Giant 555 Timer

A Working Replica of the classic 555 timer made using discrete components

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A fully functional replica of the classic 555 timer made using discrete through hole components which include only basic jellybean components like (to - 92) BJTs , 1/4Watt resistors & diodes.
All the components used are salvaged from old electronics which i collect.

Why I built this:
I always like to make circuits from scratch and especially with discrete parts.
This project is made to deeply learn each and every section of the 555 timer chip & how the IC works from inside out.
This project made me learn what are current mirrors, constant current sources, sinks, long tailed pair, output stages & flip flops. Also how internal circuit of op amps and voltage comparators work. Resulting very good practical skills and therefore this project was also part of my college mini project.

Since I am also into woodworking , i also designed a big wooden box shaped like an actual DIP - 8 package with aluminum legs. The whole box is roughly the size of an A4 size printing sheet

The Giant 555 Timer:

The main aim of making  a discrete version of 555 is to understand how the 555 works at component level.

I was inspired to make this project from EEVblog 555 timer kit video. 

I've always fascinated by OP-amps and what circuit is behind the op amps which make them a powerful component in electronics. The 555 IC contains two of these op- amps specifically voltage comparators which are similar but with a slight change in output stage.

Here I'll be listing the key details of making this project. Step by Step build guide and problems which i faced are given in the " build Instructions section." of this project page.
I Decided  to solder the components in a traditional "real bread board" way - just hammering copper pins in a block of wood and soldering components.
This helps to see each and every connections as well as it looks really beautiful and interesting. I got this idea form Collin's Lab: The REAL Breadboard video
I Also made each block of 555 IC in the shape of schematic symbols.
For connecting all the components together I used ethernet cable's bare copper wire & for the jumper connections or isolated connections enameled copper wire
( magnet wire ) was used.
The soldering junctions are made of 1mm bare copper wire -cut into small pieces 
and hammered them on wooden blocks as nails.

So to start off this project I decided to approach the 555's circuit in small parts, because it is always better to break up a big circuit in parts which helps troubleshooting easy and fiddling components around the bread board.

The voltage Comparator:

The most Interesting part of this project was learning and designing the voltage comparators used inside the 555 IC. Especially how a Differential pair works ,what is current steering.

I started from learning what are long tailed pairs which is the most important part for building the comparators by watching various lectures and tutorials on YouTube about long tailed pair or differential pair using BJTs.

 I also learned the importance of constant current sink in a diff. pair and the use of current mirror to increase the gain of the diff. pair.

The SR Flip Flop:

Now for the SR flip -flop two transistor flip flop ( or NOT gate configuration ) is used with some additional output buffers to reduce the output impedance of Q & ~Q outputs.

The Output stage:

For the output stage ,in the original 555 timer there's totem pole configuration which is good but for me its a bit glitchy and not so good performing. So I used the good old push - pull stage for output buffer of the 555 IC which is for PIN no. 3.

The Reset Transistor:

Used a PNP BJT for the reset input.

The Discharge transistor:

A NPN Darlington pair for hard shorting to ground, single NPN was working but was having some issues in the A-stable mode.

The Voltage Divider:

The Voltage divider is straight forward - used 4.7K resistors and a series diode protection for the whole circuit. 

Finally the interconnection layout was planned :

The connections are made such that it correctly matches the pin configuration of the real 555 timer.

Connecting "Giant 555 Timer" to breadboard:

I made some alligator clips with pin markings to test it on breadboard.

Demonstration videos: 

All detailed working modes with output signal on oscilloscope.


Video contents:

0:00 Build steps slideshow

4:20 Preparing to test it on breadboard

5:05 Working demonstration videos : Mono-stable, Bi-stable, A-stable modes

  • 8 × S9013 NPN BJT : General purpose Audio Applications
  • 6 × S9014 NPN BJT : low noise ,High gain ,Good linearity
  • 2 × BC556 PNP BJT : General purpose
  • 2 × BC557 PNP BJT : General purpose
  • 2 × S8050 NPN BJT : low voltage ,high current

View all 17 components

View all 5 project logs

  • 1
    Understanding the Basic building blocks of the 555 Timer

    Block diagram of the 555 timer.

    Below is the original Component level circuit of the 555 timer designed by sir Hans Camenzind

    ( not my circuit )

    image taken from Tiny Transistors website

    I will modify the above circuit and optimize it for discrete components

    If we look at the above circuit which is the actual circuit of the 555. But when tried to reverse engineer it and try to understand it , first off all its very confusing, I mean why using two different types of comparators one NPN based and other PNP based.

    So I decided to ignore the 555's original circuit and learn the fundamental blocks of 555 form scratch and redesign it for discrete component form.

