ssave@caen.engin.umich.edu (Shailendra Anant Save) (10/24/90)
Has anyone built a digital tachometer from parts?
Here is what I have:
4 cylinder car. It has wires that go to the spark plugs.
What I want:
Some way of counting the RPM.
How I have thought of approaching this:
Method 1
Wrap a coil of wire around the spark-plug wires and
get a voltage across the terminals which would be the
average DC voltage. Then DC voltmeter caliberated to
this, would give me the RPM.
Problem: How linear is this?
How accurate is this?
How easy is it to caliberate the voltmeter?
Method 2
Wrap a coil as above. Schmitt trigger the pulses into
square waves. Gate and count the number of pulses in a
time (1/10 sec). Display the count.
Method 3
The fan blades are such that there is a light path
between each blade. Use infra-red transmitter -- detector
pair and get pulses. Count as in 2.
Anyone have any experience with these? The thing is
that I want digital. Analog ones available are about
$35 each. I am hoping that I can make one that costs
even less.
Please send any other suggestions to me.
Thanks,
--Shailendra
ssave@caen.engin.umich.edu
The University of Michigan
Ann Arbor.
robin@uhunix1.uhcc.Hawaii.Edu (Robin Amano) (10/26/90)
>> Anyone have any experience with these? The thing is >>that I want digital. Analog ones available are about >>$35 each. I am hoping that I can make one that costs >>even less. Good ones are more like $80 -- ---------------------------------------------------------------- Robin Amano | Internet: robin@uhunix.uhcc.hawaii.edu UHCC | 2565 The Mall | Honolulu, HI 96822
whit@milton.u.washington.edu (John Whitmore) (10/30/90)
In article <90300.143408JXS118@psuvm.psu.edu> JXS118@psuvm.psu.edu (Jeff Siegel) writes: >I had looked into building a digital tach for my 1980 model Chevette. >On that car there is is pickup for a standard(? 8-) ) analog tach on the >primary side of the HEI coil. This signal is more or less a 12 volt pulse >each time one of the cylinders fires. I used a simple resistor divider and >a zener diode The primary side on MY car connects to a CD ignition system; it feeds either nothing or 350V to that coil. On oldfashioned points-type ignitions, the voltmeter will read 4V to 12V (depending on internal or external ballast resistor) when the plug is NOT firing, but goes to 300V when it DOES fire. The use of a zener diode and resistors is the ONLY way to connect to this signal. Big resistors are preferred (the coil might be heating the resistor instead of firing the spark plugs if you use a resistor of less than half a megohm value). It might be prudent not to touch this terminal when the ignition is operating, too. John Whitmore whit@milton.u.washington.edu