tach frequency to voltage

The first picture just shows how to hook up an LCD screen.  The second picture shows (and tells I believe) how to hook up a hall effect sensor to monitor RPM - this is basically what you want.  A hall effect sensor will pulse once per revolution so  its the same theory.  Now, his code looks like he is sending the RPM out to an LCD, you'll need to modify that slightly, but to what extent is hard to say since I'm not familiar with the platform.

I don't consider 0.17v voltage.  Anything less then about .5 volts can be considered 0.  Once you get closer to 0.7 you'll get in to a range that is triggerable as a voltage by some electronics.

Just to step in here for a moment.. perhaps not for the vehicle in question, but depending on the injection system being used, the 1:2 ratio injector pulse to RPM isn't an absolute.. I'm thinking of the older GM TBI injection systems, or 'batch fired' injection vs. true sequential. Also, the duty cycle on injection varies greatly compared to ignition pulse train.

Mentioned only for others who might be considering something like this, or just using an RPM signal for something else.. not to discourage, but to suggest verifying actual signals for your specific vehicle.

Jim

In "most" batch injection systems you will still have one pulse per fuel injector per 2 complete motor revolutions - the only difference between a batch injector signal and a true sequential injector signal is the timing (and length of signal as batch injectors open longer to make up for the ineffiencies of firing at the "wrong" time) compared to ignition.  You will have multiple injectors firing at the same time, but they will still only fire once per every two motor revolutions.

The duty cycle of the fuel injector only comes in to play under one circumstance - 100%.  Since we are measuring total pulse width (from one positive to negative transistion to the next positive to negative transition) the length of time that the pulse is positive or negative is completely irrelevent to engine RPM.  However, at 100% duty cycle there will be no positive to negative transistion (the injector will just stay negative for multiple motor cycles and make the controller "think" the engine isn't running anymore.  No fuel injection system should ever run the injectors at 100% but I've seen it done on OEM cars (like the Saturn Ion Redline) and I see it happen on modified cars all the time when people don't want to spend the money to upgrade their fuel system correctly.  But, you are correcft, ignition systems maintain a very low duty cycle, even at higher RPM while the duty cycle of fuel injectors can increase exponentially with RPM.

The duty cycle, for this applicaiton, is completely irrelevent due to the fact he is only monitoring the tach signal while cranking.  Now, if he wanted to continuously monitor tach so that the start/stop button will also stop the car but only if the motor is running it may be necesarry to consider the 100% duty cycle issue - but I doubt anyone would ever try to push the start/stop button at WOT and high RPM!

 I would respectfully disagree.. my experience has been (generally stock production stuff) that batch fire triggers every revolution.

"Injectors are typically fired in either a batch or individually in a sequential order. Batch fire is when all the injectors fire simultaneously at one time for each revolution of the crankshaft. This means each injector offers a wider response of fuel-flowing capacity since it is fired twice for every four-stroke engine cycle. A sequential EFI system fires each injector only once based on the firing order of the engine.s are typically fired in either a batch or individually in a sequential order. Batch fire is when all the injectors fire simultaneously at one time for each revolution of the crankshaft. This means each injector offers a wider response of fuel-flowing capacity since it is fired twice for every four-stroke engine cycle. A sequential EFI system fires each injector only once based on the firing order of the engine."

http://www.chevyhiperformance.com/techarticles/90778_efi_swap_part_2/index.html

"< color=#ff0000>"Most early EFI systems were batch-fire systems where the ECM fired all eight injectors simultaneously. Usually batch-fire systems fire the injectors once per engine revolution. This way, the injectors could be sized small enough to be more easily controlled at idle. Later, sequential EFI systems were refined to fire an injector a few degrees before the intake valve opened. Generally, sequential injection offers more precise fuel control at the price of increased complexity. But on production engines, the benefits are more in the area of emissions and driveability than in performance"

(quote from GM performance)

 Additionally, I'm reading that some stuff (production and aftermarket) can be set up to fire off multiple times during each rev. And to further muddy the waters, some sequential systems go to batch fire in the higher RPM ranges.

 Also 0%.. which would be seen in 'Clear Flood' mode and closed throttle deceleration. CTD 0% probably happens every time the car is driven on the road. This is easily seen with a scan tool..

