Thanks, Bob. I’ll look into that. Right now I’m struggling with learning to use my new Nano scope and with the tach output of these fans. I’m not sure I trust either yet… One day I’m getting a fairly clean square wave output and the next it’s all over the place. Frustrating.
@ Ian
Did you see the link to the DSO Nano tutorial series I posted on another thread…might help with the nano part of your problem, though I admit I haven’t watched the whole series yet:
Yea, I saw them and I’ve made it through the first couple but felt like I should try and use it some before going any farther.
Have you gone through the calibration process on yours? I think I’m going to need a wide range power supply to complete it… I have a PS3 power supply that I’ve been planning to convert into a bench power supply. Maybe this weekend I’ll get around to it.
No, I haven’t gone through calibration. Interesting thought on the PSU…I probably have a jillion laptop PSUs laying around from machines no longer in active use, plus at least a couple PC PSUs that are in PCs heading for the scrap recyclers. Bet I could turn one of those into a bench PSU…perhaps time for a new thread on that. 
I’ve seen some good write ups (Instructables.com ?) on some nice ones. I think I bookmarked one or two. Search around and you can find them or I’ll dig mine up at some point this week.
@ Ian
Started another thread on home-grown PSUs, so suggested solutions will be documented in case future fezzers are looking for the same thing. 
How about a ripple counter for dealing with high-speed pulses? Just a thought…
Thanks for giving me more homework, godefroi
Seriously, every night when I start this project up and look at the signal it’s different. Tonight it looks like this… What the heck causes that jumping around and is there anything I can do to stabilize it?
Real EE’s need to step in here because I’m going to utter ignoramious observations
It looks like something is loading the output. Like a capacitor or inductor. Do you have very long leads or maybe the scope is on the wrong setting - AC?
It could also be a poor ground. Is the scope and fan on a common ground?
Also try the following… Sit in a tight loop and read the input and output the value on another pin. If the input and output signals are both square at least you know the analog part of things are working.
Is that signal swinging from 0v to 1v or -1v to 1v? Maybe use a transistor to shift the level?
Do you have a pullup resistor to 3.3V? Looks like that might have come loose…
This is coming directly from the Hall effect sensor on the fan. I have the Panda and the fan sharing a ground and the scope is connected to the gnd of the Panda. To prove I’m not totally crazy… Everything has been off for the past hour. I turned on the fan, Panda, & scope with everything connected exactly as it was when I recorded the last video. Now this is what I get…
Note that if I hook the scope to the PWM signal on the same breadboard then everything looks nice and clean as expected.