electronic devices is Ben Eater.
Not sure how I stumbled across this, but very interesting and well explained. I've been using 555's for various projects for years, still learned a bit from this.
Thanks a lot! The output of the flipflop is connected to a power amplifier whose output is always inverted. That's why the output taken through terminal 3 is always inverted.
That was excellent and will definitely help me teach in electronics student when she goes to university in September I am very grateful for your clarity.
Thank you man, your explanation is on point, The best 555 timer tutorial on TH-cam, you definitely should do more electronics tutorial videos like this one.
Brilliant tutorial. You have good teaching skills and is delivered at an interesting pace. Well done. Best explanation I have heard. You got another sub.
Your the one that made me understand the 555. Merci beaucoup. just a green horn amateur newbie in electronics but I like to know how things work if i'am using them. I will soon promote my self to the IR2153 and hope to meet some one with your talent on my way.
Outstanding demo/explanation!!! I think what would help a ton is if you moved this circuit onto a breadboard so we can actually see the electronic pieces and their connections as you go through your wonderfully detailed explanation.
p.s. this is the best so far. and I have watched lots of 555 videos. I learned the most from your video than any other. thank you so much for making this video!
Thanks for gently introducing the 555 timer in astable mode. Deciding just how much detail to include can be a tough call. Gotta keep up the interest by imparting useful information, but not leave your viewers behind.
In most of the diagrams for the internal 555 the output of the comparator connected to pin 2 (1/3 threshold) is used as the S of the flip-flop and the output of the comparator connected to pin 6 (2/3 threshold) is used as the R of the flip-flop. Also , the output driver is connected to the Q' not to the Q of the flip-flop. But this set up still works and the video was great!
Thanks! Good thoughts about the video. Pin 6 is responsible for the S of the flip flop with the timer I'm using. Here's the datasheet: www.ti.com/lit/ds/symlink/lm555.pdf The reference is on page 3 under the description of the threshold pin. It's a bit different than some of the other 555 timer datasheets and diagrams on the internet. I'm intentionally vague about Q and Q' as the outputs of the flip-flop because the reference datasheet is as well. I avoid having to explain the existence of a second flip-flop output when the datasheet schematic (page 3) doesn't show one. Thanks again for watching and have a great day!
thank you for this great lesson. Realy made it clear what is happening and why. I alway enjoy in depth tutorials like that. Because to really learn somehting you gotta spend some time on it.
Im an engineering student and a qualified electrician, why is it that I have to look up youtube to find explanations like this to understand concepts. I pay for my uni fees and watch lectures but at the end of the day I dont get it until I find videos like this on the net.. Great explanation, why is it not taught like this in the first place????????
+Michael Latu I found that some of my professors had "the curse of knowledge". They understood so much that they were blinded to the misunderstanding of those learning material for the first time. They assume someone has previous knowledge that many times they do not possess. I occasionally fall victim to it myself. My answer to avoiding that pitfall is to ask for feedback the right way and often. Also, there are more than a few professors who have great minds but are terrible teachers. The university profits while the students suffer.
at 7:38 the video stops and talks about the voltages at V+ and V- of the comparator. trouble is there's two comparators on the board and they're both labelled with V+ and V-. so I wasn't sure which one you were talking about. would be good if you add a annotation to clarify. :)
You can save one resistors if you control the astable multivibrator's capacitor just via a resistor between pin 3 and pin 6 which is connected to pin 2. There is no need to use the discharge pin 7.
I like this tutorial very much, it's very clear and understandable but it would be better for me personally if you gave some explanations about the formula and where the 1.44 value comes from as well as how we can control the output by picking up the value of the capacitor on pin 5.
Hi, Great video!! I have a 555 timer circuit that doesn't use pin 7. Instead I have the capacitor discharging through a speaker that is also connected to pin 3. This gives me a siren sound. What is the frequency calculation without using pin 7 and R2 in your diagram. Thank you
+Anthony B The simple answer is that the 1.7V is dropped inside the 555 timer and it doesn't always have to be 1.7V. A certain amount of voltage is used to run the timer, but internally not all of that voltage will make it to the output when the output goes High.This drop will vary. If you are running the timer with a 5V source, the drop on the output will be less than 1.7V. If you are running the timer on 15V and are drawing 200mA of current, the drop could be 2.5V. For a definite answer of how much you could lose in an application, the first table on page 6 of this datasheet gives you an idea of what to expect (Output Voltage Drop High): www.ti.com/lit/ds/symlink/lm555.pdfFor a more complicated answer as to where it's losing the voltage internally, check out the internal schematic diagram on page 1 of the datasheet. I suspect it's lost somewhere around component Q28 but I haven't done any analysis to verify it yet. Hope this helps!
