I have a 3-pin 12 V computer fan and I want to interpret its speed sensor output. At the yellow wire I get something that looks like pulse-with modulation. How would I interpret the output without actually connecting the fan to a computer?
Brief background: The tachometer output comes from a Hall-effect sensor mounted on the motor driver PCB on the fan frame. One or more magnets embedded in the fan rotor hub activate the Hall-effect sensor as they pass by. The sensor is amplified, and eventually drives a logic circuit. The fans that I have seen use an open drain/open collector output.
One (or more) pulse is generated every time the the fan rotor completes a revolution. The number of pulses counted in one minute is directly proportional to the RPM of the fan. In your fan's case, I think it would be reasonable to guess that there are two pulses generated for each revolution. With the frequency that you have measured, about 1500 RPM sounds right, given that you are running it at 10V (12V nominal) and the typical is 1800-2000 RPM.
If you want a more visual approach, you can make a crude strobe tachometer using just a LED and resistor. Connect a LED (brighter is better) and an appropriate current-limiting resistor between power and the tachometer pin. If you mark one of the fan blades with something easy to see, like a sticker, you should be able to shine the LED on the fan blades and see the sticker illuminated in two places. You can use this technique to count the number of times the tachometer output goes low each rotation, and to approximate the duty cycle of the signal.
All the needed infos are published here:
https://noctua.at/media/wysiwyg/Noctua_PWM_specifications_white_paper.pdf v1.3, including example schematics
Voltage 12 ± 1.2V
Peak current (@13.2V) 2A
Speed reading: 2 pulses per revolution
Open-collector or open-drain type output
Mobo has pullup
PWM frequency: 21-28 kHz, target 25 kHz
logic low: <=0.8V
Imax: 5 mA
PWM duty represents the speed output compared to full speed, linear relationship
If PWM is lower than minimum accepted value for that fan, undetermined behavior according to specs
Fan should match PWM control signal ±10%
Rotor lock and polarity protections are expected
Pins: 1, 2, 3, 4 are black, yellow, green, blue and their function is GND, 12V, sense, control
In most fans that I've worked with, the yellow wire is referred to as the TACH or tachometer wire. It is similar to PWM output but it is the frequency that is related to rotation of the fan. Sometimes it is 1:1 and one period output on the TACH line is equal to one revolution of the fan; sometimes there are 3 periods on the TACH to 1 revolution of the fan, you need to check the datasheet.
You can connect the TACH signal to an I/O pin on a microprocessor and determine the RPM value of the fan pretty easily.
The fan signal is the rate of rotation, 1 Hz = 1 RPS (rotation/revolution per second.) Connect a PIC or your favourite brand of microcontroller to the signal, count each rising or falling edge in one (or however many you want - more seconds, more accuracy) second and multiply to get RPM. If your processor is fast, you could even measure the period of the waveform and from this determine the speed to a high degree of accuracy (1/t = f).
For most fans the 1 Hz represents one rotation, as it is more expensive to include multiple switches in the fan, but don't rely on this.