Reading Tach (Plug Wires, coil, or....?)

[cswilsn]

Hi, I've been reading, and looking around and found:
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http://forums.parallax.com/showthread.php?p=508237 (http://forums.parallax.com/showthread.php?p=508237)
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http://www.youtube.com/watch?v=KOH2n1YhCOc (http://www.youtube.com/watch?v=KOH2n1YhCOc)
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is it possible to read engine RPM's from an active plug wire, safely? or has anyone come up with another way to get a reading from the sensors/coil?
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I'm trying to keep things as basic as possible, i just need a servo controller for a specific RPM range to open a valve to certain points.
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Any help would be greatly appreciated, even if its just a starting point.
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Thanks! :)

Chris
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[GICU812]

What type of car are you looking at?

Example, mid 80's GM, there is a white wire from the module to the coil, it is grounded to fire the coil, about 28hz at 800-1000 RPM, assuming a V6.

Newer DIS systems just have the white wire running to the tach. The tachs run on the same signals so the frequency should be about the same.

You might be able to use a freqin command just tapping the white wire.

The full system consists of a darlington transistor inside the module that grounds the line. Inside the coil is basically a transformer with 12v on the other side.

Personally, Id use an optocoupler just to be safe

[dazed&confused]

This Maybe of help to you cswilsn. I 've included a little short program that uses the Melexis Hall Effect Senc. the current used by the IGN. coils should have enough flux(in guass) to make the Melexis go _LOW_ on fireing the plug.If you look @ the Melexis DATA sheet you'll· find that this is an ACTIVE LOW circ. other wise the input pin on the stamp$ is held HI "1" w/ a 5.6kohm res....Just AS GICU812 recommend I do to the use of a optocoupler or @ least some zener/res. protection if you decide not to use the HallSen.

[dazed&confused]

· This may also be of help to Chirs. I have included a short little servo controller from $stampworks
Manuel. It uses a 10Kohm POT. -2 .1uF caps and a 220ohm res. on the wiper of the POT. This is a neat lil. program that uses the RC time Constants,A very clever method!!!!