read engine rpm from battery/alternator???

[Alexander Wenning]

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. 

Does anybody know how I can read the rpm from the battery using a 
microcontroller like PIC or Basic Stamp II?

Kind regards 

Alex Wenning

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e-mail:	mail at wenning.de

website: www.wenning.de
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