really enjoying the 555 timer series, i'm experimenting with lighting my model kits using leds, so far ive pretty much figured out flashing single and multiple leds and going to try your cascading one but wondered how would i go about making a led start off off and then slowly come up to full brightness? for an effect i want to use, any help on that would be great thanks ok and how to make a strobing circuit, would that just be like the cascading one but on a very short timer?
Awesome explanation. The best I've seen. But... regarding the 555 itself. Couldn't you pretty much do the same exact thing with a simple RC circut with a Comparator or a Zener diode?
You could also do it with a few NAND gates and some ingenuity. I've seen one design with 2 transistors. The 555 timer is nice because it's easy to design a wide range of frequency outputs.
I've been staring at 555 timers all day, and this has been the most helpful description of how they work. Thank you!
"I've been staring at 555 timers all day, and this has been the most helpful description of how they work. Thank you!" - Comment from below. Same for me :)
It really is a pleasure when you're trying to learn something and you're in new territory and you find someone that can explain something in a clear and illuminating manner. Thank you.
You're welcome. Thanks for watching!
AMAZING. This is the kind of step-by-step top-notch shit my University should be giving me for what I pay. This is real teaching, understanding, learning. Thank you very much, I must integrate every detail into my understanding of the topic or else I understand nothing at all, and you went through every piece of this circuit. Exactly what I was looking for. Thank you again I just felt a wave of relief because I know I can now complete this lab and I have learned something.
nice tutorial, I have watched a hundred 555 tutorials trying to get a grasp on how it works, well done. very helpful in explanation thanks.
+Joshua Cox Thanks! I'm glad the video was helpful.
The other 555 timers make one or two good points before they lose me, so I watch many to figure this chip out. Skinny's video is the most understandable out there, step by step I learned more from his than any other. Keep up the good work!
One of the better 555 timer videos to explain it to someone with basic electronic experience. Well done!
I am at the second year of the university of applied science to piraeus at GREECE and i am study electronic engineer .Your video for 555 informed me very very well for the 555 !!!.....
This is way easier to understand in 'ordinary English' compared to any of the 'best tutorials' which assume people already know various electronic terms. Thanks
I am an Industrial Electrician student and found the video is excellent.
It really helps to see the breakdown of the 555 internals.
+Lawnie Denbow Thanks and I'm glad you found it helpful!
Phenomenal. The way you stepped through the logical process made it make perfect sense. I went to trade school for electronics and no one could adequately walk the class through just what made the ol triple nickel tick, but I understand it now. You've earned a new subscriber.
thanks Skinny R&D! Having finished a tech school back in 1980, the 555 timer has been around for a while. Your explanation has been so-o-o-o much better than trying to decipher the data sheet!
Man, you're a great teacher. Loved it. Now I know the inner working, the general current flow and most importantly the connections. The formula is perfect and thanks for the last tip on making Rb a potentiometer to vary the output frequency. More from you will be very appreciated at any time.
just one thing to say to you..
"you r just awesome"
i have not seen any tutorial as clear as this.
Thank you so much
Thank you! This is my first attempt at trying to understand the 555 chip. Because I'm not formally trained and/or schooled in this field, ( I've had an inexplicable fascination with microelectronics and the physics behind it for over 55 years... I just can't explain why...), I'm not fully understanding the tutorial.
I must reiterate that this is because of my ignorance in this field and not due to any failure on your part to explain this chip. I'm sure that those who have a far better grasp of this topic will find this tutorial 100% understandable.
I noted that my first point of confusion was when you kind of glanced by the Capacitor for pin 5 (around 5':30" on the video). I think I understood you to say that this pin should ALWAYS have a cap connected to it, (I believe that's the negative leg), but I don't see why it needs to be there if it's not going to close this, or any other circuit in this type of A-Standard mode 'clocking-timer' example.
