August 19, 2019

Astable 555 timer – 8-bit computer clock – part 1

Our computer’s clock is built using several 555 timers. The first is configured as an astable oscillator. See for more.

Part 2:
Part 3:
Part 4:

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You can get all the components used in this video from any online electronic components distributor for a few dollars.

Complete parts list (everything in this video):
1x 555 timer IC
1x 1MΩ potentiometer
2x 1kΩ resistors
1x 100kΩ resistor
1x 330Ω resistor
1x 2µF capacitor
1x 0.1µF capacitor
1x 0.01µF capacitor
1x LED
Solderless breadboard
22 gauge wire
USB charger and cable or some other 5v power source


33 thoughts on “Astable 555 timer – 8-bit computer clock – part 1

  1. It is to your credit that you've spent all this time to explain how you've built your 8-bit computer. That is no easy task. You deserve way more than just great comments. Thank you for taking the time to explain this complex area so well. Very well done Ben.

  2. God this is just gold. You are explaining these concepts so well and clearly. I wish College Professors would learn from you, and explain things like you do.
    It's so rare to find a good teacher these days. Everything is left on the students to figure out on their own.

  3. ok i have a question i always wanted to know….
    why on earth do you guys (i assume Americans? but dont know) draw GND connections with the Earth symbol?
    i know it with just one big line

    it confuses the hell out of me every time i see such a schematic. the funny zigzag resistors i kinda got used to because they don't look like something else

  4. I do have a question that I dont know if it was asked already. Why on the discharged phase, the capacitor gets to discharge into the transistor rather than the ground it's already plugged into?

    And while asking this question, another came to me: Why the capacitor is even discharging, to begin with? As the way I see it, it's just constantly plugged in the 5V then into ground.

    I feel like I missed something there >_<

  5. The 555 was my mainstay in the 80s.  Two of these was the heart of an automatic car starter which was pretty cool since in the mid 80s they were not all that common.   One in the Bi Stable mode was triggered by a radio link from a cheap remote controlled car from RS which supplied power to the ignition and triggered a second 555 in the Mono Stable mode that powered the starter for a few seconds.  It was pretty simple by today’s standard but back in the 80’s it was pure magic .

  6. Those capacitors, judging by the "MFD" unit label, are older than dirt and have become electrically leaky, hence the high reading. Allow me to explain.

    Imagine an ideal capacitor. Two plates, with a gap. No DC current will flow through them once the field is built up. Non-ideal capacitors have a characteristic called leakage current, which is the amount of expected DC current flow between the plates due to things like electrolyte conductivity, ionization and metal migration over time under the force of the electric field, etc.

    As some types of capacitors like electrolytics and the old paper and wax capacitors age, they leak more and more DC current across that they shouldn't, and slowly start to build a parasitic resistance that decreases as the capacitor ages. Since the parasitic resistance is in effect in parallel with the capacitor (because the leakage current occurs between the plates) it causes the capacitor to take longer to charge. Most multimeters read capacitance by passing a small, known current between the leads and then sampling the voltage over a specified time (basing the reading on the fact that larger capacitors take longer to reach any given voltage at a constant current), meaning that any leakage between the plates of the capacitor will cause the reading to look high because of the parasitic current draw keeping the voltage down.

    Slap one of those capacitors between the leads of an old capacitor leakage tester and see how badly they leak. Some of the older caps I've seen can leak current at a charge as low as 2-3 volts. No doubt those are the same.

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  9. Oh man, I just realized – so, in Minecraft, there's a commonly used redstone device called an Etho hopper-clock, which serves basically the same role as the 555 timer of being a cheap, easy-to-use clock where the period can be varied easily depending on how you set it up. And what I just realized is, it's not just the role that's analogous – the mechanism is analogous too! You've got a set-reset latch, a couple comparators feeding into both inputs of the latch, and an accumulating device where stuff can either flow into it, or flow out of it, depending on the state of the latch. Items flow into a hopper until one comparator says it's full enough, which sets the latch, allowing the hopper to "discharge", and then items flow out the same way they came in until the other comparator says it's empty again, which resets the latch.

    This ubiquitous circuit was independently reinvented in a videogame by someone with no electronics knowledge. I just think that's neat!

  10. when i plug my probe to check the cap on pin 2 the output of my 555 goes stable, the LED remains on and doesnt pulse. Whats going on there?

  11. I love your vídeos, and understand pretty much that you say, but im struggling a bit on understanding the way you place resistors and the value of this resistors. Thats because my lack of electronics knowledge. May you recommend me some book or tutorial to follow in order to understand this? Thank you in advance!!!

  12. Asking just by curiosity. At 12:00 you talk about the rate between on and off time (duty-cycle). Wouldn't you get 50% duty-cycle if you placed another 1k resistor before the discharge-transistor? Nice video btw!

  13. why would you even need the first resistor if you want to have the same time on as off, couldnt you just connect it straight to 5v with wire? or is there something im missing

  14. I was wondering when you connected that small 0.1 mF cap to pin 5 and ground which I think is called the "Control Voltage" pin, how did that smooth out the transition ripple so well? Also these R/C timing circuits are notoriously unstable in frequency with change of input voltage and temperatur. A crystal controlled circuit with a bunch of dividers in order to divide the higher frequency down to a couple of hertz would have been more stable.

  15. On the schematic for the clock module, there are two capacitors labeled C1 and C10. I'm having trouble understand where the second one fits in to this and its purpose. Does anyone have any insight?

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