Variable frequency generator using a 555 timer chip

1. Variable frequency generator using a 555 timer chip

Hi

i would like to make an frequency generator using a 555 timer chip. The frequency should vary in the range of 100hz to 1kHz with a reasonable duty cycle (20-80%). I am using the circuit below. Varying r2 alone would not do the job because at r2=0 the duty cycle is 100%.

I found out if i started with r1=2k2 and r2=4k7 and c=1uF. The freq =124hz and duty cycle is 60%.
At r1=700R r2=200R c=1uF. The freq=1309hz and the duty cycle is 81%.

But i cant find a way to change both resistors with one potentiometer. Is there a way to do that or should i use a different topology?

2. Re: Variable frequency generator using a 555 timer chip

There is a simpler topology with only one resistor. It has a duty cycle close to 50%. See the changed picture below.

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3. Re: Variable frequency generator using a 555 timer chip

There are a few tricks you can try, to obtain a variable duty cycle. Some are at this link:

One method uses a potentiometer dialing a position between two diodes arranged in anti-parallel.

Another method uses a 556 dual timer IC. You select frequency with one 555 and the pulse length with the other.

4. Re: Variable frequency generator using a 555 timer chip

Originally Posted by godfreyl
There is a simpler topology with only one resistor. It has a duty cycle close to 50%. See the changed picture below.

I am simulating this option and it does not seem to work. Shouldnt pin 7 be connected to something?

5. Re: Variable frequency generator using a 555 timer chip

Shouldnt pin 7 be connected to something?
No. In your original circuit pin 2, pin 6 and the capacitor were driven by pin 7 and a pull-up resistor. Now they are driven by the output (pin 3) instead.

It should work. If you vary the resistor between say 10K and 100K, it should give a frequency range of about 10 to 1.

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6. Re: Variable frequency generator using a 555 timer chip

Thanks guys. This porblem is solved.

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7. Re: Variable frequency generator using a 555 timer chip

I want to generate frquency from 17 Hz to 17KHz. how to calculate frquency by above circuit?
can anyone sugges me the values of R and C?

8. Re: Variable frequency generator using a 555 timer chip

But this uses two resistor,Do you also need one transistor?

9. Re: Variable frequency generator using a 555 timer chip

I want to do it by one resistor. and transistor for what purpose? I am using this frequency in one unit called tachometer calibrator

10. Re: Variable frequency generator using a 555 timer chip

Okay... sorry it is resistor not transistor

11. Re: Variable frequency generator using a 555 timer chip

Now please help me to do that.. I have used 100k pot and 1uf capacitor. Now for 84.70k I got 17Hz but for 17KHZ it should be 84.70ohm but by 100k pot its not possible to achieve 84.70ohm. I am trying another calculation. If you got any idea then please tell. And one another thing by these values only on time is of required frequency but i want total on time +off time equal to 17 Hz to 17KHz

12. Re: Variable frequency generator using a 555 timer chip

A good way is to use 3 capacitors, say 1uF, 100nF and 10 nF, with a switch to select which capacitor is in circuit. Then put a resistor of say 6K8 in series with the potentiometer to limit the frequency range for each switch position.

That should give frequency ranges of about 15Hz to 200Hz for the first switch position, 150Hz to 2000Hz for the second switch position, and 1500Hz to 20000Hz for the third switch position

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13. Re: Variable frequency generator using a 555 timer chip

Thanks.. I have got the solution..

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14. Re: Variable frequency generator using a 555 timer chip

Thank you godfreyl.....I made it!!! 3 capacitor solution is good one

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