Monostable 555 timer - 8-bit computer clock - part 2

Our computer's clock is built using several 555 timers. The second is configured as a basic monostable circuit to debounce a pushbutton. See eater.net/bbcpu8-clock-module for more.
Part 1: • Astable 555 timer - 8-...
Part 3: • Bistable 555 - 8-bit c...
Part 4: • Clock logic - 8-bit co...
Support me on Patreon: / beneater
You can get all the components used in this video from any online electronic components distributor for a few dollars.
Complete parts list (everything in this video):
- Circuit from part 1
- 1x 555 timer IC
- 1x 1kΩ resistor
- 1x 1MΩ resistor
- 1x 330Ω resistor
- 1x 2µF capacitor
- 1x 0.1µF capacitor
- 1x 0.01µF capacitor
- 1x Momentary "Microtivity 6mm tact switch"
- 1x LED
- 22 gauge wire

[@user-yb5cn3np5q]

The awkward moment you find series of videos on quite technical topic in non-native language and it's perfectly clear, while the same kind of things in textbooks in your native language didn't help at all.

[@yoshwarzilla]

Its because in a series like this you can see the theory and practical use alligning next to each other. Going hand to hand making sense for each other.. This is actually the way people are supposed to learn. It was invented longtime ago and it still isnt used correctly. Which is also a reason why i even wrote this comment :)

[@networkmist883]

Philip Polkovnikov For a moment I thought there was a fly on my screen. Fortunately, good sense kicked in and & I scrolled down to make sure.

[@JamesBos]

That awkward moment when you put a fingerprint on your screen to try get rid of an non-existent fly!

[@zes3813]

not awkward at all, just learn it. and no such thing as perfx.

[@Ghost572]

Yeah I agree WarzillaM, I was thinking this guy is actually showing us how things work instead of just generalising it in a theoretical way. Even the small things like showing us on the oscilloscope how adding capacitors to the power supply or switch debouncing is just great. Its like little gems of information for people new to electronics. Really glad I came across this series of videos and even more so that ben made them in the first place.

[@jiehuan233]

I am a middle school student in China.I think your video is easy to understand and it can help me to learn the hardware.I am learning to make a 6502 computer and run BASIC on it.Thanks for your videos!

[@j_m_b_1914]

It isn't a lot, but the holidays are here and I wanted to express my profound appreciation for all the knowledge, passion and interest you generate and spread via your amazing videos. Your videos are absolutely the best tutorials and knowledge based videos I've ever viewed related to computers and electronics. As a software engineer, you really have bridged my knowledge gap between the software world and the hardware it runs on. Thanks again for all that you do and I hope that you and your family and friends have a safe, happy and healthy holiday season

[@gameboyv1790]

Awesome!

[@garrettgold]

Thank you!

[@sayantanmaiti2513]

No text book nor any college class can make this understand in just 12 min!!

[@nchutchind]

This is so cool! I've used "debounce" methods in web programming for years and years to limit how often a function is triggered within a time frame and never really thought about why it was named that. Knowing that it comes from a physical phenomenon in electronics is super cool.

[@tailslessons9634]

It's not the only software term based on physical phenomenon. "Debugging" was coming by the removal of a moth from a Mark 2 computer.

[@GTLugo]

@@tailslessons9634Fun as that would be, the term “debug” actually predated that incident!

[@tailslessons9634]

@@GTLugo Really?

[@DrewWalton]

​@@tailslessons9634the term "bug" as it relates to a technical error predates that, yes, but the term "debugging" did actually start with the Mark II moth incident.

[@topquark22]

"Tie the RESET_ pin 4 of the 555 timer to +5V" Completely solved the problems I was having with my circuit. You are a godsend. Thank you!