The second point is around the 8':50" spot in the video. You mentioned that when the Flip Flop = 0, then the Output Stage voltage will be high. That voltage would be approximately equal to 9v minus 1.7v, (or otherwise, 7.3 volts?). I'm not clear as to where you got the 1.7 volt figure. Is that some sort of formula constant? (Kind of like the 63.2 figure when measuring a sine wave from an oscilloscope grid?).
And finally, the third point where you reference the Frequency to equal 1.44 / (Ra + 2Rb) x C. Where did the 1.44 figure come from? I understand this video was posted some time ago and don't really expect a reply, but I'm just taking a shot at it and maybe you'll find my comment at some point in time.
Thanks again for a great tutorial and a very important topic to me.
Rich
Seriously that's the first time someone has explained a 555 to me in an understandable way. Thank you!
Hey guy, good job. I have watched about 6, 555 tutorials and was still clueless. Now I get it. Thanks so much.
I'm very impressive the way of your teaching. you are the best presenter of 555 among other TH-cam presenters. wish you good luck. hope more.
Awesome video, explained it far better than the supposed "best 555 timer on youtube" video... thanks for your staggered approach and giving the learner chance to take in the information. You would make an excellent lecturer.
+Matthew Dobing Thanks! I am pleased that everything was comprehensible.
Thanks so much man. Just started my electrical engineering degree and they threw a 555 at us in the third week. You made it very clear compared to many other tutorials I've seen.
Sweet! Glad it could be of some help.
This is probably the best 555 timer explaination I've seen.
That was brilliant... I wonder what are the calculations involved in finding the frequency of the output signal
Just found this little gem, absolutely brilliant description as to the understanding of how this timer works. Thumbs up and subscribed. THANK YOU.
Thanks for the kind words. Glad you enjoyed the video.
At first thanks for this good video and i want to add notes :
1. we can control the time of on and off of the led by :Ton =.693*(Ra+Rb)*c , Toff=.693*Rb*c
2. the output of flip-flop is connected to the pin 3 not on Transistor and the complement of the output of f-f is connected to the transistor.
3.Trigger pin is responsible to make set not rest , and the Threshold is responsible for rest not for set .
:)
Best explanation of how a 555 works bar none, and I have been in electronics for many moons!
that was great, I built several 555 projects and kept searching on youtube to get a explanation on on how it works and all I would get is either people just hooking it up and saying "this is how it works " or some engineer with a way over complicated explanation of theory and formula tables. Thanks for making it simple, now I get it to the point I can even modify in a different circuit and drive a mosfet with it because I "get it"...
Sweet! That's my intention with all these videos. I'm glad you understand what's going on. Thanks for watching!
Your channel deserves about 2 million subs, i think... THAT IS GOOD
+RedBirdRedstone I'm glad you think so. Thanks!
I followed this vid. in a deep mood to understand in detail about this wonderful chip and gained much from the class today ! many thanks ! good day .
The world needs more teachers as thoughtful as you
Thanks!
Best video explaining how the 555 works and how the components affects its function! Thank you.
One of the greatest electronics videos I have ever watched
Thank you so much. I've been stressing about my coursework all day and this finally cleared it up
This explanation of 555 circuit is amazing. Congratulations .The other one on youtube who does perfectly explaining the mechanics of the electronic devices is Ben Eater.
This is the most useful explanaton of a 555 and it helped me with my coursework. Also great that this was posted on my birthday 16/08 ;)
Glad it helped. I made this video with students like you in mind.
Not sure how I stumbled across this, but very interesting and well explained. I've been using 555's for various projects for years, still learned a bit from this.
Thanks for the kind words!
this is the best and most informative video I've seen yet. I'm a beginner and it helped tremendously in my understanding of thes ic. thanks!
+MrKillanthony Thanks!
I've seen a lot of 555 timer videos on YT, this is the most clear and easy to follow one. Thanks so much!
Thanks a lot! The output of the flipflop is connected to a power amplifier whose output is always inverted. That's why the output taken through terminal 3 is always inverted.
The best explanation i have ever got here. Please make more videos on electronics.
That was excellent and will definitely help me teach in electronics student when she goes to university in September I am very grateful for your clarity.
Thanks for watching!
After trying tutorial after tutorial for hours this was incredibly helpful and my project finally works. Thanks, you definitely earned my sub
best explanation of 555 on youtube..!!! Well Done..!!!!!!
