Electrical – Changing frequency of 555 timer with varicap or capacitor

555ne555oscillatortimervaractor

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

Best Answer

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.

Related Topic