I have a boat from 1970 that I have been fixing, and I decided that I wanted to use a GPS based speedometer, a digital tachometer, and a fuel level sensor. The boat has a MERC 1350 engine that seems to have absolutely no regard for what the stator coil is putting out (14 - 30V). I did not really consider noise at all, but it is now a big concern, as I am nearly done with my project, and when the engine starts chaos ensues.
So, I have two questions:
How would I filter/regulate the output from the engine for minimum cost? That can't be good for the battery, and it is causing problems for me. (I realize this is not an engineering forum, but there are a lot of intelligent people here)
What is the best method of reading the speed the engine is going at? The engine has a Tach line, but it averages about -2.6V and has a peak to peak of something like 19V if I remember. (The engine also has a "Thunderbolt" ignition coil which is supposed to have a significantly stronger spark. When I pulled the plugs the first time there were a couple millimeters of black crud on the plugs)
Look at your tach line with a scope. Some of them are square waves peaking on each spark. Some are emulated primary coil patterns, some are real coil patterns. Once you see what the pattern loos like you can get a better idea how to deal with it.
I know with car stereos if you are getting noise through the speakers you can put a cap in parallel from the battery to the amp, I have read you can also use a Ferrite bead or choke to help eliminate noise.
Hi,
A popular solution is to use a pre-regulator running on the car's battery power. I'd suggest:
1 to 3 amp diode and small value (1 to 4 ohms) resistor from battery to two paralleled capacitors. One > 1000 microfarads or more at 25V, one .1 uf at 25V or more. (For both low and high frequency filtering)
That gives you a voltage with little noise, that is protected from reverse-voltage spikes from the starter motor, and will "hold up" for a short time if the battery voltage drops way down when cranking begins.
THEN follow that with a voltage regulator like a 7809 or 7808 with another .1 uf capacitor on the output, to give you clean power to plug into the Arduino power jack.
@jimlee
Thanks for pointing out that there may be multiple signals overlapping (I purchased a universal tach and it only actually gets a reading on rare occasions) Once I figure out what the signal is supposed to look like, how do I actually read it? I have heard of people wrapping wire around a spark plug wire, and I could try getting a hall effect sensor on the distributor, but I don't know the best/most reliable/easiest way to approach the situation.
@terryking228
Thank you for the suggestion, I will give that a try. I am thinking of seeing if the engine has a voltage regulator at all, and if not, finding an aftermarket one. Between the two it ought to fix my problem.