What is the signal on fanspeed pin?

[walshlg]

I'm trying to design a Hall-effect sensor to stick on the outside of my D5 pump (watercooling) to act as a pump speed sensor. Does anyone know what the signal from the fan lead consists of? The fans get gnd and +12v, is the fan speed analog votage, if so what range? Is it 12v pulses? 100mV pulses?? If noone know would someone with an oscilloscope please check for me, please!?

[walshlg]

Sigh, still working on this - at the risk of talking to myself in public:
so far I've found out that fan speed is a tach at 2x/ revolution as an open collector. That means that Mobo pulls the pin high and the fan sinks it to ground with each pulse. What is unclear is whether this is actually an open collector or not and how much current you need to sink to pull it low. Advantage to this design is that mobo high signal can be anything, heck even negative voltage as long as the fan will sink current on pulses. Nothing yet on noise. Are there really Hall Effect sensors on every 3pin fan? Interesting! So where do I find a Hall sensor with an open collector output?? (Must destroy a fan in search of knowledge!) BTW the reason you have a 4 pin fan is because doing pulse width modulation with the Vcc would effectively mute the fan tach signal whenever the pulse is low, effectively chopping the signal. PWM typically runs at about 20kHz so each tach pulse could become 4 or none depending on timing.