Automotive Tachometer input

Ok, further to my little project with a Pro Mini in a car i need it to have a tachometric input. The ECU has a simple 12v pulse every ignitor firing sequence that the gauge uses for reading the tacho. I was simply going to splice off this signal and put a 20k resistor inline with a 5V zener bridge to ground to filter the signal down for one of the Interrupt latched digital pins on the Arduino.

Does that sound appropriate? Or do i have it all wrong?

According to section 12 of the datasheet for the hardware externa interrupts (INT0 & INT1 -> D2 & D3), if edge triggering is used (rising or falling edge) pulses that last longer than 1 clock period will generate an interrupt. Shorter pulse are not guaranteed to generate an interrupt.

So, how long will that pulse be?

12V/20K = 0.6mA - is that enough current for the zener to actually kick on? Check the datasheet of the part you plan to use.

A whole lot longer than 62.5ns i can assure you. I havnt punched it in on an oscilloscope but the switching hardware in a standard tachometer isnt ns accuracy.

The 20k resistor is only there to buffer the input against spikes, could probably drop it to 10k. But 0.6mA should be well enough for a 1/4w Zener to kick in.

More just after a sanity check, i havnt done much circuit design for about 12 years now.

What are you building? Just curious. I'm building some projects for a Jaguar to include an updated HVAC system and also to retrofit, the new dial shifter into an older Jag. Your efforts may help me in the future, so post if you solve something. Thanks.

Im building a very simple exhaust valve controller for the car. Takes two inputs, throttle position (0-5v analog) and RPM. In turn it has a little bit of logic smarts to controls= a Hbridge relay to open or close the exhaust valve, and has a little timer so its not slapping it back and forth all the time.

Its really something i could do with a couple of logic chips and a 555 timer, but i had a spare Pro Mini from another aborted project so i figured i may as well use it instead.

Ill post up more details when i get into building it.

For what its worth, im using a 1N4732 Zener diode.

Datasheet here: http://www.jaycar.com.au/productView.asp?ID=ZR1402&keywords=4.7v+zener&form=KEYWORD