Remote Vehicle Starter Help

[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.

[timmers]

I built an auto starter for a generator.
Used a hall effect sensor looking at the starter teeth of the flywheel. Fed this into TIMER 1 external clock input, and used it to count the number of pulses (teeth) over a time period. I then did some tests and measured the fastest it could crank when warm, then set a threshold about 20% higher. Never had a problem with it.

For the record, I built in lots of safety features, which included low voltage cutout, under speed cranking, low oil pressure, over temperature, with visual and audible warnings before starting and an emergency stop switch.

I was able to have a remote control panel in the house where I could display and configure the parameters or manually start the engine and switch over the electric contactors. Never used it in manual, auto was fine.

[Art]

I can understand why people would do this with generators. Who wants to walk outside to start an engine

My alternator may not produce voltage straight away (although I'm pretty sure it does),
I am able to see an initial voltage, then a drop while cranking, then a gradual rise to a higher voltage.

I think this is nice because I should be able to use the supply leg of the 7805 to take
the voltage measurement, and need no extra wires.

[b1arrk5]

I used a voltage divider. For my test circuit, I had the ADC continuously read the voltage, and send the result out the serial port. With my laptop watching the serial port, I started the car, and had before and after battery voltage readings. Merry Christmas,Jerry