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.

18 thoughts on “Building a 555 timer from discrete components

  1. 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

  2. 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.

  3. @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.

  4. 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.

  5. @Juani_c,@Jeri

    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.

  6. @Esalazar
    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.

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