# Inverted pwm with 555

#### valepe69

Joined May 28, 2019
15
Hi, i would like to build a circuit generating an inverted 25 kHz pwm with 555 ic and a trigger to regolate the pwm.
Is it possible?
I must drive a pwm fan with inverted Logic (0V full Speed), it's a fan from a hp rack server.

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#### crutschow

Joined Mar 14, 2008
32,064
Is rotating a pot to change the PWM duty-cycle, as the 555 circuit does, sufficient for your purposes?

#### crutschow

Joined Mar 14, 2008
32,064
A question: It seems a standard pwm not a inverted one (full speed at 0V and stopper at 5V)
The PWM duty-cycle goes from 0% (0V average) to 100% (5V average).
What you want to call full speed is just an arbitrary definition for that duty-cycle.
It has no effect on the waveform.

#### valepe69

Joined May 28, 2019
15
Is rotating a pot to change the PWM duty-cycle, as the 555 circuit does, sufficient for your purposes?
I suppose yes. I saw a sketch for Arduino where the pwm is generated with this code:
C:
pwm = 255 - out * 255
Where out is a range from 0.0 to 1.0

#### crutschow

Joined Mar 14, 2008
32,064
I suppose yes. I saw a sketch for Arduino where the pwm is generated with this code:
C:
pwm = 255 - out * 255
Where out is a range from 0.0 to 1.0
That should work if you want to use the Arduino to generate the PWM.

#### valepe69

Joined May 28, 2019
15
That should work if you want to use the Arduino to generate the PWM.
Well, I'd like to build a discrete circuit so my question. The code is to illustrate a working solution. But re-reading the code i think that it doesn't change the duty cycle but the voltage. Am i wrong?

#### crutschow

Joined Mar 14, 2008
32,064
Well, I'd like to build a discrete circuit so my question. The code is to illustrate a working solution. But re-reading the code i think that it doesn't change the duty cycle but the voltage. Am i wrong?
Not that familiar with Arduino code, but the code line starts with PWM so I would assume it generates a PWM signal that varies in duty-cycle by 1/255 step size changes with an out value of 0.5 giving a 50% duty-cycle for example.

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