This is related to a previous topic: "Counting A/C peaks from a hub dynamo to work out bike speed" http://forum.arduino.cc/index.php?topic=87390.0
In that one, someone was thinking about the same thing, but I don't think they realized the power output of their hub.
So, these hub generators produce A/C, fixed at around 500mA, but with a variable voltage output. Voltage varies from 0 at standstill to around 20V at significant speeds. You can supposedly, if you don't have some load attached, reach voltages as high as 100V. Also, it's AC of course, so half of your signal is negative voltage.
So, I'm looking at using the hub to 1) provide a frequency signal to the Arduino to use as a speedometer and for some circuit control needs, and 2) use the power output to charge batteries. So what would I want to do to the output to convert it to something I could safely read on an Arduino pin? It seems like if I use a bridge rectifier first, put some over-voltage shunt in, and run that through an appropriate voltage divider, I can safely get a signal varying from 0 to +5 volts. But the voltage is varying with speed, so I can't just read it with a digital in. Any advice on that? Analog in with a pull-up (or down)? Digital same way?
Bigger power question - anyone familiar with load dump surges? How much power would I need to draw to avoid that power spike? Is my teeny-tiny little draw at the Arduino to read that frequency going to do it? The Arduino (nano) will always be drawing a load for its own power and to read the frequency (plus the display), but I'm planning to connect/disconnect the power to the charging circuit to reduce parasitic load on my engine (me!)
Last question, what would be a reasonable way to track the frequency? Interrupt with a counts/second counter?