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Set the 555 timer to 40 seconds idle, 2 seconds trigger - possible?

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Hi @kabl,

  Neat diagram!

I used to find work got in the way as well .. very inconsiderate!

I'll try to discuss what your schematic shows.


  There should be decoupling capacitor(s) connected across the power input to the 555, VCC to Gnd pins in this case.

Decoupling component values are a bit of mix of science, art and luck, but I would suggest something close to 100 microFarad electrolytic and 0.1 microFarad ceramic in parallel, as a starting point. Note, to be effective the wire lengths beween the capacitor and the respective pins of the 555 should be as short as possible, otherwise the inductance of the wire can be enough to effectively 'insulate' the capacitor from the 555.

Do not be surprised if adding decoupling capacitors makes no apparent difference ... however, their absence can cause all kinds of weird effects, which can come and go for no apparent reason!


The LED is connected backwards. Clearly your LEDs were working, so this is just a 'typo' on your drawing.


As a 'sanity check', I attempted to simulate the circuit using LTSpice, and it seemed to oscillate with LED off for 1 sec, on for 1.81 secs.


Which I assume is what you were expecting.

(Ignore the changes to LED and Mosfet part numbers, compared to your drawing ... they were just conveniently available in the simulator library.)



Simulations and 'real life' often differ, but this looks 'sensible' and doesn't have any obvious reasons why you should get strange effects.

I would definitely add the decoupling capacitors to your circuit -- I have put them on the simulator schematic, but ironically, it is difficult to demonstrate their effect on a simulator.

If you still have strange effects, then I would suspect the wiring .. sometimes those breadboards only make a poor contact, so that the 'real' wiring is different to what it appears to be.

You might be able to track down the problem with a multimeter, but it is one of those cases that an oscilloscope could be very handy.

Good luck and best wishes,


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Joined: 2 years ago
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Thank you for all your feedback! I will play around with decoupling capacitors when I get the time, to see what exciting and erratic effects they will bring. You are right, the LED is drawn the wrong way.


With the resistors I have plotted, I get off=40 seconds, on=1.6 seconds? If you get off=1 sec, and on 1.8 seconds, I need to recheck my schematic vs the breadboard.


I just got an Siglent SDS1104X-E oscilloscope, it`s not even unpacked yet! Very kind wife! So that is now also on my to-do list; learning how to use an oscilloscope, without blowing it up.


Kind regards and all the best, Knut

This post was modified 1 year ago 2 times by kabl

Joined: 3 years ago
Posts: 1794

Hi @kabl,

   Sorry, my mistake ... R1 on my diagram was 20k instead of 1.2 Meg.

Re-running the simulation gave LED off 61.7 seconds, LED on 2.0 seconds, which is a much more reasonable agreement to your 40 / 1.6 seconds measurement.

The most likely suspect for most of the difference in timings is the 47uF electrolytic capacitor. 

Electrolytic capacitors are usually best avoided for 'accurate' timing circuits as they have huge tolerance in values, plus high leakage current. Of course, some timing circuits do not need to be accurate, just in same 'ball park', when their performance may be 'good enough'.


I certainly recommend getting that Siglent out of the box!!!

The only 'obvious' way to damage it, (excluding by mechanical trauma), is by overvoltage, and I would hope it has at least reasonable protection against that. If you are working on a low voltage circuit, that does not have significant inductances, you would be very unlucky to damage it.

(I don't know about Siglent's in particular, but it is common to be able to install new firmware .. just like a mobile phone or a PC ... and obviously that can go wrong, but if you are careful, it should be ok.)

Best wishes, Dave

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