 It's not so much that the ignition system is a low duty cycle, more that it's a stable one. Using an injection trigger, at any given RPM you could have multiple duty cycles.. depending on engine temp, throttle position, etc. If the actual transition event is being read it's one thing, but if it's a voltage reading being measured (because at any given RPM, apparent voltage would increase with an increase of duty cycle) then changes due to cold enrichment etc would result in an incorrect reading.

 It looks like he(she) is using some sort of PLC or ucontroller.. if you're gonna have an input, might as well use it for all you can, right? It's just a matter of writing some code. Using an injector signal would allow you to drive a light that brightens with throttle (somewhat like the old Nissan 300ZX digital dash did). Ignition signal would let you program shift lights, multiple stage nitrous activation, overrev protection, etc. Road speed could allow you to program movable body panels like the Boxster does with the spoiler.

 consider this.. seeing some of the widebody conversions I see running around.. set up a controller with brake light switch input (digital), road speed input (analog) and override switch (digital).

If OVERRIDE is 'OFF' , BRAKELIGHT is 'ON', ROADSPEED is > 30mph - 'deploy airbrake panels'

if OVERRIDE is 'OFF' BRAKELIGHT is 'ON' , ROADSPEED is <15 - 'retract airbrake panels'

 Or do what this guy did with his Christmas lights..

http://www.snopes.com/photos/arts/xmaslights.asp

 Regards..

 Jim

Like I said, in "most" systems batch injection is still only once per motor revolution.  That's not to say that there weren't a few systems between 1982-1985 that did fire once per crank revolution, I just feel that they are not the "norm" and that the majority of batch firing were at least twice per full motor revolution (a form of batch firing called bank firing).  By the mid 80s the technology was more then capable of at least bank firing.

That all being said, when I laid out my calculations I showed how the end number was achieved and anyone should be able to follow that guide and calculate the correct value for their specific car.

There are distinct reasons though that I recommended a fuel injector over an ignition source.  I have never seen (or even heard of until you posted) a sequential combustion engine that fires the fuel injector more then one time per crank revolution but I have seen cars that fire the spark plugs multiple times per crank revolution - this is used to decrease emissions by making sure all fuel is burnt before being exhausted.  Ignition systems, generally, are much noisier as well, making it hard to distinguish between a spark event and noise in the system.  Fuel injectors give you a wide window to work with where you can debounce the turn on by 10-20mS where a spark event generally doesn't last long enough to do much verification on.  Lastly, ignition systems can vary greatly from one car to the next, but generally all vehicles use the same fuel injector principal (tie one end of a solenoid to 12vdc and pulse the other side with a (-) output from an ECU).  This gives predictability from one vehicle to the next.  The last thing I have to consider is the fact that sequential (and batch) systems always fire at about the same time (x amount of degrees after TDC generally).  Spark events differ with timing and if you have a knock event on a late model car you going to see extreme retarding of timing which will cause at least two cycles to report erronious RPM values (the first cylce of the retardation and the last where timing goes back to normal). 

On my TT G35 coupe I'm using a fuel injector signal to trigger a shift light and to trigger a two step module that can hold RPM to the exact value programmed.  I had once tried to hook my shift light up to the ignition output but found it too noisy to be compatible with the hardware/software I was using.  If you choose to monitor the ignition pulses much more hardware/software filtering will be necesarry.  On a similar note I just recently read on this forum that it is not recomended to use ignition triggers on the tach input of DEI remote starts - I guess they have had similar issues monitoring tach signals that way. 

If you want to get really technical neither the fuel injector nor the ignition is the "best" place to get RPM data - the cam or crank angle sensors would provide the absolute most accurate data as they are mechanically bound to the actual RPM of the motor.  The issue there though is that most crank and cam angle sensors don't report linear data so you have to decode the information - this makes it almost impossible to make "universal" interfaces for any RPM based product as all cars use slightly different pulses per revolution and timing flags.

I've honestly never worked on a batch injected car - the closest I came is when a friend and I were talking about converting his Caddy to sequintial from batch.  However, when I told him we would have to retune the entire fuel table he opted not to go through with it!

The hall effect sensor can't be seen in the picture - it is the sensor that measures actual RPM so it should be built in to the fan motor.  The hall effect sensor sends a signal back to the processing board through one of the wires.  I'm not that familiar with computer fans so I'm not exactly sure how they do it but I do know the hall effect sensor needs to be mounted by the rotational object that you are trying to measure RPM of.