Conditioning input signal for tachometer circuit

[hgboy]

I know the tachometer thing has been beaten to death ( I searched all the listings for it already), but noone really asked what I need to know. I have a PIc 18f4550 that I will be using as an led shift light. 7 leds are connected along port b and are set to light up at certain intervals. The only thing I dont know how to do is condition the tach signal for use by the pic. My tach signal wire is connected to the ignition coil and outputs 12 Volt pulses. Could anyone give me some examples of how to clean up this signal, and drop it to a stable 5 volt pulse?

 

[Diver300]

If you use this circuit, with R1 at 10 kΩ, D1 a 4.7 V zenner, and C1 at 10 nF.

You should connect the output to a schmitt trigger input of the PIC. It would also be a good idea for the program in the PIC to ignore any edges that are too soon after the previous one.

 

[hgboy]

Thanks for the diagram and the values. regarding the schmitt trigger pin (I will be using RA4 on my 18f4550), do I need to do anything special with it other than setting it as an input in my code?

 

[Diver300]

No more than any application where you want to use RA4 as a digital input.

The pin is shared with comparator 1 output, Timer0 output and the USB receive input, so you need to turn all of those off to allow you to use it as a digital input.

 

[hgboy]

thanks a lot. I write my programs using swordfish basic, which has a setalldigital command which sets all of the pins to digital i/o. Should work nicely for this particular case.