read engine rpm from battery/alternator???

[Clint Sharp]

In message <m10vKf3-0003GGC at fwd12.btx.dtag.de>, Alexander Wenning
<wenning.motorsport at t-online.de> writes
>Recently I´ve seen that Bosch´s latest rolling road reads the engine rpm from 
>the battery voltage. When connecting my multimeter to the battery I can read 
>different frequencies at different rpm´s caused by the alternator. This seems to 
>be easy but when connecting my 20 MHz-oscilloscope to the battery I can only see 
>a flat line around 14.0 volts. 
>
Select AC coupling and wind up the gain on the scope, you should see a
different trace altogether. As to reading RPM with a pic, should be
fairly easy but you'll need to AC couple, amplify and filter the signal,
this is the hard bit. 
-- 
Clint Sharp