The 555 timer. A chip so versatile that it has been used in everything from toys to spacecraft. A chip that can act as an oscillator, a schmitt trigger, PWM driver, a siren/alarm, a light or dark detector, and much much more. It is the most popular IC of all time having been around since 1971 and now selling over 1 billion annually.

This instructable will show you how to build your own 555 timer using only transistors and resistors, no ICs!

Why build this?
Good question.  There are a few different reasons:
1. To Learn:
Learning may be a scary thought to some people, but this project has taught me a lot about comparators and analog circuits as well as a lot of the basics of the 555 timer. The 555 timer combines both digital and analog circuitry  and while digital circuitry is taking over, analog is still important.
2. To Understand:
The 555 timer is a very versatile and useful chip. That's why it is the number 1 most produced chip. It is used very often so it is important to understand how the thing works. Now, you can read about it or even see a simulation, but nothing it quite as good as actually making it yourself.
3. It's Fun:
If you like working with electronics, especially breadboarded electronics, this should be a fun little project. You will break the black plastic barrier that stands between you and your integrated circuits and see the circuit in all its glory (well almost, making your own transistor could be difficult)!

Now that I've hopefully convinced you somewhere within that intro, lets get started!

Step 1: 555 Internals

So what the heck is going on inside the 555 timer? Well here are a couple of schematics from the National Semiconductor datasheet to help explain it.

In the first picture we can see that there are two comparators, one on the trigger pin and one on the threshold pin. We can also see that they are connected to a voltage divider. One input of the Threshold comparator is at 2/3 Vcc and one input of the Trigger comparator is at 1/3 Vcc. The outputs of the comparators lead to a flip flop. Although it doesn't say on the image, the flip flop happens to be a SR flip flop. From the flip flop there is the output stage which leads to the output pin and the transistor that controls the discharge.These are the basic parts of the 555 timer.

This is the basic theory of operation:
When the trigger voltage goes below 1/3 Vcc (its reference voltage) the comparator Sets the flip flop, which pulls the output high and turns off the discharge. When the threshold swings higher than 2/3 Vcc (its reference voltage) the comparator Resets the flip flop, which pulls the output low and turns the discharge on. This basic operation allows the 555 timer to operate in various ways with various configurations.

I don't want to get into all of the ins and outs of how the 555 timer works, so if you know, great! If you don't know here is a good tutorial with lots of theory and operation information. It is my personal favorite.

If we look at the schematic diagram in the datasheet (second image), we can see what is actually happening inside the chip. The comparators are differential amplifiers, or long tailed pairs with a few added components to increase gain and sensitivity. The differential amplifier is the basis of the comparator, it greatly amplifies the difference in voltage to the point where millivolt differences result in rail to rail swings (voltage swings between 0v and Vcc). What's interesting here is that the threshold comparator uses NPN transistors whereas the trigger comparator uses PNP transistors. I don't know if that has an effect on the operation, but I just kept them like that in my circuit. The threshold comparator also has several extra transistors not present in the trigger comparator, along with a slightly different configuration. They perform the same function though, so I just replicated the trigger comparator but using NPNs instead.

The flip flop circuit is rather interesting. There is a lot going on for what is just a SR flip flop with a second force reset. That can be made with 3 transistors so I discarded that circuit and made my own.