    The first and the most important circuit of the 555 is the two voltage comparators. Which brings me to step 2. understanding the differential pair.

  • 2
    The Voltage comparators

    Reference - For designing these differential pairs to serve as a very basic voltage comparator i found a gem on YouTube for understanding the internal working of Op -amps and fundamentals of differential pair - w2aew YouTube channel video on long tailed pairs. ( Not my video/Channel  ) . He also has many fundamental videos on electronics for any beginner like me . Very very nice channel on YouTube ,his way of explaining things is just outstanding and very different.

    Firstly I grabbed some random NPN transistors form my parts bin and just tested the behavior of the simple long tailed pair - in this case used some S9013 NPN BJTs

    I used NPN because I am much more comfortable working with NPN designs

    I was surprised at the common mode input response. The output was quite balanced but not as good as a real op amp.  

    Even if the two transistors are not matched ,as per my experimentation on breadboard  it works fairly well as a voltage comparator and we only want to use this circuit to operate so that it slams the power rails 0V and the supply rails ( Digital output ) so matching here doesn't  make any significant difference. ( But for analog application it is a must to precisely match those transistors.)

    So if you plan to build a DIY voltage comparator out of discrete components it is not important to match the diff. pair transistors.

    But there are many problems in the above circuit -

    • Output doesn't remain stable if common mode level is changed. Also by varying Vcc the steering current changes.
    • The gain of the above long tailed pair is low
    • input impedance is very low
    • No single ended output 

    Lets solve them one by one-

  • 3
    Adding a Constant current sink instead of tail resistor

    Here I used a classic BJT constant current sink instead of the tail resistor. in this case about 1.5 to 1.7 mA tail current  depends how the BJT is biased.

    After the constant current sink the input impedance got slightly higher that is what we want.

    Also by changing common mode level the output is slight more stable than before.

    And as a plus point by varying Vcc the steering current remains constant.

View all 13 instructions

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Gareth wrote 05/05/2022 at 03:01 point

one of the best projects !

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Rudraksha Vegad wrote 05/08/2022 at 05:12 point

Thank you soo much for appreciation 😁 !!!!

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Rudraksha Vegad wrote 01/20/2022 at 17:40 point

Hello everyone 😀

Remember, I participated in Hackaday 555 Timer contest 2021 , I am really happy to say that I came out to be one of the winner among the top three winners.

Thank you soo much to the judges, Hackaday staff , all the enthusiasts all over the world for giving so much love to my project Giant 555 timer and choosing me one of the winner. It was a fascinating experience participating this contest.

I also congratulate to the other two winners.

Thanking Digikey and Hackaday for organizing this legendry 555 commemorative contest.

 -- Rudraksha Vegad

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cjhonlyone wrote 12/26/2021 at 15:26 point


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Rudraksha Vegad wrote 12/26/2021 at 15:57 point

Thank you !😀

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Henrik Östman wrote 12/26/2021 at 02:05 point

What not to love... Amazing!

Could you make a Zilog Z80? ;-)

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Rudraksha Vegad wrote 12/26/2021 at 10:34 point

Thanks 😁!!!!

Zilog Z80 would be too crazy for me , I know some people have made similar microcontrollers with transistors, but I don't have any plans of making such an insane circuit.

Since I just wanted to learn how internal circuit of op amps work and that resulted me making 555 as a fun project. 

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Henrik Östman wrote 12/27/2021 at 11:55 point

I'm just kidding. A Z80 would be a fotball field sized circuit. :-)
Keep up the good work!

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Rod Nussbaumer wrote 12/23/2021 at 16:42 point

One of the most unique and truly instructive projects I've ever seen on here. Bravo!!

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Rudraksha Vegad wrote 12/23/2021 at 18:09 point

Thank you so much !😁😀

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Evan Salazar wrote 12/22/2021 at 17:53 point

This is really cool, I did something similar 10 years ago on the first 555 timer contest. However mine was not nearly as sophisticated as yours. I only build the most basic versions of the comparators and SR latch. 

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Rudraksha Vegad wrote 12/22/2021 at 19:02 point

Thanks !😊😁😀

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Will Donaldson wrote 12/16/2021 at 20:38 point

This is really cool! I love how you broke the internal components into discrete sections!

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Rudraksha Vegad wrote 12/17/2021 at 10:34 point

Thanks 😀😁😍

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Ken Yap wrote 12/15/2021 at 23:33 point

Wow, that is a big achievement, literally. 👍👍

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Rudraksha Vegad wrote 12/16/2021 at 16:25 point

Thank you very much Ken Yap 😊😁

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