I think you project is neat!
Would you be willing to share your code (just for personal
non-commercial use).
tks

Mike Montaigne
montaignem@aecl.ca

>----------
>From:  N STEENKAMP [M.ING E&E][SMTP:STEENKMP@FIRGA.SUN.AC.ZA]
>Sent:  Tuesday, January 13, 1998 4:58 AM
>To:    PICLIST@MITVMA.MIT.EDU
>Subject:       Re: pic tachometer...
>
>Hi,
>
>> Once again I'm drawing upon the infinite wisdom of the PICLIST...
>>
>>   I'm building a simple pic-based automotive tachometer and would like
>> to count pulses from the primary lead of the ignition coil and display
>> the RPM on an LCD.  For peace-of-mind (and other reasons), I'd like to
>> isolate the input pin of the pic with a 4N35 optoisolator.  I can handle
>> everything but the connection to the coil...
>>
>> I have 2 questions -
>>
>> 1.  What is the maximum frequency that the 4N35 will reliably switch at
>> (will it keep up with the pulse train from the coil)?
>>
>> 2.  What do I need to do at the optoisolator's input to handle the
>> ignition pulses?
>>
>Just a comment:
>I have also constructed an automotive tachometer about a year ago.  I
>have found that counting pulses in a fixed period is not a very good
>approach, since the frequency of pulses can be quite low.  At 600 RPM it
>is 20Hz for a 4cyl engine.  That means that your counting period must be
>quite long to be able to get sufficient accuracy, which means a
>slow update rate of the display.  The method I used was to time the
>period between two pulses and calculate the RPM from that.  I
>wanted to keep the refresh rate between 5 and 10 times per second.  Since
>the higher the RPM's the shorter the period, I was also able to average
>samples at higher RPM's for a stabler display.  From 0 to 1200RPM's I
>used 1 sample, from 1200 to 2400 I used 2 samples.  From 2400 to 4800 I
>used 4 samples and from 4800 upwards I used 8 samples.
>
>The project uses a 16C54 running at 20MHz and the display is a 16x2 LCD.
>It has a resolution of 10RPM's.  In the top line of the LCD I display the
>RPM's in digits and in the bottom line I have a bargraph display (made by
>using the programmable characters of the LCD).  The display is
>really fast and responsive.
>I guess that using one of the PIC's with a capture and compare unit
>would have been easier, but I wanted to see what I could do with a 54!
>
>Regards,
>Niki
>