Electronic – arduino – Safe connection of a 4pin Fan to an Arduino

One thing to note is that

if (pwmVal != 255) {
    pwmVal += 10;
    ...

is not going to stop at 255, since pwmVal will be 250, then 260, e.t.c. One way to fix this would be write

if(pwmVal < 255)
{
   pwmVal += 10;
}
else
{
   pwmVal = 255;
}

By the way your code is written, if DEBUG is set to 0, nothing will happen (pwmVal will be set to 0). If DEBUG is set to 1, the fan speed will increase at a rate set by "wait" (which isn't defined in this code), until it gets to it's maximum. The value will also be printed to the serial port.

By far the lowest component count solution to the problem of simulating the TACH signal from a fan is to use a small microcontroller (MCU) that can measure the PWM input signal duty cycle. The PWM signal can be measured two ways with the MCU either by using a timer to directly measure the high / low pulse widths OR by filtering the PWM via an R/C circuit to produce an analogue level that you read into the MCU via an A/D converter channel. The output TACH signal can be generated easily using an MCU timer that is capable of driving a pin. In fact I have even done TACH signal simulation using a timer that generated a periodic interrupt and then let the MCU software toggle an I/O pin in the interrupt service routine.