RPM Sensor

[natedawg]

I am trying to hook up a sensor to detect engine RPM on a small (<3hp) engine. What methods are out there to make this as simple as possible to hook up to the engine and microsquirt? I have a small optical sensor but it does not work for some reason. I have seen the hall effect sensor method used and I am curious if this works on a small scale.

What methods would you suggest for this application?

Thanks for the help.

Nathan

[Matt Cramer]

The engine size won't make a big difference. The most important question is if you are trying to control fuel only, or need something to control ignition too. Ignition timing needs a bit more precision.

[natedawg]

Thanks for the reply Matt.

As of now, my ignition is being controlled via stock magneto coil (standard small engine ignition) so I would only be interested in the fuel control. I have an optical sensor that seems to be working but MT will not register any RPM. I found an issue with MegaSquirt where no rpm would be read if a jumper was not installed, I assume this only applies to MegaSquirt and is not an issue for Microsquirt though? Not sure if this is a common issue or not.

[Matt Cramer]

You may need a pull up, as most optical sensors are like a switch to ground and the MicroSquirt is expecting a voltage. Try wiring the OptoIn+ to 12 volts through a 1K resistor and see if that helps.

[natedawg]

That did it! Thanks for the help Matt.

[natedawg]

OK so now the optical sensor seems to be working but it is showing some really high rpms, like around 15k...my engine should top out at 8k. Along with the optical sensor I am using a disc with a slot in it that is mounted to the crankshaft. Anybody have any ideas why the rpms would read so high?

[Matt Cramer]

Since you're getting 1 pulse every rotation, tell it you have a 2 cylinder.

[natedawg]

Thanks, I'll try that...with skepticism. It seems like that might cause other troubles? Are there any other calculations based off the cylinder quantity?

[mfro]

OK so now the optical sensor seems to be working but it is showing some really high rpms, like around 15k...my engine should top out at 8k. Along with the optical sensor I am using a disc with a slot in it that is mounted to the crankshaft. Anybody have any ideas why the rpms would read so high?

Nathan, you seem to have similar problems than I do (single cylinder, double RPM displayed). I will try Grippo's advice from the other thread tonight and see how it goes.

[Matt Cramer]

When running just fuel, the number of cylinders is really the number of tach pulses per engine cycle.