[@emanuelpaz4954]

saw this channel when I'm in junior high... not really interested back then. Didn't Know I'll stumble back here to learn ^^ I'm in 2nd year in Electronics engr course

[@danman32]

I noted a flaw in the typical 555 monostable circuits, particularly for debouncing. The denounce is only 100% effective in the rising edge of the button. On the falling edge when you release the button, it is only effective if you released it sufficiently before the 555 times out. This is because the timer is not retriggering in this configuration, other than staying in triggered state as long as the button is pushed. What you really want is the output logic to stay high a certain amount of time after the button is released and not sooner. This can be accomplished if a circuit prevents the capacitor from beginning to charge until the button is released. Although button release bounces can't be eliminated, adding a .1uF capacitor across the switch cut it down considerably. Lengthening the on time from the current. 110mS or so to .250mS or more would do better without being annoying. Subbing the .1uF with 1.0uF electrolytic was effective but lasts over 1 second which can be annoying.

[@sfperalta]

Best explanation of the operations of the 555 timer I've ever heard. Thanks to Ben for his clear and easy to understand delivery. He makes 555 timers fun!

[@streamindegenerate4339]

One of the best tutorials on youtube, for those confused the d capacitor reading/marking for mfd means double, MicroFarad Double..

[@larrybud]

While clever, let's not confused newbies. That is not a true statement. mfd= MicroFaraD

[@rgbplaza5945]

I can't help but think that the ceramic capacitors are made of lentils!

[@djtoddles8750]

I've always thought the same thing, and the electrolytics are made of mini coke cans

[@arashmousavi]

In fact in Persian language they are called Lentil capacitors (خازن عدسی)

[@DRSDavidSoft]

It doesn't help that the ceramic caps are essentially the same color as raw lentils!

[@dijoxx]

@@arashmousavi Same in Turkish

[@emirhandemiral7832]

@@dijoxx yes (evet)

[@zaviermiller8980]

Maybe been said before, but coming from web dev I've implemented/used debouncing functions a lot. So cool to learn where the term comes from!

[@AceBruuDofSpades]

Loving this video series!!

[@neilolif]

My goodness this takes me back. WAY back.., 1974 study of CPU Logic Design at IBM.

[@odeldodelhorst7549]

Whoa. Im sure you could build something like that?

[@bradkassing616]

This is by far the best 555 video out there. Thanks for adding bits step by step.

[@subliminalcastillo2126]

I remember stumbling across your channel a long time ago & thinking to myself "One day I will be ready to come back and learn this stuff" I started programming in C#, and after being constantly dissatisfied with not understanding what the heck was happening underneath all the abstracted high level concepts, I worked my way down. I ended up getting an arduino last christmas. Now I am back to your videos & learning everything. Thanks for your channel, you are a blessing. You have the best videos that explain everything so thoroughly and well.

[@kjamison5951]

This is what I was delivering to K-12 students for 15 years. With sketches in resources and examples. Great set of videos - subscribed.

[@th3pr0ject]

These are possibly the best videos on electronics I've watched. Thanks a lot. Amazing work!

[@AnthonyFrancisJones]

Outstanding explanation! We are so lucky to have people share knowledge like this on KZbin.

[@ah2347]

I've started building along with your videos. Extremely appreciative of all the hard work you've put into sharing this knowledge. Thank you.

[@thesillyhatday]

This series is properly well sorted. Beginner topics and expert topics are well covered on the internet. Those of us looking to find some intermediate content are pretty stuck. I'm looking forwards to learning somre more computer related topics in future videos. This start has been a great refresh on astable timers

[@louisadelwiche22]

I really appreciate your videos. I struggled to get my head around basic electronics for a long time, and your explanations have been very helpful to me. Thank you!

[@ojonasar]

A beautifully and simply explained description of the 555 - thanks.

[@jkobain]

Can't stop thinking of you as of a teacher I'd like to learn from. Thanks for your efforts!

[@LazyEngineer88]

Excellent tutorial. I have studied about 555 timers 12 years ago. I never understood its working clearly until now. Thanks a lot mate.. :)

[@jwbranham2009]

I played with those 555 timers growing up. I really appreciate your knowledge and the way you share this information. You may be the best teacher of electronic circuits in the world for enthusiasts.

[@gameboy-bt7nk]

you are able to explain everything so good in these videos! im glad i can easely follow and understand these videos without being an expert in this kind of stuff myself

[@Calm_Energy]

I love how you say we have the "guts" of the 555 timer laid out.

