Hey all i am trying to make a tach that i can read the cars RPM's. I am only looking for the chip to see what the RPM's are and not display it out to the user.I've draw up how i think it should be hooked up but I'm still now 100% sure.
I am using the LM2907 chip and tapping into the tach wire from the ECU of the car. I'm looking at it as if the RPM's are 200+ then the car is started. Anything lower then the car must be off. I'm checking because i will be remote starting the car and need to know when the stop cranking it.
To measure frequency, count the number of ignition pulses in a fixed time. It's all done in the programme on the microcontroller.
Warning no.1:- At low frequencies like 200 rpm, with say 2 ignition pulses per revolution, you are only looking at 6.666 Hz. To measure to 5% accuracy you need to count 20 pulses which takes 3 seconds, so the response time will be too slow.
The alternative is to measure the time between pulses and take the inverse of that. That gives you the frequencies much faster.
Warning no. 2:- You need some way of debouncing the signals or you could get noise being detected as ignition pulses.
I'm just going after the nicest way to get what the rpm's are. It doesn't have to be 200. I just wanted to tell if the car was started... be that the cars idle at 200,300,400,...1000. I would guess that any number above 50 would be considered started as 0 would indicate that the engine is not running.
I'm just going after the nicest way to get what the rpm's are. It doesn't have to be 200. I just wanted to tell if the car was started... be that the cars idle at 200,300,400,...1000. I would guess that any number above 50 would be considered started as 0 would indicate that the engine is not running.
David
You could count time between pulses or count pulses per given time. If you only want a trip-point, you can connect the timers reset to the pulses. Let the timer run (if it can run slow enough). If the timer overflows, your motor is revolving too slow. This method is actually the same as a watchdog circuit as i mentioned in another post.
Cos he's probably not making a tacho, maybe just a car bomb. If he was making a tacho with a microcontroller he would not need a silly "engine is running" detector.
Why not use an old tried and true method to see if the car is running? Like battery voltage has risen above 13.5v or test if the exhaust manifold is hot.
Sorry for the delay.. it never sent me a notice that someone posted something on the forum...
blueroomelectronics: I do not think the ATMEGA168 chip can convert frequency to voltage?
crutschow: So what help do you need? I need to be able to bring the "RUN" down to 0v when the car has started. Just not its about .40 to .12 and it bounces up and down here and there.
What's 203K? That is the resistors in series. 100k + 100k + 3k. It took the place of the R5 on the schema.
Are the voltages what you expect? Sure. It's taking the constant 5v on R5 and once the engine starts running then it should go down. But like i stated, it needs to be 0v instead of hovering above it (or less than 1v)
Can you post a schematic? yes. **broken link removed**
Chippie: It doesn't *exactly* have to be 0v and i dont think it ever could. Just want to get as close to it as possible. In other words, lower than 1v constant.