Remote Vehicle Starter Help

[Charles Linquis]

It is my belief that anytime you are controlling machinery of any sort, you need to use interrupts. People can wait, machines can't. It sounds like you aren't using DT-INTs, on a timer which would make pauses unnecessary, and which would allow you to read the RPM (assuming you have access to a signal - like the crankshaft position sensor) simultaneously with everything else. Search the forum for my tachometer example. Since uses a timer to poll pins, you can use any pin at all for your tachometer.
If you have a belt pulley on the engine, you can use that as well. Years ago, I glued two magnets on a pully (using JB Weld - a good epoxy), and read an engine's RPM with
a Hall-effect sensor mounted on a bracket that was about 6mm away.

[amgen]

Some alternators have a tach or "R" output that has different voltages during cranking and running. Some diesel or gas generators use that output to stop cranking.
Don

[Art]

Charles, I did see your post with that code before posting this. Some of it's description seems ambiguous to me, but that's likely my own fault. PRELOAD value for example... I don't know what my shortest tach half cycle is, and then in code comments it says "depends on clock speed", but I don't know what to change if using a 4MHz Xtal. Seems a lot to work out when I only need to measure the time between two pulses. The current program is cycling all the time as said in my first post, or the power LED wouldn't work... as I also said, it's a POV display. I'm not using any pause commands at all. Timing is done by counting program cycles so there is opportunity to multitask. The alternator does sound like a good idea, even if it doesn't have the right output as described, the output of the regulator wouldn't reach it's full voltage until idling. With the right resistor divider, I assume I could produce a logic high or low state on a pin so there would still be no halting program execution.

[Charles Linquis]

I think voltage out of the alternator is probably not an accurate measure. What if the battery is discharged?

PRELOAD is simply a var I load into TMR0 on every interrupt to get the interrupt period to be what I want. If you are running 7000 RPM and your tach signal is a square wave that has one cycle per revolution, then the period is 8.5 mSec, and a half-cycle is 4.25mSec. In order to make certain that you don't miss a pulse, your interrupt rate (period) would need to be 4mSec. At 4Mhz, PRELOAD would be 0xF067 (PRELOADH = 0xF0, PRELOADL = 0x67). Directly from MisterE's PIC MultiCalc.

Measuring the time between two pulses is OK if your signal is not "noisy". But the signals I had to measure were very noisy. Sometimes the time between two pulses was 1.5 mSec, sometimes it was 5mSec. That really screwed up my program. In order to get an accurate reading, I had to average a large number of pulses. Then my "quick and dirty" routine wasn't so quick anymore. And the slower the input signal, the longer my routine took.

Timer interrupts to the rescue. I run an interrupt at 500uSec. Now, I can measure any number of signals simultaneously - on any pin. I count transitions over a time interval - say half a second, and I automatically get the average number of pulses over that time interval. The best part is, it all takes place entirely in the background. I can read a variable anywhere in my program, and it has the latest RPM value. The accuracy is limited only by the sampling time.

[b1arrk5]

With a gas engine, I energize the fuel pump, wait a few seconds for preessure to stabilize, energize the ignition, read and store the battery voltage and then start a timer and energize the starter. The timer limits how long the starter can be engaged. As soon as the starter is engaged, I start to continuously read the battery voltage. When it rises a half a volt above the stored voltage value the starter is disengaged. I've used this system for years now, in two different vehicles, and it works great. If the timer times out, then the engine didn't start. In this case, I'll try again twice more.Hope this helps,Jerry

[Art]

I do have the device installed in the car now, but haven't done anything other than a timer for the starter,
and you can manually disengage the starter by pressing the button again while the starter is engaged.
This is not acceptable for the long term, but at least the hardware and existing software is all working well.

You would need a resistor voltage divider at the ADC input yes?

[spcw1234]

A voltage resistor divider would be needed! I would use 33K/12K or similar changing your 0-15VDC to 0-5VDC. I would first verify that your alternator produces voltage as soon as the engine starts. Newer vehicles with computer controlled alternators sometimes don't turn on immediatly. I have an automatic engine starter I built on a diesel engine and I am using a 5PSI oil pressure switch to check if the engine is running or not. Perhaps your motor has one of these already for an idiot light on the dash to show that there is no oil pressure.