I need an astable multi-vibrator at exactly 50 or 100 Hz with exactly 50% duty cycle. using circuits like 555 can't give me that with the common well-known values of components R and C , let alone the tolerance of components .

so , any suggestions ??


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    \$\begingroup\$ You can't have exactly 50 Hz, but you can get arbitrarily close if you're willing to spend arbitrarily large amounts of money. How close do you need? \$\endgroup\$ Nov 13, 2021 at 3:08
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    \$\begingroup\$ The closest you can practically get would involve getting a caesium reference standard and deriving your clock from that. That will set you back several hundred thousand dollars. \$\endgroup\$
    – Hearth
    Nov 13, 2021 at 3:16
  • \$\begingroup\$ Crystal oscillator, clock divider? Still won't be exact but you can get quite close. \$\endgroup\$
    – nanofarad
    Nov 13, 2021 at 3:20
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    \$\begingroup\$ @AbdAlhaleemBakkor Yes, exactly is impossible. Almost exact, for example to an accuracy of 0.005%, is possible. Come up with a numerical spec. \$\endgroup\$
    – nanofarad
    Nov 13, 2021 at 3:22
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    \$\begingroup\$ @AbdAlhaleemBakkor If you're not able to calculate how much accuracy you need, it would be better to describe your underlying problem and then ask for help solving it directly. \$\endgroup\$ Nov 13, 2021 at 4:30

1 Answer 1


Exact is not possible. How precise does your oscillator need to be and how much jitter can you withstand? What is your budget? Since you're mentioning a 555 timer, I'm assuming that spending over €1000 is not desirable.

Some inexpensive options:
You can use a temperature compensated crystal oscillator (TCXO) and get 1 or 2 ppm accuracy for under €5 up to a very fancy meal at a 5 star eatery. TCXOs do drift, perhaps no more than 1ppm per year, so you may want an adjustable TCXO if accuracy is important. However, you'll need an accurate source to calibrate the oscillator.

If you need good precision at low cost, you can buy GPS based timing systems that have a disciplined oscillator, often a 10MHz output with 0.001 ppm accuracy. It usually takes a couple minutes for the oscillator to reach specified accuracy after satellite lock. If you do an Internet search, you can find kits for perhaps €100.

Divide the output frequency down to 100Hz, then use a flip-flop to give you a 50% duty cycle at 50Hz.

  • \$\begingroup\$ +1 for GPS based idea. It is probably useless for OP but might come in handy for someone with tight requirements. \$\endgroup\$
    – Maple
    Nov 13, 2021 at 5:56
  • \$\begingroup\$ Thank you all for your time and effort, I had a lot of good ideas and information from your answers . Though there was no reason for all the negativity and down votes , I’m asking about an exact principle and the circuit following the signal has nothing to do with the answer to my question , you could have considered it for scientific purposes and just answer it. \$\endgroup\$ Nov 13, 2021 at 21:40
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    \$\begingroup\$ @AbdAlhaleemBakkor If you want to read up on how the exact second is derived, see The International System of Units (in French & English). Unfortunately, to generate an exact frequency is not possible without having a specialized laboratory. Even then, there is a problem with jitter which causes short time issues. Do not construe the comments as negative. They are only trying to impress that exact frequency generation is not possible. We can only state that the frequency is within an error band. \$\endgroup\$
    – qrk
    Nov 14, 2021 at 0:01

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