Inductive pickup?

[Mosaic]

Hi guys,
Need a little help with designing a Tachometer. Got a few questions.

Would a simple inductive pickup work off a spark plug cable?

These newer cars come with plug packs, so i am not sure where to wind the pickup.


Would a 2.7k resistor inline with the inductive pickup and a rev biased schottky diode clamp be sufficient to handle transients? Should I include a .01uf near the input pin of the MCU?


Lastly, the 4 cycle engine fires 1 spark/cylinder for 2 rotations, not so? therefore I need to double the actual spark count to get RPM?

 

[colin55]

Just a wire near the lead is sufficient. Then you need a high impedance input to your circuit.
Just put the lead near the spark plug lead, connect it to a transistor with a LED and resistor and battery and watch the LED light up each time the spark plug is fired.

 

[Mosaic]

Hmm,

A transistor isn't really HIGH impedance. If that worx shouldn't the pic input pin be sensitive enough as it is a MOS device?

Perhaps I could configure a tmr as external clock and use it as a counter with the overflow interrupt?

 

[colin55]

You really need a buffer device between the pick up and the micro

 

[Gnome]

If it is a modern engine with coil packs it will have a tach signal wire. Just tap into that. Also when it comes to determining the RPM it depends on how many cylinders the engine has.

 

[colin55]

You don't have to connect to any leads. Just lay the input of the project near the spark plug lead and the pulse will be picked up.

 

[Mosaic]

Any suggestions on handle CDI variable multispark ignitions?

 

[colin55]

Use the micro to sort out the multi-spark signals.

 

[burgerfeet]

There should be a signal out of the ECU of the car that either goes directly to the coid pack(s) or to the pack(s) via an amplifier. Either way you will have typically clean switching levels of +12V. Be careful if you pick straight up from the input of the coil pack as there can be a lot of 'ringing' as a result of inductive switching; an oscilloscope would be useful to test for this. If you can pickup inductively easily then go for that. Just remember that on a modern engine you will have two sparks (known as wasted spark) per full revolution. If one spark per revolution everything is nice and easy. After than you will need some sort of division, either in a microcontroller or frequency divider.

 

[Gnome]

Why not just go off of the CAS ( crank angle sensor )? With the CAS you would just need to count square pulses for RPM.

I still say you tap into the TACH signal for ease of installation. If you ever install an aftermarket fuel computer you will notice that it doesn't need any crazy contacts or connections. Look those up and read about the installation. That may give you some insight.

 

[Mosaic]

I think Gnome is right, I need to tap into the OEM pickup.