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I've been struggling with building a 555 timer for a project. I get the expected voltage at pin 3 (output), but the problem is that it's not oscillating. The Vout is constantly 8.4V, while I expected a 70% duty cycle - 5s on, 2s off.

I've pinpointed the problem to be C1 not charging up to 2/3 of Vcc so it can't start discharging. It goes up to 5.3V - 5.4V and stays there. Vcc is 9.8 V, so 2/3 of it would be approximately 6.5V.

Things I've tried so far (apart from scouring the internet for all kinds of 555 troubleshooting problems):

  • Trying with a different capacitor - both an electrolytic and a non-electrolytic 10 uF cap gives the same result.
  • Testing all connections with and without the circuit, with and without power on, and measuring voltages across different points.
  • Buying 2 additional ICs to test with them, in case there's something wrong with the 555 chip itself.
  • Trying with a 43kO, 30kO, 100uF combination, instead of the 430kO, 300kO, 10uF combination I had originally used.

All of the above to no avail. So here's my plea for help! Below you'll find a list of parts and the diagram itself:

  1. R1 - 430 kO
  2. R2 - 300 kO
  3. R3 - 200 O - just a test load to see if it's working.
  4. C1 - 10 uF electrolytic. This is the one that's getting up to 5.4V only, instead of 2/3 Vcc (6.5V).
  5. C2 - 10nF
  6. IC1 - the NE555P chip.

All the calculations were done based on this handy calculator: https://www.allaboutcircuits.com/tools/555-timer-astable-circuit

Here's the diagram:

enter image description here

EDIT: Decided to try to try again with the lower resistor and higher capacitor values. It worked. Perhaps the capacitor was faulty, or there wasn't enough current to charge it all the way up to 6.5V? Either way, these are the values that work, for anybody with the same problem:

  • R1 - 43 kO
  • R2 - 30 kO
  • C1 - 100 uF
Rumen Rumenov
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  • First of all, read this post: https://electronics.stackexchange.com/a/28255/41856 . Second, you show a headphone jack for the power supply, and on top of that you are using the ground connection for VCC. Are you sure you don't have things connected backwards in the real world? Without drawing a neat, accurate schematic that matches what you actually built, answering questions like this is next to impossible. – DerStrom8 Jul 27 '18 at 10:38
  • There, I've fixed the diagram. Of course I don't have it connected backwards in the real world, it's obviously not consistent with the measurements I've made and described. The jack was there for illustrative purposes only. – Rumen Rumenov Jul 27 '18 at 11:03
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    Rubbish schematic! – Leon Heller Jul 27 '18 at 11:29
  • Leon Heller Would you please elaborate? – Rumen Rumenov Jul 27 '18 at 11:34
  • @RumenRumenov If the supply was connected backwards then it could have damaged something within the IC and the result could be just about anything, even the output being constantly on. That's why I mentioned it. Next time draw a schematic that's consistent with your actual problem. How can we know that you haven't mis-wired your actual circuit? I can't trust your schematic because you already proved it was inconsistent with what you built. – DerStrom8 Jul 27 '18 at 12:38
  • @DerStrom8 Thanks for taking the time to answer. The current schematic is 100% accurate to how it is wired. I've checked numerous times that I've wired everything correctly (I've done several other more complex circuits, so I'm not a total newb) and even bough 2 new ICs just to check in cause I'd damaged it or it was faulty. – Rumen Rumenov Jul 27 '18 at 14:26
  • @RumenRumenov The problem with replacing potentially-damaged old parts with new ones is that if there is a problem with the wiring, the new parts may become damaged in the same way. Anyway, your note mentioning that it worked with different values for the Rs and C suggests this isn't the problem. Good thing! – DerStrom8 Jul 27 '18 at 14:56
  • 555 timers are notoriously unstable when high-value timing resistors are used. Makes sense that it started working once you lowered their values. – DerStrom8 Jul 27 '18 at 15:23

1 Answers1

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Just one problem really. You used a NE555 timer instead of a CMOS TLC555 timer which would have allowed for high resistive values up to 10 M. Then the leakage of the timing capacitor becomes an issue. As a one-shot timer a few minutes is the limit, and less than half that for the NE555.

If you wish to work with the NE555 keep R1 + R2 under 200 K ohm.