# Build Your Own 555 Timer

20 Steps
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!
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## 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.
robot797 says: Apr 14, 2013. 2:10 PM
no do this with tubes :P
Teslaling (author) in reply to robot797Apr 14, 2013. 2:26 PM
Challenge Accepted!
robot797 in reply to TeslalingApr 14, 2013. 3:47 PM
i look forward to the result and i am even willing to supply triode tubes
Teslaling (author) in reply to robot797Apr 15, 2013. 5:12 PM
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.
robot797 in reply to TeslalingApr 16, 2013. 1:51 PM
well send me a pm and we will arange is so that the rest to happen
spikec says: Mar 27, 2013. 5:24 AM
Nice job, you are not only very clever but a great teacher. Thanks!
ZilentKiller says: Oct 23, 2012. 4:23 AM
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.
nodoubtman says: Aug 17, 2012. 6:36 PM
ondrikczech says: Aug 4, 2012. 3:57 AM
Nice work,try make Core2Duo Procesor :-) !
Teslaling (author) in reply to ondrikczechAug 4, 2012. 6:42 PM
Hahaha!!! That would be nearly impossible!!!
waldosan in reply to TeslalingAug 5, 2012. 2:34 PM
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.
astroboy907 in reply to waldosanAug 7, 2012. 11:55 AM
Nah, do and Xeon or an i7. Get some speed goin :)
powerman666 in reply to astroboy907Aug 8, 2012. 6:57 PM
powerman666 says: Aug 6, 2012. 6:39 PM
AWESOME! So AWESOME! This is THE best ever 555 timer instructable!
jptrsn says: Aug 4, 2012. 4:01 PM
Really great instructable! I love building analog circuits from salvaged parts, and this project is right up my alley. You've done a fantastic job of breaking a complex subject into smaller more understandable components. Really well done.
Teslaling (author) in reply to jptrsnAug 4, 2012. 6:50 PM
Thanks!!! Digital has all but taken over electronics so I've been going analog!

I've been practicing breaking complex things down for a while now! None of my friends can bear more than 5 minutes of me talking about electronics unless I speak to them like they are 6 and my dad is a chemist so trying to communicate what I'm doing involves lots and lots of simplifying!

ynze says: Aug 3, 2012. 2:30 PM
FANTASTIC! I love this kind of projects!!! I'll never actually build my own 555 timer (I'd rather buy one for almost nothing :-)), but I salute you for taking the time and effort to actually DO this!

Thanks for making me smile!

Y.
Teslaling (author) in reply to ynzeAug 4, 2012. 6:34 PM
Yeah, its much easier and cheaper to buy one than build it and quite honestly, I just wanted to see if it could be done!
pfred2 in reply to ynzeAug 3, 2012. 4:25 PM
Yeah I already knew that buying ICs is the best way to go. It is amazing just how much goodness they manage to pack into each one. At one time the 555 was the largest selling chip in history. I'm not sure if that is still true or not, but I wouldn't be surprised if it was. There is no way I'd ever build a 555 from discrete components. I've seen them for sale for like a nickel a piece.
nibrobb says: Aug 4, 2012. 9:44 AM
You deserve a medal. You have gone really in-depth on this topic and you have written a lot. Really good pictures and very "goodly" explained. Thank you, I'll go order all the components now.(probably not right now; but in the near future).
Teslaling (author) in reply to nibrobbAug 4, 2012. 6:28 PM
Thanks! Tearing things down like this is something that I enjoy very much. I delve down very far into this project to fully understand everything I could about the workings of the timer so I took the time to share all that I could with the world!

Good luck to you if you give it a go! I would like to see pictures!
Topcat2021 says: Aug 4, 2012. 2:34 PM
Amazing job you have done recreating a 555 timer. I love your detail and explanations in the instructable you have made, this should be part of any high school or community college electronics class, for its value in explaining theory,practical application, and trouble shooting circuits.
Keep up the good work.
Dan
WWC says: Aug 4, 2012. 4:10 AM
WOW
I applaud you for making this and your efforts. I have thought about this several times but just never could talk myself into it because there is many connections and many things could go wrong, but my hat is off to you!

W
lookwhatjoeysmaking says: Aug 3, 2012. 10:11 PM
LOL you dead a grate job
astroboy907 says: Aug 3, 2012. 10:56 AM
Looking at your board vs. the chip... ICs are the only reason our computers aren't still the size of a barn....
pfred2 in reply to astroboy907Aug 3, 2012. 5:33 PM
If you are referring to historical computers then you should compare them to their peers now, which are physically large systems. The most powerful supercomputer 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 ENIAC which was dubbed by the press a "Gigantic Brain".

Back then there really were no "our" computers so your comparison is basically flawed in that respect.
Teslaling (author) in reply to astroboy907Aug 3, 2012. 11:10 AM
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!
pfred2 in reply to TeslalingAug 3, 2012. 4:34 PM
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.

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.
astroboy907 in reply to pfred2Aug 7, 2012. 11:58 AM
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...
pfred2 in reply to astroboy907Aug 18, 2012. 8:54 PM
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.

The way forward seems to be parallelization, multicores and clustering. Maybe teaching kids how to program again.
amandaghassaei says: Aug 3, 2012. 10:47 AM
nice work! this is really well documented.
Teslaling (author) in reply to amandaghassaeiAug 3, 2012. 11:11 AM
Thanks!