tachometer output on car ECU

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Thread Starter


Joined Mar 8, 2009
I have a PE ECU installed on a motorcycle engine (Honda CBR600F4I) and the tach output states that a 12V square wave is outputted with a 30% 'on' and 70% 'off' duty cycle with 2 pulses/crank revolution. I understand that this means that the voltage read from a volt meter would be 3/10 of the actual output correct?

I guess I am just trying to get a better idea of what the 30% 'on' duty cycle really means to the values being outputted from the ECU. I plan on using an LM2907/LM2917 circuit to output the RPM to an ADC port on an ATMEGA8 chip using its ADC.

I don't have the exact frequency range that the ECU outputs so I am not 100% sure about the LM2907/LM2917 would look like. Do I really need these frequencies in order to build the circuit. I have models from LTSpice simulating similar circuits but I need to be more sure on the whole 'duty cycle' business.

Any input would be greatly appreciated!


Joined Apr 16, 2010
I doubt that this thread will survive; it likely conflicts with the site's rules regarding automotive modifications.


Joined Jan 28, 2005
I am going to lock this thread for the moment while the moderators consult over whether it violates our rules against automotive modifications.

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