hi all im new here i am studying electronics in A-level and i am wondering how i could make a rev counter display so basically it needs to be about 20 LED's in series, the input will go from 12v to 0v 4 times for one revolution, i am thinking of doing it by PIC chip but if there are any other suggestions then plz help out, also i want to make the LED's come on one at a time and the last one to stay on after the next comes on every 500 revs so 500, 1000,1500... and so on so if i make it up to 8500 revs ill use 17 LED's.

Difficult to know what to suggest with this enquiry. Yes I would use a PIC, but short of writing the code for you, there is little help any of us can offer you.

On the front end, you need to divide down the 12V to 5V and remove some of the noise that might be generated by the ignition contacts. You can either do this by adding a small capacitor across the bottom resistor, or replace the bottom resistor with a 5V Zener diode.

Use a 7805 regulator to take down the battery voltage to 5V, but keep an eye on the power dissipation if you change your mind and want more than a couple of LEDs on at once.

It makes sense to look for a falling edge using something like:

label1:
btfss portpin;
goto label1;

label2:
btfsc portpin;
goto label2;

; falling edge has been detected


Drive the LEDs with a 220 ohm series resistor from the PIC port pins and you should get good illumination

Hope this helps