Crystal clear and what I've been missing in all the other videos. Thanks, Skinny R&D!
Thanks for watching!
Thanks, from a mecheng student having to throw together a timed pneumatic valve! This helped me finally put my breadboard to use.
One of, hte easiest, well explained video about 555 timers of all time ! THANKS a LOT !
Thanks!
Thank you man, your explanation is on point, The best 555 timer tutorial on TH-cam, you definitely should do more electronics tutorial videos like this one.
+Mohammed Marwan Al-Atassi I'll do my best. Thanks for watching!
By far, the best, and I mean the best explanation yet
Brilliant tutorial. You have good teaching skills and is delivered at an interesting pace. Well done. Best explanation I have heard. You got another sub.
+IN3 Thanks for subscribing!
Your the one that made me understand the 555. Merci beaucoup. just a green horn amateur newbie in electronics but I like to know how things work if i'am using them. I will soon promote my self to the IR2153 and hope to meet some one with your talent on my way.
Outstanding demo/explanation!!! I think what would help a ton is if you moved this circuit onto a breadboard so we can actually see the electronic pieces and their connections as you go through your wonderfully detailed explanation.
p.s. this is the best so far. and I have watched lots of 555 videos. I learned the most from your video than any other. thank you so much for making this video!
No problem. Thanks for watching!
Best 555 tutorial on TH-cam..I've ever seen..!! Subscribing..!
Thanks for gently introducing the 555 timer in astable mode. Deciding just how much detail to include can be a tough call. Gotta keep up the interest by imparting useful information, but not leave your viewers behind.
In most of the diagrams for the internal 555 the output of the comparator connected to pin 2 (1/3 threshold) is used as the S of the flip-flop and the output of the comparator connected to pin 6 (2/3 threshold) is used as the R of the flip-flop. Also , the output driver is connected to the Q' not to the Q of the flip-flop. But this set up still works and the video was great!
Thanks! Good thoughts about the video. Pin 6 is responsible for the S of the flip flop with the timer I'm using. Here's the datasheet: www.ti.com/lit/ds/symlink/lm555.pdf The reference is on page 3 under the description of the threshold pin. It's a bit different than some of the other 555 timer datasheets and diagrams on the internet. I'm intentionally vague about Q and Q' as the outputs of the flip-flop because the reference datasheet is as well. I avoid having to explain the existence of a second flip-flop output when the datasheet schematic (page 3) doesn't show one. Thanks again for watching and have a great day!
It was the best 555 timer lecture ever. !!!
+Chris Choi That's some a high praise. Thanks for watching!
You are awesome, thanks for the clear detail! Best electronics tutorials that I have found on-line.
Saw a couple of 555 timer ic working video's and this seems to be the best one out there. sorry great scott😅
Thanks!
That was awesome! Loved the step by step detail. You earned a sub sir!
jason and future jason , both of you guys did a great job !!! . Great explanation
This is the 555 tutorial I've been looking for, Thanks !
That video just made the operation of the 555 'Click' for me, thanks!
+NovakinCore You're welcome. I'm glad it helped!
certainly the best explanations I've ever got.great job!!
Thank you so much for your concise and free information. You are a positive force in the world brother.
+TheNoBSZone Thanks!
thank you for this great lesson. Realy made it clear what is happening and why. I alway enjoy in depth tutorials like that. Because to really learn somehting you gotta spend some time on it.
Best turorial for 555ic. Thanks for clearing my doubt in 555 timer.
+Mayank pratap singh No problem. Thanks for watching!
Easy, simple and clear concept. Thank you, sir.
You made that so simple to understand. Well done!
Im an engineering student and a qualified electrician, why is it that I have to look up youtube to find explanations like this to understand concepts. I pay for my uni fees and watch lectures but at the end of the day I dont get it until I find videos like this on the net.. Great explanation, why is it not taught like this in the first place????????
+Michael Latu I found that some of my professors had "the curse of knowledge". They understood so much that they were blinded to the misunderstanding of those learning material for the first time. They assume someone has previous knowledge that many times they do not possess. I occasionally fall victim to it myself. My answer to avoiding that pitfall is to ask for feedback the right way and often. Also, there are more than a few professors who have great minds but are terrible teachers. The university profits while the students suffer.
very good in-depth explanation of the 555...
