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I have created a 555 oscillator circuit where the output is set to have flashing LED light. Now I have a problem when I change the voltage of the potentiometer, the output frequency does not change. Here is the schematic of my circuit that I implemented in breadboard.

I used fixed ceramic capacitor of 10nF instead of varactor here for now. But If I change the capacitance and add 0.33uF, the flashing of LED doesn't change. Now LED only flashes with constant frequency. When I change potentiometer, I checked with multimeter that the voltage changes. I want to control the capacitance through the potentiometer so that the output frequency changes and LED flashing rate changes according to the changing of capacitance. Can anyone help me what is the problem here?

Components: CMOS 555 timer R1 = 1 kΩ, R2 = 4.7 kΩ, C1 = 10 µF and C2 = 0.01 µF 1 MΩ Potentiometer.

enter image description here

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  • \$\begingroup\$ Consider copying a working example. \$\endgroup\$
    – st2000
    Jul 25, 2018 at 13:24
  • \$\begingroup\$ That link ^^ from instructables does not even show a proper schematic which always worries me. Proper circuits are shared through schematic drawings, if you don't include it then unfortunately I cannot take the "design" seriously. \$\endgroup\$
    – Bimpelrekkie
    Jul 25, 2018 at 14:06
  • \$\begingroup\$ @st2000 Ummm.. do you believe that's actually a working example? If it does change the flash rate, it's got a lot more to do with the pot loading the capacitor through a forward-biased diode than any change of pF in parallel with an electrolytic capacitor. \$\endgroup\$
    – Spehro Pefhany
    Jul 25, 2018 at 14:11
  • \$\begingroup\$ ...honestly, I had my doubts that changing the depletion region of a varactor diode created enough capacitance change to make that much difference in a 555's time period. That's why I stuck the link in the comments rather than make an answer of it. Just about all examples of a varactor diode that I have see use them in a RF tuners where the frequencies are magnitudes higher than what a 555 is normally run at. \$\endgroup\$
    – st2000
    Jul 25, 2018 at 14:30
  • \$\begingroup\$ Rather than trying to control the capacitance of the timer, consider controlling the charging current of the capacitor. You should get a wide dynamic range and good stability. \$\endgroup\$
    – lakeweb
    Jul 25, 2018 at 16:28

2 Answers 2

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If you did everything well then the frequency does change but the change is so small that you don't notice it.

What Varicaps are you using?

I bet that their capacitance varies between say 10 pF and 200 pF.

So how much would that make the frequency change?

When the varicaps are at 0 pF, only the 10 nF capacitor is there determining the frequency.

When the varicaps are at 200 pF, combined with the 10 nF capacitor you would get 10 nF + 2 x 200 pF (2x since there are 2 varicaps) = 10.4 nF

That's a difference of 0.4 nF on 10 nF which id 0.4/10 x 100% = 4%

That's too little difference to notice.

To make the difference more noticeable you would have to lower the 10 nF in value. You could lower it to 1 nF resulting in about 40% frequency change. That should be noticeable but it's probably still not what you want.

You cannot go much lower than 1 nF as the 555 chip simply isn't suitable to work with such low values.

You really should be looking at a different way to change the oscillation frequency of a 555. There are plenty of examples to be found if you look for them. Google for "555 VCO", VCO means Voltage Controlled Oscillator which is what you're trying to make.

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  • \$\begingroup\$ Hi, thanks for your detailed answer. A confusion about the capacitance that you mentioned 10 nF. Which one is it in my schematic, you mean C2? As far as I know the output freq is 1.44/[(R1+2R2)C1]. I didn't get the 10nF component. Can you explain a bit further? Or am I probably not getting the point? \$\endgroup\$
    – aguntuk
    Jul 27, 2018 at 9:08
  • \$\begingroup\$ Your question isn't very clear on the timing capacitor you're using. I thought you used 10 nF in parallel with the varicaps. Using the varicaps on their own as timing cap. will not work, the capacitance is too small. \$\endgroup\$
    – Bimpelrekkie
    Jul 27, 2018 at 10:51
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The simplest way to control the frequency of a 555 with a voltage is through the CONTROL pin (the hint is in its name). This is pin 5, and people usually connect it to a decoupling capacitor, or even leave it open.

Try changing its voltage. Or better still, read the datasheet for what it does, and then try changing its voltage.

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  • \$\begingroup\$ thanks for your reply. I know your suggestion to change voltage along the control pin. But I have to do it with varactor as the project is about the performance of the varactor with a small real time project. \$\endgroup\$
    – aguntuk
    Jul 27, 2018 at 10:48

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