RPM monitoring with PWM?

[sevisehda]

I've been trying to figure out a way to monitor my fans speed while still controlling them. I think I finally found out how. I first trying it with a BasicStamp as to not accidently fry a fan header on my MB. Then tryed it on my DD5 and finally on my MB. They all worked, how well I'm not sure.

Basically I cracked open a fan then cut the traces between the main power input and the tach power input. I then soldered a 12v lead to the tach chip. This allows the fan to output a constant tach signal at 12v, while the fan can be supply either a pwm signal or a lower voltage.

The Basic Stamp gave me great results because it polled 5 times a second. Even after the fan power was removed it showed a nice decrease in RPMs until it went below 500 at which point iit reported ridiculous RPMS(above 40k). The DD5 gave some nice numbers but unfortuneately it won't display a single RPM for very long. Once I plugged into into the MB I ran MBM5 which polls every 10seconds. It showed some reasonable numbers as well.

Does anyone have any comments/suggestions to improve this, or is there an easier that I missed?

[Boli]

Have you looked into light gates? I mean they are used in mice all the time so they must be pretty cheap and if you have opaque fans they must work fine. OIf course you'll have to make a circuit and find a way to fit them onto the fans but hey its another option.

~ Boli

[starbuck3733t]

Can we get a diagram on this? What fan did you hack up?

The only other way I've found of rpm monitoring a PWM'd fan is to only read the tach line when the fan has +12V applied to it.

[sevisehda]

I tried it on 2 fans, the first was a Delta from a AMD HS, the second was a Vantec stealth.

There really isn't a diagram because each manufacturor/fan has has its own circuit. All you have to do if fallow the 12v lead, and cut it where it splits to the tach chip. The solder on a second 12v lead.

I tried to take pictures, unfortunately my camera didn't like taking pictures of something that small.

I tried using the winbond utility that came with my NF7S, its polling time is much smaller and shows the RPM changes much better, it however must have a lower limit around 800.