I was trying to create a sine wave with this circuit but with different values.

enter image description here

When I just measure the 555 timer, I get a more or less proper square wave:

enter image description here

But the moment I add my LC low pass this happens:

  • Green = 555 Output
  • Yellow = LC Output enter image description here

Is the lowpass too much of a load for the 555 timer or what else could cause this?

What can I do to fix this?

When I connected a proper triangular wave from a generator the sine from my LC low pass was fine.

The simulation with my values:

enter image description here

Another thing I am wondering, the yellow line is the output of the LC.

Why does it have a flat maximum? shouldn't it slow down its slope reaching its maximum?

enter image description here

Ok I changed the values to: 2.2mH for the inductor and 2.2muF for the capacitor getting a resonant frequency of 2287Hz. I measured the generator with a frequency of 2300Khz. It's looking still better but still has this weird flat maximum. Interestingly I can actually hear a high-frequency noise from this circuit now:

enter image description here

  • \$\begingroup\$ how exactly did you change the values of the lowpass and what kind of load are you driving? Because the 555 timer strongly depends on the proper feedback signal to function correctly. \$\endgroup\$ Jun 20, 2020 at 17:02
  • \$\begingroup\$ @ChristianB. Sorry forgot to add my simulation \$\endgroup\$
    – Tobias
    Jun 20, 2020 at 17:10
  • \$\begingroup\$ It would be handy if you would note which waveform correspond to which node. I assume yellow is alway the node between the L and C designated "output" in this circuit and that green is ... the output of the 555? What is the last picture about? \$\endgroup\$ Jun 20, 2020 at 17:16
  • \$\begingroup\$ I am sorry. Yeah. Green = 555 OUTPUT , Yellow = LC Output. \$\endgroup\$
    – Tobias
    Jun 20, 2020 at 17:17
  • \$\begingroup\$ okay here is a riddle for you: a) what happens if you try to drive an LC resonance tank with a frequency strongly diverting from its resonance frequency? b) why does the 555 timer oscillate without the LC tank as well? Which components define the switching frequency? P.S. actually the answer is there at the end of the tutorial you linked. \$\endgroup\$ Jun 20, 2020 at 17:21

2 Answers 2


So what lesson did I learn? Not only the circuit can cause the problem but your measurement equipment. I didn't realize that On the x1 Setting of my probe the input range was +-1V which caused the sine wave to clip, which I referred to as "weird maximum". When I changed it to x10 it now has an input range from -10 to 10 V displaying the sine wave properly. Fun thingy, the circuit was actually producing a hearable sine noise. enter image description here

  • \$\begingroup\$ If you have solved this problem, don't forget to accept your own answer, so the system can mark the question as solved. \$\endgroup\$ Oct 5, 2020 at 5:09

To cite from the link you have given yourself:

Obviously, there are modifications you can do to this circuit. You can change the frequency of the output signal by changing the values of the RC network. If you increase the values of the RC network, this decreases the frequency. Likewise, if you decrease the values of the RC network, you increase the frequency.

To create a 6Hz signal, R1= 10MΩ and C= 10nF.

To create a 600Hz signal, R1= 100KΩ and C= 10nF.

To create a 134Hz signal, R1= 470KΩ and C= 10nF.

To create a 1.7KHz signal, R1= 33KΩ and C= 10nF.

To create a 43KHz signal, R1= 1KΩ and C= 10nF.

To create a 180KHz signal, R1= 150Ω and C= 10nF.

To create a 252KHz signal, R1= 100Ω and C= 10nF.

But remember if you modify the frequency of the square wave signal, you have to change the values of the LC network, so that resonance can be achieved for that frequency.

So the answer is: you need to modify the feedback part as well.


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