Has anyone tried to make a digital tachometer with thier stamp?

bcoomes

I have a bs2sx and hope to make a digital tachometer for my vehicle. I think it can be done using the count command, but was hoping to find out if it will work or if someone has done it. My biggest worry is the stamps ability to sink the incoming current. I haven't measured the signal yet, but I would think it would have to be pushing the 30mA? limit. I'm also worried about possible spikes that won't be seen under normal circumstances. If anyone has ever tried this or has any insight to the situation, please let me know.
Thanks

steve_b

You don't actually have to grab the signal off of existing equipment. You can use a hall effect sensor. Basically, you put a little magnet on your flywheel or some other "rotating" part and you put this little "pickup" near enough so that the magnet triggers it. THen you figure out the equivalent RPM per switch of your sensor.
you can put multiple magnets on one wheel/pulley and count more than one per revolution...but just be wary that the stamp might not be fast enough to count 36 switches when the pulley is moving at 5000RPM already!

I've got a project on the shelf (waiting for better weather) that I'm planning on interfacing to my Jeeps' OBD2 interface and picking off live data.
I've only build the circuit to 'talk' to the interface chip....haven't gone past that!

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Steve
http://ca.geocities.com/steve.brady@rogers.com/index.html
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

KenM

Yes I have.

What make & model year is your vehicle?

bcoomes

Steve_b
Thanks for the replies!·· I didn't think about attaching my own sensor. That's a good idea that should eleminate all of the current worries.

KenM
It is actually for a friend's racecar. It's a Chevy 350 with a MSD ignition box.

Thanks Again

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lightsout

Even though I still consider myself a beginner, I consider that an easy one to do,(I didn't say quick)· . First of all what kind of car, (auto electronics is my specialty), you need to identify the type of ignition system. You could do a seperate "pick up" as steve_b said although I think for most ignition systems it would be easy enough to tie-into. Safest would be an opticoupler (see component sales page). And some of the LCD's I've read about sound as easy as programing DEBUG info. if you want that type of display. And I don't see where you would need to sink any current. although if directly conecting it ,I think a voltage divider should be used and depending on what your tied-into, you might have to deal with ahhh....whats the word..(voltage-flyback?) from the flux colapse of the ignition coil.

steve_b

I think most ppl would agree that, even though a car is running on 12V DC, it's quite the noisy environment!
So, when you go to power your setup (after realizing how you want to read/pick-off the tach data) then you'll have to go about setting up some filters and such to protect against coil collapse..etc...

lightsout:
A voltage divider may not be good enough. The voltage in a car can go from 12V up to 14 or even 16V if you have a HOT alternator.
Run a regulator off of the battery (ACC switched). This way, regardless of how much the voltage varies, you'll have a nice steady supply...AND, it offers some protection as well. Not much...but some! lol

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Steve
http://ca.geocities.com/steve.brady@rogers.com/index.html
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

bcoomes

Thanks to everyone who responded. The information should really help with my project.

Brad