PWM-Frequency Booster

Convert low pwm frequency of ~980Hz up to ~32Khz

If it’s necessary to have higher frequency on puls width modulation than a standard MCU is able to and its not practicable to increase the controller timer frequency, you can use this solution.
If your fan works like a speaker, you know what I mean

An ATtiny85 or similar MCU, convert the low pwm-frequency of 976.5625Hz (e.g. a standard Arduino pwm frequency) up to
32KHz. You need only a cheap ATtiny model, this program and a 10k resistor. The duty cycle of the puls is not touched, that means:

Input Duty Cycle = Output Duty Cycle

In this example an ATtiny85 is used.

Schematics:

• The low pwm input is on pin3 (PB4), the high pwm output on pin5 (PB0)
• On pin1 a 10k resistor is connected direct to VCC
• The two caps are standard blocking capacitors

Software:

In the header section you can find some values to adjust the program according to your situation:

#define PWM_INPUT      // PWM low frequency input pin
#define PWM_OUTPUT     // PWM high frequency output pin
#define PWM_FREQ_HIGH  // PWM frequency: 0 = 16KHz or 1 = 32KHz
#define DUTY_MIN_LIMIT // min limit according your fan 
#define DUTY_MAX_LIMIT // max limit according your fan

To get more or less precise values you can adjust the SAMPLE_RATE. More accuracy needs more time, try with some values to find the best for your project. A good guide value is times of 256, 512 or 1024.

// some examples:
#define SAMPLE_RATE 512  // verry fast reaction
#define SAMPLE_RATE 1024 // fast reaction, but a smaller accuracy
#define SAMPLE_RATE 1280 // little bit slower reaction, but better accuracy
#define SAMPLE_RATE 1536 // good agreement of speed and precision
#define SAMPLE_RATE 1792 // slower reaction of pwm changes, but high accuracy

If you try out some diffent values of SAMPLE_RATE, its a good idea to deliberate about SAMPLE_TIME. This time is in dependence of the rate. Higher sample rate needs more time to calculate the duty cycle, so the sample time should also grow and vice versa.

Programmer:

One hint to program the MCU: Its important to erase the Fuse Low Byte CKDIV8 of the MCU to get 8MHz Clock frequency!