As promised but I have to say this first:
You have to work on inside your PC, removing your fan, connecting wires to mobo, using a solder, buying or borrow some measurement equipments (Volt meter or oscilloscope if you rich), etc...et.....
And no guarantee you will success because neither do I or nor somebody else can control the quality of work you do. So, you have been warned and I am not responsible should any damage occurs for any failure or irregularities.
This RPM circuit will also introduce some voltage drop to the fan (about 0.5 volt) so if you are using a fix 5 Vdc to the fan, your fan may not start after introducing this circuit. You will need to increase the voltage to your fan.
This circuit is also use 0Vdc as a common. So if you are using illegal method of 7 Vdc supply (using +12 Vdc and +5 Vdc line) or using fan supply with elevated common voltage, this circuit may not work unless it is modified.
OK. If you choose to proceed, here we go.
Motherboard RPM sense input
Most of the motherboard provides a pull-up resistor and need an open collector transistor connected to this input. Since the open-collector transistor (inside the fan) is turned on/off at proportional rate to the fan speed, the same pulse frequency is then created at the pull-up resistor (TTL level). A pulse counter will measures this frequency. To check if your mobo RPM sense requires an open-collector switch, measure the DC voltage to between the sense pin and common (0 Vdc - black wire). It should read about 5 Vdc. If not, yours may not be applicable to this mod.
Alternative RPM signal for 2-Wires Fan
Brushless type fan (such as axial fan on computer) will have a negative pulse during the pole switching inside the fan. Most of the fans have twice pole switch in a single rotation as you can see in the picture below. During the pole transition, the fan draws much less current compared to exciting mode, which then creates those nice negative pulses as you can see on the graphs below (take from an IC datasheet):
Our objective is then to extract this signal. The easy way to get this signal is via a sense resistor in series with the fan. The resistors size and the fan rating do matter. The higher the sense resistor, the higher the spike voltage measured, the more reliable the signal. On the other hand it also reduces the voltage to the fan for a given fan voltage supply. This is a trade-off. But it should not be a problem for SilentPCReview forum since we never want to run our fan at full speed anyway.
Here some guidelines for the sense resistors (again taken from an IC datasheet) but remember, this table is for nominal 12Vdc supply. So if your fan is running at 6 volts, you may double the value of the sense resistors.
The next step is to feed this signal to some kind of switching amplifier. This signal conditioning is needed since the spike voltage at the sense resistor is probably only 200 mV typical and even lower for down volted fan.
1st Project: A simple circuit for Fan RPM
This circuit is only used 1 transistor, 2 resistors and 1 capacitor. The circuit is below:
The R2 (edit: 10 M ohms, it was not showed on the picture for some how - sorry) bias and keeps the transistor 2N3904 in on condition. C1 blocks the DC signal from the sense resistor and passing only the AC signal (negative spike). When the negative spike reaches the base, the transistor will be cut-off and becomes high impedance and on again when the signal is 0.7 V or more at the base. Eventually, transistor will be on and off synchronously with the fan speed.
Please be aware that this circuit may not be reliably enough to switch on/off for low fan speed because the negative spike voltage is too weak to change the state of the transistors. Using FET instead of BJT probably is good idea but never tried because I have moved to the next circuit, using a comparator. If you are after a simple circuit and not too worry on low rpm accuracy, this circuit may suit you need.
2nd Project: A Less Simple Circuit for Fan RPM
The circuit is below.
An IC LM393 low power dual comparator has been used. A single chip IC is good for 2 RPM circuits since it has dual independent comparator. I have used it for CPU and PS fans in my computer, the most critical fan IMHO.
Again, the principle is no different from the 1st project but it is more reliable in low fan speed, thanks to the comparator. However, a comparator will require a reference (where do you think the name comes from?).
I use a voltage divider (R2 and VR) to get the reference. C1 (edit: 10 microFarad - not shown on the picture) capacitor is used to stabilize the voltage. You will need to adjust the VR to get the best result. I found the reference voltage (pin 2 of the IC) is best to be 60-75% of the dc voltage level on sense resistor (R1) during normal operation. It gives a good result for entire range of fan speed.
However, if you set the VR incorrectly, you will measure the noise or the harmonics of signals (very high RPM reading) or no RPM signal at all (If you do not understand, see the fan graph again, it will more make sense). So, spent a good time to tune this.
The good thing about the above setup is that the reference and fan are intentionally connected to the same voltage supply. This will maintain a proper reference level (once has been setup properly) for entire range of the fan speed with assumption that the fan is a linear load. And it has been proved to be rather linear in my experience.
R1 (sense resistor) is, again, depend on your fan rating. I have been using 5 ohms value for 200mA fan with a good result. My fan voltage is running from 6-9.5 volt, depending on the case temperature. If you are using 80 mm Panaflo (45 mA), you may need a sense resistor about 10-20 ohms depend on your fan operating voltage. Try the 10 ohms first.
As far as for comparator theory, it may take long time to explain it. I would rather skip it. You can find good information of this on the net and make it as your homework.
Well it took my 4 good hours. I do not know either this project is worthed to pursue or you just buy readily 3-wires fan. You decide. Should you proceed, OK. tell me your comment or if you have success to build this project
The pictures found a new home
Many thanks to Fancontrol