[@rodneyjaygarrett]

Very nice video's. I am following along and doing the projects. You have an excellent way of explaining things that even I can understand. Thanks for taking the time to share your knowledge with us.

[@dmsalomon]

Using 2 of those 2uF capacitors in series will yield an equivalent capacitance of 1uF if you ever need it.

[@hrnekbezucha]

I just love those videos. I'm about to make z80 computer and most likely some analog synth boxes. Thank you for explaining the basics so well. Great job!

[@VoidHalo]

I just bought a Z80 CPU the other day. I've yet to receive it, but my first planned project is the LED flasher Julian Ilett made out of one. Tbh I never thought I'd do anything with the Z80, much less use one in my first CPU based circuit. I always thought I'd end up using a 6052 or 8086 since I'm more familiar with them. I know almost nothing about the Z80. But it'll be a fun adventure learning.

[@rty1955]

@@VoidHalo the Z80 is a great cpu. I built my first computer using an 8080 with front panel switches and LEDS, serial I/o, 16k of memory, and 16k of ROM for a custom o/s I wrote. I wrote hundreds of thousands of lines of code on it. And later modified the machine for. Z80 and 64k of memory and adopted CP/M on it.

[@piotrk8369]

Not so long ago, I came across your YT channel quite accidentally. :) Among the hundreds of other creators, the quality and high level of your films arouses my admiration and true respect. I have seen so far what you have prepared for 555 timer. Thanks a lot for that.

[@arthurfricchione8119]

I built the first one works great and will now build the second setup. Thanks for sharing and explaining what exactly happening. Artie 👍👍👍

[@TheUnofficialMaker]

Awesome, your explanations and actual performance using the scope is fantastic, thanks.

[@fingersmcoy]

I really love how you explain things. i am learning so much from your videos

[@runforitman]

thank you very much! this circuit was very useful for me as the debounce circuit for the input of a t flip flop! it works very well and your explanation really made me understand why it works

[@ericbeard7007]

Been watching these breadboard, arduino and computer videos and these are the best explanations ive seen yet

[@ThePilhuhn]

Your videos make me understand why people like the 555 so much.

[@brianofoblivion]

Great content from all angles. The presentation is so organized. The audio is clear. The video is clear. The lighting is even nice and diffuse. Exposure is correct. What's supposed to be black is black. What's supposed to be white isn't blown out. And nice closeups. I like the approach of "not asking you to take anything on faith." Well done, and thanks.

[@jorgerangel2390]

The best tutorial on clocks with 555 that I have seen

[@jeanlucmouetaux6713]

Thank you so much taking time to share your knowledge. Really appreciated.

[@emeraldthunder]

I really love this kit, it's so much fun to build.

[@depsshenanigans7406]

This is a really handy debouncer trick! Thanks a lot!

[@danielkoszta1295]

I've been fiddling around with this circuit for a while and I think I found a design flaw. Consider the scenario where we're in manual mode and continuously pressing the button. After a very short time, the capacitor for this 555 will be fully charged. If the button is released and it bounces (thus the trigger first goes high for a moment, than back low, before it finally goes high again), then the output goes low for a moment, but this will be followed by another high pulse (duration: about R*C*1.1). I solved this by also connecting the trigger input to a PNP transistor's base (BC556) through a 10k resistor, grounding the collector of this and connecting its emitter to the positive terminal of the capacitor. This way, while the button is pressed, the capacitor will be continuously discharged through this transistor and its charging will only start when the button is released.

[@danman32]

You beat me to it.

[@pjcappaert]

You can also use a transistor as a switch for the input voltage to the capacitor so that the capacitor is disconnected from 5 V when you're pressing the switch. I've tested this and it seems to work reliably. See a modified schematic here: imgur.com/a/7uHehRF

[@suuuken4977]

When the button is released don't we want the capacitor to discharge at that time, and then when the button is pressed the capacitor charges and then discharges quickly for the circuit to work? I don't understand why continuously discharging the capacitor through the transistor when the button is pressed and charging it when the button is released would make the circuit work. :/

[@olacolega7067]

@@suuuken4977 That's because if we let the capacitor charge while we press the button, after a while, the charged capacitor will activate that "threshold" comparator. That would activate the "reset" on the SR latch. But if we keep pressing the button, you can see that the "trigger" comparator would also be on (the 1,67V on the "+" terminal at the comparator is bigger than the 0V on the -). At that point, we would have a signal both on the S and R terminals of the SR latch. On this type of latch, we can't have both terminals (S and R) set to 1. This would make the outputs unpredictable (Ben also has a video on the subject). For that reason, we need to make sure that the capacitor won't charge while we press the button, to avoid having the reset terminal bein on with the set terminal.

