Basic OBD2 decoder to get tach signal?

Other Hardware Device Hacking
1

Has anyone produced a basic OBD2 decoder that can be used to output specific engine data, as opposed to the bulkier units that transmit all data?

For instance, is there a way to tap into the data wire and get ONLY the RPM data without needing to capture any other information, transmit via bluetooth, etc?

What I'm trying to do is create a controller for under-tank LED lights on an early J1850-protocol Harley-Davidson that will dim or brighten according to engine RPM. I understand the PWM control portion of it, but I still need to be able to get the actual tach signal and would prefer it to be in as compact a package as possible so as to fit neatly under my bike's electronics cover. As it will be wired directly there's no need for bluetooth transmitters or anything else of the sort, the output can come directly from the arduino.

2

Sorry you will also have to deal with the other stuff. OBDII is a network and has traffic other then the RPM on it. You need to decode enough to be sure you have the RPM signal. It might be much easier to tap into the RPM sensor.

3

You don't need to use J1850 to get the engine RPM.

Why not use an inductive pickup at the spark plug cable and calculate engine RPM?
Like this: SportDevices. Chassis and Engine Dynamometers