The output driver is fairly simple. It is composed of two transistors, with the signal to one inverted so that when one is on, the other is off. This allows the output to operate in push-pull mode. This means that the output can source current, the output is shorted to Vcc, when it is high and sink current, the output is shorted to ground, when it is low. 
hi buddy :) very nice and illustrative instructable thank you very much. ☺ from your bio and instructables, i can figure out that, you are also Nikola Tesla fan like me, is it? <br>nice?
<p>It did not work for me. Starting having problems at the threshold comparator. At the end, the guide just stopped, at the part where I was supposed to connect the wires like the IC datasheet which was hard because I didn't know what the names were. Overall a good guide on the beginning, but at the end it seemed a bit rushed.</p>
<p>Hi, @Teslaling!</p><p>I'm really interested in building one 555!</p><p>Do you think I could use 2 resistors (4k7) in parallel instead of the 1k resistor?</p><p>Thanks a bunch. Great work!</p>
<p>A great video that explains the 555 and one of its uses, <iframe allowfullscreen="" frameborder="0" height="281" src="//www.youtube.com/embed/kRlSFm519Bo" width="500"></iframe></p><p>It's only part 1 of 4, and the 4 parts go together for making a computer clock. In fact, the whole computer series and the YouTuber are great.</p>
<p>I am currently building this, and am running into what I think is an issue. I am testing the threshold and trigger comparators, but the LED is always on. However, when I plug it into VCC and Ground respectively, it does brighten up significantly. Is this an issue or what?</p>
<p>There were no problems at all. Worked on the first try! Excellent instructions.</p>
<p>I have a problem. I have been building this for 3 days and I cannot get it to work. Also, there is a problem with the parts list vs. your schematic. The list shows 11 4.7k resistors and 2 1k resistors. The schematic shows 10-4.7k and 3-1k. Which one is correct? (I suspect the schematic is the right one). I don't expect that my first sentence is worth a darn for asking for help.Oh, also, I am using 2n2222 for NPN and 2n5551 for PNP. Is that part ok? The tests all pass (after correcting a few mistakes). It is obvious that everyone here is smarter than I am. Congratulations to all those who made this and it works. I am very impressed. BTW-Your 'ible is fantastic. I know the problem is between the chair and the workbench. TIA. </p>
How many amps or Ma can I get in the output
Awesome, have a question, the output drivers can put the Vcc on the output???<br>I See a resistor divider in the base of output drivers so i think the output should be 4.7k(Vcc-0.7V)/5.7k. <br>
<p>what i understand is that you made 555 timer ic circuit using transistor that cool, but not everybody can make this i means its not that simple. techmess(.)page(.)tl</p>
<p>that's nice</p>
<p>this is very complected &amp; very expensive </p>
<p>Why so complicated ? You can make one without all those transistors, or did i miss something ?</p>
<p>This is an awesome tutorial! It took me a few days, but I was finally successful, after I realized a wiring mistake I made. I only did the breadboard version, I don't think I need to solder up the permanent version. I learned a lot, thanks!</p>
<p>That is cool. Thanks for taking the time to make this awesome instructable. I will try this and things like this in the near future. Favorited and followed.</p>
<p>I've bought 2 of these online and I'm trying to make a blinky for fun. Here's the problem: it wont blink. But it lights up.</p>
<p>try changing your capacitor size to arouns 22-100 uf</p>
<p>I made it and I did an instructable on it giving an explanation on exactly where to put all the components on a perf board. You can check it out here: <a href="https://www.instructables.com/id/Remix-build-your-own-555/" rel="nofollow"> https://www.instructables.com/id/Remix-build-your-...</a></p><p>Thank you for the inspiration, and I hope that I can get you vote for the remix contest as well as the others. This project is awesome and it worked perfectly, once again thank you.</p>
<p>Wow! This is of the best instructable i ever seen! </p><p>I can't wait to make one! :)</p><p>You just earn a Follower!</p>
<p>Hi!! May I know where are those arrows connected to? I'm trying to simulate the circuit that you made because the other 1 is more complex than yours! Thanks! Cheers</p>
The arrows represent the connection to the positive supply rail.
<p>That is awesome. Certainly the best how-to I've seen on here....</p>
i was wondering what the capacitor on the pcb version was for, stabilising power supply?
Yes, exactly.The 555 is notorious for being very noisy, especially when the output changes. I threw the capacitor on it because there was room for it and so I didn't forget during testing. I must not have mentioned it anywhere in the instructable.
thanks for the help :)
Hi! great work. I'm kind of new to electronics and started to experiment with building oscillators with the 555. my question is would an oscillator (for a synth) sound better with a discrete 555? Thank you!
It would make no real difference. The discrete 555 and an IC function the exact same way so it wouldn't matter. You would be better off using an IC because they are cheaper, easier to use, and don't take an hour of soldering to be ready to use.
should make an atari punk console all on one board without any ICs
Nice work,try make Core2Duo Procesor :-) !
Hahaha!!! That would be nearly impossible!!!
it'd make a very interesting project, it'd only take up half a room, though if you do it right you could overclock it to massive degrees.
Nah, do and Xeon or an i7. Get some speed goin :)
How about an ATmega128p (Arduino)
yes!!!!! do the atmega then make an arduino board around it
Wow great tutorial! <br>This can show how IC's are all actually simple electronic circuits compressed together.
no do this with tubes :P
Challenge Accepted!
i look forward to the result and i am even willing to supply triode tubes <br>
The first thing I have to do is figure out how some of the elements can be implemented using tubes and how they would connect together. Actually building some circuits wouldn't be for a while but if you would be interested in supplying some triodes, I only have a few on hand and I know that I will need more. This should be a very cool project. I haven't worked with VTs other than with a guitar pedal.
well send me a pm and we will arange is so that the rest to happen
Nice job, you are not only very clever but a great teacher. Thanks!
Thank you very much dude, i really appreciate it. You helped me to finish a school work and i also understood how a ne555 works.
Looking at your board vs. the chip... ICs are the only reason our computers aren't still the size of a barn....
Exactly! What is really amazing is that the 555 IC only has about 25 transistors in it. A 8 pin PIC or AVR has several hundred thousand to a few million in the same exact package!
I'm sure due to manufacturing constraints there is an upper physical limit to just how many components can be put on a die in a particular package but in practice there is very little relationship between the two characteristics. Something the examples you cited point out clearly. What is amazing about the 555, that you failed to mention, is that such a simple device has been so popular for so long. <br> <br>What is more amazing is that many of the tasks the more complex micro-controllers are put to today could easily be accomplished using things as simple as the 555 is.
It does get smaller every year pretty much... We are fitting some 3 Billion transistors into a processor now (thats just average), but with Haswell and intel going down to 5nm production (hopefully by 2015!), we will see massive speed increases...
5nm by 2015 is pretty optimistic. There are physical limitations they've run up against at 22nm. Being as less than 22nm is shorter than the wavelength of light they use with masks to process dies. There are interference games they can play to get a little under that but I don't think the technique can be extended very far. I've heard of some 18nm stuff perhaps that is as low as they can go? The way the industry has been going lately I wouldn't hold my breath for any massive speed increases either. First off no one needs it, second there is no competition on the high end anymore to drive it. <br> <br>The way forward seems to be parallelization, multicores and clustering. Maybe teaching kids how to program again.
If you are referring to historical computers then you should compare them to their peers now, which are physically large systems. <a href="http://en.wikipedia.org/wiki/IBM_Sequoia" rel="nofollow">The most powerful supercomputer</a> today takes up 3,000 square feet of floor space, which is a square almost 55 feet on a side. Which is almost twice the physical size of <a href="http://en.wikipedia.org/wiki/Eniac" rel="nofollow">ENIAC</a> which was dubbed by the press a &quot;Gigantic Brain&quot;.<br> <br> Back then there really were no &quot;our&quot; computers so your comparison is basically flawed in that respect.

About This Instructable




Bio: I like to make things that move, sense, calculate, compute, blink, and make noise. I like making things that create high voltages, electrical arcs, and ... More »
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