RPM measuring and OUTPUT trigger

Other Hardware Sensors
1

hi
im building an EFI system for an engine. i have a 76 tooth sprocket attached to the crankshaft. And im using and automotive crankshaft position (hall effect) sensor. my original plan was to fill between a few teeth, and monitor the frequency coming from the sensor. once the frequency dropped from passing the fill in portion, i would know when the engine is in the top dead position.

ive had a hard time finding info on frequency monitoring, and most info ive found RPM/Tach is on reading rpm rather then just using it to trigger an output.

i have an UNO R3 and im still fairly new to Arduino. any good material on frequency would be appreciated.

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You are adding an Electronic Fuel Injection system ?
I guess you also want to know on which cycle the engine is on, maybe from the camshaft.
Is the hall effect sensor positioned in such a way that it triggers at the appropriate point ?

You should be able to "reverse engineer" an Arduino hall sensor based tachometer design to work out how to trigger an output. Look for one in which the hall sensor is used to trigger an interrupt. In the interrupt service routine, you can do non blocking activities like say switching led on as a test.