[@suuuken4977]

@@olacolega7067 Thanks for leaving that comment, helped open my mind ;D

[@Stoneman06660]

You content is brilliant, Ben. Many thanks from Australia.

[@ShanyGolan]

That bounce!!! this is one wizardry found!

[@hupa1a]

I already learned so much in your videos! Great!

[@raphaelrockus1303]

I'm currently studying for an exam on Op-Amps and it turns out that the schematic in my textbook was wrong! The guy had the SR latch upside down. You made it absolutely crystal clear. Nice channel. You got a new subscriber. Cheers from Belgium.

[@Dhivakar]

Understanding Electronics is an Art. you are an Artist 🤴we love to hear and learn from you. Lots of love and respect for you! ❤❤❤ Thank you for making me a better engineer.

[@YEWCHENGYINMoe]

The videos connect so perfectly

[@omarbrokeit]

Thanks for these videos! They're great!

[@Oinikis]

aaaaaand subscribed. I really enjoyed your explanation of logic gates, and now thanks to you I'm building my knowledge on how computing works. Maybe for some basic aplication I won't need to use and arduino no more.

[@km-sc4kz]

Wow , this is really cool, thanks a lot for explaining it.

[@PaulMcQuade]

you can replace 2 555 timers for one NE556 Dual timer. Thanks for the videos. I learned a lot.

[@alexmcd378]

I just realized, the 555 timer is basically the circuit version of a Japanese deer scare fountain.

[@nihasilev]

everything in the video is so simple; I can easily focus on the issue .. Like ASMR video .. Health in your hands👍👍

[@indrarana9392]

I like the way you explain

[@gzitterspiller]

Mech engineer here, I want to build a little 4 bit proc, started watching these videos... so far so good. You explain all things in a very intuitive way with examples and real measurements, you should consider being a teacher as a professional.

[@davidsantiagoalonso]

PLEASE START AN ONLINE ELECTRONICS COURSE... everything is easy to understand and builds as it goes.

[@josedominguez2021]

Thanks part 2..... I love Relays....

[@industrialdonut7681]

this is so much easier to understand than (or maybe in consideration of, after the) the last video lol

[@juliannicholls]

Brilliant explanation.

[@rdriskie_lopl]

I'm a electronics student, and the past week my teacher assigned us this circuit as our project for the 1st partial. I was a one step away of quitting the project because my circuit was a disaster, and i was so stressed. But then, i discovered your channel, and i watched your Breadboard 8bit computer. Then, i told myself "if this guy can make a freaking computer using breadboards, then, i can finalise my project" and i did it. And if i hadn't see your video, i would fail the project. You're a source of inspirarion for me bro, thank you.

[@kaitlyn__L]

That’s rough! I built these circuits on breadboard in my first weeks of high school EE, but in class with the teacher’s help, not as a homework assignment! I didn’t start to really “get” the rhyme or reason for circuit layouts until I was a dozen or so circuits deep in the course, all done in class with access to help. This knowledge has been really helpful in life, but I probably would’ve failed if I had to do it at home from the start! Still, I’m glad you’ve been able to hang on and continue the course!

[@skittermckitter05]

If you hold the button for longer than the RC constant, the output will become unstable and cause the computer to stop working. Although neat as a simulation for a bug, it doesn't work well for the intended purpose. I found the easiest way to eliminate the issue was to place a capacitor in between the button and the rest of the circuit (with a resistor to v+/gnd on either side). That way, only the negative-going edge makes it into the circuit and it can only pulse once per button press.

