Building a 555 timer from discrete components

The 555 Design Contest shook a whole bunch of really creative circuits out of the trees, hence the 555-heavy content lately. While not technically part of the contest, [esalazar] wanted to know what made the 555 tick, literally! He started working on the project in a circuit simulator, then ultimately ended up building the three main logic blocks inside the familiar timer on pieces of copper-clad board. He’d built a 555 using discrete components.

While this isn’t 100% compatible with the classic 555 IC, it covers the basics pretty well, and [esalazar] gets extra-credit points for embracing the hacker spirit of seeing for himself how stuff works while documenting it well and citing his references.


  1. walt says:

    cool idea. this could help others understand what happens inside of a 555.

  2. biozz says:

    i always wanted to do this but i could never understand soem of the included data sheet schematics with these strange dual base transistors and such and there all different XD

    i THINK it works by charging the capacitor until it hits a threshold threw a resistor than discharges it threw a second resister than using a comparator it turns the triangle unto a square wave
    im not sure its to early in the morning to poke threw all the data talk to me again at noon XD

  3. bootnecklad says:

    Damn it! I was thinking of doing this a while back. Never got round to it though…

  4. Here’s a good animated diagram of the 555 internals. It’s very nice:

    It does have an error though, the inverter pictured that feeds the RST to the Flip Flip should be an AND gate, with non inverted inputs (just a regular old AND gate)

  5. j_jwalrus says:

    i had this thought too. not surprising that there are no more original ideas for 555’s. its 40 years old!

  6. fotoflojoe says:

    I already did this on a breadboard last year while going through the Make: Electronics book.

    Only difference: I failed miserably…
    I tried to be faithful to the 555’s pinout, but never got it to work.

    I’m still such a n00b at all of this electronics stuff.

    Kudos to esalazar!
    This is how Hans Camenzind developed the original design.

  7. Esalazar says:

    @Fight Cube

    I looked at many 555 block diagrams and they all seemed to be different when it came to the flip flop. If it was !Q or Q driving the drain transistor, if the top inverter went to RST or SET and exactly how the reset was wired (AND,OR or direct to the drain). Some diagrams didn’t even bother and had a block with F/F that the pins went in and out of.

  8. juani_c says:
  9. Matthew says:

    You should also check out Paul Falstad’s beatiful simulator.

  10. Jim Narem says:

    Linear’s LTSpice simulator has pretty close
    implementation of the 555 timer in the examples
    directory, file NE555.asc. The multiple collector
    transistors are simulated by using one transistor
    per collector.

  11. j_jwalrus – There are tons of original ideas for the 555! We’ve been getting awesome entries.

  12. juani_c says:

    @Jeri Ellsworth
    I’m trying to build a small Art project with a couple of 555 made with discrete components.
    Do you think that sort of things it’s against the rules?

  13. Esalazar says:


    I was wondering the same thing, I submitted because the rules said any 555 is allowed so that could perhaps include the HM555.

    BTW, Juani I like how you decided to go with the current mirrors in your testing. That may have allowed you only use one design for the voltage comparator. I had to use a different designs for each one(NPN and PNP versions) however we did use the exact same flip flop design.

  14. juani_c says:

    With the first op amp I made I had the same problem as you, It worked only when the negative input was connected to reference and the positive to a variable voltage, so I end up using this other design that it’s a little bit more complicated. I also think that it shouldn’t be a problem the HM555, comercials 555 timer have several different circuits and different packages, this is just another one.

  15. qwerty says:

    I was looking for that tool he uses to cut small pcb pads since ages, what’s its complete name? Searching for punch in Harbor Freight’s site or Ebay leads to nothing.

  16. strider_mt2k says:

    Oh that is so cool.

    I had to laugh when I read the headline just because the 555 projects have been rolling out lately.

    Well done!

  17. kabukicho2001 says:

    I tackled about 15 years ago, the mc1374 using many 2n2222 transistors to work like tv audio modulator all in a breadboard.

  18. Esalazar says:

    I could not find the punch on the HF website either. However ever you can get the same one from Grizzly.

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