Thanks for taking the time to educate me on any kind of chip I really enjoyed it and feel better about electronics.
at 7:38 the video stops and talks about the voltages at V+ and V- of the comparator. trouble is there's two comparators on the board and they're both labelled with V+ and V-. so I wasn't sure which one you were talking about. would be good if you add a annotation to clarify. :)
Thank you very much for the tutorial. We are having a project for a stop sign in my industrial tech class that uses a 555.
really best tutorial ever dude ,i loved it keep up the good work .
thank you so much for this video it has all the details that I need for my lab experiment tomorrow
You can save one resistors if you control the astable multivibrator's capacitor just via a resistor between pin 3 and pin 6 which is connected to pin 2. There is no need to use the discharge pin 7.
Awesome, extremely detailed description of the ICs function. Thank you
Your welcome! Thanks for watching.
I'm new at this and your video was very informative, thanks!
+Jesus Garcia Glad you found it to be helpful!
I like this tutorial very much, it's very clear and understandable but it would be better for me personally if you gave some explanations about the formula and where the 1.44 value comes from as well as how we can control the output by picking up the value of the capacitor on pin 5.
Best way of teaching about 555 👍
Great video. Now I need to purchase a few 555s for some hands on experience.
Awesome video, helps me understand what I am learning in school. Thank you!
I feel you bro, teachers in school make you see everything going in front of you without making you understand.
I wish I went to your school, I'm learning this at university...
Thanks a lot man, helped me on my Theses and was just the right moment. keep them coming like that
+Johennessy1 Cool! I'm glad it was helpful!
Best explanation I've seen. Thanks!
Thank you for watching!
Cheers mate. This is a great explanation. Very helpful.
Hi,
Great video!!
I have a 555 timer circuit that doesn't use pin 7. Instead I have the capacitor discharging through a speaker that is also connected to pin 3. This gives me a siren sound. What is the frequency calculation without using pin 7 and R2 in your diagram.
Thank you
it is so nice of you to explain everything....
'The brilliant tutorial i have ever seen'
Good video , I am a beginner if you can what capacitor value did you use. Anyway thanks for the videos.
very well made video!! really made me understood well in clear way how basically this timer works, Thanks!
Glad it was helpful!
Such goodness. Damn i actually understood every single part. You are a Amazing teacher! Subbed.
+Inesophet Thanks for the sub!
Really good tutorial. I didn't understand what the voltage drop of 1.7 was at the output. Was that to do with the LED?
+Anthony B The simple answer is that the 1.7V is dropped inside the 555 timer and it doesn't always have to be 1.7V. A certain amount of voltage is used to run the timer, but internally not all of that voltage will make it to the output when the output goes High.This drop will vary. If you are running the timer with a 5V source, the drop on the output will be less than 1.7V. If you are running the timer on 15V and are drawing 200mA of current, the drop could be 2.5V. For a definite answer of how much you could lose in an application, the first table on page 6 of this datasheet gives you an idea of what to expect (Output Voltage Drop High): www.ti.com/lit/ds/symlink/lm555.pdfFor a more complicated answer as to where it's losing the voltage internally, check out the internal schematic diagram on page 1 of the datasheet. I suspect it's lost somewhere around component Q28 but I haven't done any analysis to verify it yet. Hope this helps!
Glad to watch again.
Go on, Skinny! You make a great job!!!
really enjoying the 555 timer series, i'm experimenting with lighting my model kits using leds, so far ive pretty much figured out flashing single and multiple leds and going to try your cascading one but wondered how would i go about making a led start off off and then slowly come up to full brightness? for an effect i want to use, any help on that would be great thanks ok and how to make a strobing circuit, would that just be like the cascading one but on a very short timer?
Thank you. How would you modify it so that the on and off time periods are not the same, eg. LED flashes for 100ms every 5 seconds?
Awesome explanation. The best I've seen. But... regarding the 555 itself. Couldn't you pretty much do the same exact thing with a simple RC circut with a Comparator or a Zener diode?
You could also do it with a few NAND gates and some ingenuity. I've seen one design with 2 transistors. The 555 timer is nice because it's easy to design a wide range of frequency outputs.
This was such a good explanation, Thank you!
thnx for the video , what all other things can be made from this ic ?
awesome video... great work and keep posting such videos!!!
excellent video, most informative i've seen