[@kyle4897]

Yeah I just found this too. I gather that the debounce RC timer starts ticking on *button down*. That’s great for button down. But if hold any longer than the RC time it’ll just bounce on button up! So a) make quick button presses. And/or b) adding extra capacitance at pin 6 made it feel better. I wasn’t racing to get the button down and back up.

[@camelliascholl6564]

Ben sounding absolutely defeated by the 2 microfarad capacitors that say 1MFD is so relatable

[@chrissold5957]

One of the best explanation and teaching person on youtoube, very good demo and right tempo for me thx a lot

[@russellbertrand3242]

awesome stuff. Enjoyable

[@lautarolavecchia5995]

Awesome content, you are great

[@penguinator7453]

I do not have the parts, but this is a great video!!! You made 23 videos in the playlist I am watching. I focused more usually on analog/digital synths.

[@jankleine1319]

Wesley Mays

[@chrisbroome]

I experienced this bouncing when learning about a shift register (8 bit) that I was trying to manually clock. I noticed that at least 1 out of the 8 clock cycles it would just "freak out". I thought button was faulty but swapping it with other ones exhibited the same behavior. Now I now why! I had no idea contacts on the switch bounced! Thanks for these videos!

[@ivarnordlkken8082]

Pgysical switches are not perfect, and will never be. I had to build a debouncing board for my IC-testing using 7400 as a RS-FF.

[@sumedhburbure4173]

Thank you SO MUCH!!!

[@easonzhang662]

that's fairly amazing

[@adriansikora6556]

Who is so stupid to give a dislike on this video???!

[@rycka88]

You just made me to dislike the video just to make you pissed.

[@dijoxx]

Someone who isn't happy to realise that their level of knowledge is not enough to understand what is being explained.

[@aldux]

bad aim

[@bobdagamer640]

Aldo Monteiro lol

[@bobdagamer640]

dijox probably

[@AlanMedina314]

Your videos Rock!

[@akashmondal382]

Theory+Well Explanation+Practical ..

[@CST1992]

Hi Ben, a question. What happens when the capacitor is too small? If its time period expires before you release the button, does it stay on? Or does the time period expire only after you release the button? Specifically I'm confused about what happens when S goes high so Q'(Q bar) goes low and the transistor doesn't discharge current anymore, charging the capacitor up. Eventually it'll exceed 3.3 and cause the R to set, and so S and R both are set to high. What happens then?

[@IceMetalPunk]

Thank you for such a clear explanation! One thing I'm still confused about: if you hold the button long enough for the capacitor to fully charge, then let go, why doesn't the charged capacitor instantly trigger the reset?

[@PetPeePee]

Thank You

[@Gabagool22]

Goddamn this is a great explanation !

[@Martin.Krischik]

Yesterday I measures a 1MΩ resistor at about 600kΩ. Tolerances aren't what they used to be.

[@ivorodrigues6931]

Hi Ben, This is by far one of the best explanations series I've ever seen in 555 / Logic ICs!! Cristal clear and well balance between theory and proactive! Hats off! NOOB QUESTION: Why use 1K pulldown resistors VS 10k ?? is there any technical reason or any particularity of the TTL family ??

[@nurahmedomar]

I remember seeing somewhere that 555 timer is the number one most sold chip, which is the most successful chip in semiconductor history!

[@odissey2]

Yep, some 40 billions (!) were sold since the beginning.

[@tehedx]

Thanks, very helpful for somebody who's just curious and want to appreciate better the technologies we use nowadays and take for granted. I wonder if there are concerns about the size of the capacitor? The discharge transistor is getting its full capacity at once when it resets, I figure it could burn out if the cap is too big?

[@adammm142]

Very well explained! The 555-timer is starting to make sense for me!

[@davidcourtney1398]

3:48 - Quick question as I'm trying to make sure I fully understand this. Is it R*C, or is it 0.693(RA+2RB)C as it was in the previous video? I realize the difference is very small (0.1s vs 0.0693s), but I'd like to know just for my own understanding.

[@vibodhj349]

Check the datasheet for the LM555(This is the IC Ben is using).

