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Simple RPM Switch

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well, not necessarily 350rpm, but its for a device that will control starting the engine, so it needs to know when to open the circuit to the starter motor and if the engine is on or off. I was just going to try 350 rpm and adjust it up or down as needed.

I thought about the battery indicator light, but it doesnt seem consistent enough. Plus if the alternator failed, the starter would never disengage when I try to start it.
 
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Sounds like a home made remote car starter project. if you alternator is dead you wont get very far on battery only! Or at least my vehicals never do!

Put a cranking time out function on it. If its still cranking after so many seconds give up. Either its running but not seeing it or it wont start and all your doing is running down the battery and wearing out the starter.
 
there is a cranking time out, but the battery light just seems like a bandaid solution. To me, RPM input would be the best way. Plus I'd like to know for my own knowledge how to have an rpm input for a pic for future projects.
 
I dont recall saying anything about the alternator light. What I was talking about was using the alternator windings themselves. Many of the big diesel trucks get there tach signal this way. Its just a matter of figuring out the ratio of your alternators to engine gear up then how many poles there are in the alternator.

The other option I am thinking of would use a simple 3 wire hall effect sensor to watch a magnet attached to the crankshaft. That would give you an independent and isolated sensing system for your project.
Or use it to sense the magnetic field of the alternator stator.
the frequency will be higher but thats still not a big deal just change the reference point in the program.
 
sorry, i know you didnt say anything about the alternator light, but I figured it would be along the same lines.

I'm trying to keep it simple. I have a feeling that something maybe I didn't have the lab scope connected properly, or something, and maybe there is more voltage passing through that wire than what i was reading. I'm starting to wonder if I should go back to the LM2917 idea. If it is 100mV, is that going to be enough for the LM2917 to work properly?
 
There is something not right here. Nothing works with a 100mV signal and so I suspect something else is wrong. When you look at the original signal do you have the existing tachometer connected? I ask as the sensor could be a simple (transistor) switch to ground that requires a pullup in order to work.

To answer your question, no, a transistor will not be switched on by 100mV. It needs several times that voltage.

Mike.

Ah, I completely missed what you said about using the a pull-up resistor in order to work. I get the idea, not sure how to apply it though. You mean one of these setups?
**broken link removed** or **broken link removed**

If so, how do i find out if this is the case?
 
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If so, how do i find out if this is the case?

Connect your scope from 12V to the signal wire and see what you get. Also, while your there, have another look at what you get between 0V and the signal line.

Mike.
 
checked the signal wire again. It turns out I am an idiot after all! I had the leads for the scope in the wrong spot. they were in the spot for the meter function, so the 100 mv it was reading was just being picked up without a lead in the scope "input". After connecting it correctly, I saw the pulses actually go from 0v to battery voltage (~12V-14V).
 
Hi, you have a very interesting topic and i would like to build the same type of project. Can you help me with this please? I am new here and also do not have the know hows of it. Thanks
 
Well, to be honest, I am kind of at a stand still until I find some time to do some testing on the RPM input. My wife and I are searching for our first house, so this project had to take the back burner. :(
 
RPM switch

Howzit buddy, i have found what we both are looking for, it just need a small change in parts. Send me your email then i can mail you the diagram with list of parts and assembly details. Chiao:D
 
Howzit buddy, i have found what we both are looking for, it just need a small change in parts. Send me your email then i can mail you the diagram with list of parts and assembly details. Chiao:D

How about posting it here so everyone benefits. ;)

ken
 
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