[@KarateSnoopy]

Thanks for the great vid series! At 10:00, Ben says "it stays off". Now if you are holding the button down, doesn't that mean the bottom compare is still on and thus S = 1 which starts the process again, so won't start slow cycling on and off while the button is held? But maybe I'm missing something. Thanks!

[@Mayank-mf7xr]

god of electronics

[@VoidHalo]

I've just tried this on a counter with a 7 segment display I've made from a 4026. I was initially still getting some bouncing, but when I added a bypass capacitor it seemed to fix the problem.

[@blahdelablah]

@SuperPetrichor That's what I was wondering as well. I imagine the clock keeps switching states if the button is held down.

[@julienbongars4287]

3:06 - For a moment, I thought you were going to say to use an SR flip-flop switch (I was taught to do it this way). I actually didn't know you could use a 555 timer for a debounce circuit but I guess you could jack the SR latch inside of it and take advantage of the discharge circuit to create that automatic falling edge... Nice

[@braetonhenzel3502]

Would you mind making a video about op-amps and how they work?

[@jojodi]

Hi Ben! Can you say anything about what camera and setup you are using for these videos? Is it a typical DSLR etc. What are you mounting your camera on to get this viewpoint? Fantastic series :)

[@NeilRoy]

"The guts of it and the green stuff".... starting to sound like a medical show. ;)

[@BillAnt]

Ahh the wold famous "debouncing" circuit. Can you imagine if it was missing on our phones, every time you're trying to blank the screen it would flicker randomly. ;)

[@lextatertotsfromhell7673]

Bill A huh?

[@eternaldoorman5228]

I think it's great that you have 1.16M subscribers. It seems amazing given the technical level of your videos. But I do sometimes wonder, ... KZbin is not necessarily right about these things. How could we ever know?

[@connorwatson7162]

Could you also accomplish a denouncer with a high pass filter using an RC circuit?

[@WardXmodem]

Oh, does this bring back memories - of me considering parts for a TTL based computer in 1974. My only computer experience was IBM mainframes, so my clock was going to have a start, and a stop button, and the mainframes had a "single step" button. I decided to go one better - so holding stop and pressing start, single stepped it. My clock was 2 flip-flops decoded to 4 states - (1) fetch instr; (2) decode instr; (3) execute; (4) increment program counter. WELL, looking on my scope, it clocked 0 1 2(0) 3 -- I was getting a 2nd phase 0 because of using ripple carry - sort of like your pushbutton debounce. The answer was to (I wonder where I found it) change from ripple carry to synchronous clocked. Unlike you, my "build" stopped at this point - because the Altair came out on the January '75 Popular Electronics, and I bought one, then built my own selectric and floppy interface boards.

[@pauldshan]

I know a lot about what you're doing in these videos, having taught myself as a youth in much the same way you're walking through the lessons in your videos. These are fantastic instructional videos. I encourage you to do as many of these as you can. They are more valuable that you might realize. I do have a nitpick about this monostable clock video. You mention that when you hit the button, as soon as the capacitor charges, the flip-flop switches the LED off. That isn't exactly true, as the video shows. It only turns off the LED after the capacitor charges AND YOU RELEASE THE BUTTON. That indicates that if both Set and Reset are asserted in the 555, the Set takes precedence. As I said, it's a nitpick...please don't see it as a criticism. These are all great videos!

[@chuckbenedict7235]

Thanks for this observation. I was scratching my head...

[@ehjones]

What happens if you hold the button down for longer than the time it takes for the capacitor to charge (i.e. longer than 0.1ms)?

[@StrangeHairJoe]

Hello Ben! Thank you for this great video (and all the other videos as well, of course). I don't really do anything practically right now with your videos, I'm just studying them for fun to get a better understanding for electronics, so I can't really try out for myself and gotta ask instead: what if, in the monostable approach, you just keep the button pushed? Wouldn't that lead to both of the inputs of the rs-latch being 1 and therefore cause unpredictable behavior? That's at least what I found checking the truth tables of rs-latches. Thank you so much for your help! I wish you happy holidays and a great new year :)

[@cameronarchibald7213]

I had that exact thought! SR both being high has undefined outputs, maybe the 555 has some prevention inside