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### Topic: LM2917 tachometer calculation help.. (Read 5159 times)previous topic - next topic

#### Joy

##### Sep 13, 2011, 10:00 am
Here I attach the datasheet of the lLM2917
http://www.national.com/ds/LM/LM2907.pdf

I want pick the capacitive pulse with a ne555 like here http://www.sportdevices.com/rpm_readings/index.htm
and feed it to the LM2917..

now in my single cylinder bike there is one spark in every rotation..

so if the engine is running at 2000 rpm then what will be the frequency..??
2000 Hz or (2000/60 )Hz...??

#### dc42

#1
##### Sep 13, 2011, 12:24 pm
It will be 2000/60 Hz.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

#### robtillaart

#2
##### Sep 13, 2011, 07:32 pm

Be careful as this can be truncated by integer math

2000/60 = 166 Hz in integer math, but with proper rounding you get 167 Hz.  ALthough the difference is not much it may add up.

To get max precision you should measure over longer time, an unsigned long counter that counts the pulses; divide it by millis() and you get the average freq over the longer time.

Of course if you want a very reactive freq measurement you better restart counting every second.

Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

#### prairiemystic

#3
##### Sep 14, 2011, 04:01 am
:~  if your bike is a 4-stroke engine, you get a spark every 4 engine revolutions. If your bike is a 2-stroke engine, you get a spark every 2 engine revolutions.
2000RPM/60=33.3Hz crankshaft speed = 8.33Hz (4 stroke)  or 16.67Hz (2 stroke) ignition frequency.

#### Joy

#4
##### Sep 14, 2011, 05:00 am

:~  if your bike is a 4-stroke engine, you get a spark every 4 engine revolutions. If your bike is a 2-stroke engine, you get a spark every 2 engine revolutions.
2000RPM/60=33.3Hz crankshaft speed = 8.33Hz (4 stroke)  or 16.67Hz (2 stroke) ignition frequency.

I have a bike which has single cylinder... And there is one spark at every revolution.. Its a Royal Elfield bullet

#### Techone

#5
##### Sep 14, 2011, 05:49 am
@Joy

Thank for the link about to build and idea to get a signal from the spark plug wire.
I will try that setup using my car and a lawn mower. A very interresting project.

prairiemystic is correct that type of set-up to get "RPM" from a spark plug wire depend of the type of motor you have.

I check the circuit, it use a 555 to "clean" the pulse and send to a uController like the Arduino.  The 555 use R1 = 18 K and C1 = 100 nF  Time of the 555 = 1.8 mS pulse.  I just hope that the spark plug pulse from the sensor is shorter than the 555 pulse.

@ prairiemystic

Do you think Canadian Tire will carry those type of sensors ?  If not, I will no choise to build the "inductive" type.

#### dc42

#6
##### Sep 14, 2011, 09:39 am

:~  if your bike is a 4-stroke engine, you get a spark every 4 engine revolutions. If your bike is a 2-stroke engine, you get a spark every 2 engine revolutions.
2000RPM/60=33.3Hz crankshaft speed = 8.33Hz (4 stroke)  or 16.67Hz (2 stroke) ignition frequency.

Incorrect. If it is a 4-stroke engine, you get 1 spark in each cylinder per 2 revolutions. if it is a 2-stroke engine, you get 1 spark in each cylinder every revolution.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

#### Techone

#7
##### Sep 14, 2011, 04:33 pm
@dc42

My mistake. 2 revolution per spike ignition.  http://www.oldengineshed.com/forcycthry.html

So dc42, you are correct. Sorry prairiemystic....  I still whant to know how can I get that sensor ( capacitive or inductive ) from Canadian Tire  or UAP in Toronto.

#### dc42

#8
##### Sep 14, 2011, 04:39 pm

I still whant to know how can I get that sensor ( capacitive or inductive ) from Canadian Tire  or UAP in Toronto.

Why not either order it from the USA, or build it yourself?
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

#### Techone

#9
##### Sep 14, 2011, 07:45 pm
@dc42

I will go to Canadian Tire, here in Toronto ( they have a few of them here ), ask for that type of sensor, if not available, well I will build an inductive type ( a circle ferrite - a donut -( not from Tim Horton  XD )  big enough to fit the spark plug cable , I try to find an open type ferrite( like a current meter )) , or a radio type ferrite, loop some enamel wires ( from discard TV yoke ), maybe 50 turn to 100 turn and a circuit to "pick-up" the signal and send to a 555 monostable ( to clean the signal ) and send to an Arduino to count and display RPM.  Experiment is a must in this case. And I maybe do a program on a another Arduino to simulated a pulse - like an engine, sending through a opto-coupler, the receiver part connect to the other Arduino with the counter program to test the software.

That will be my setup.

#### prairiemystic

#10
##### Sep 14, 2011, 08:06 pm
Sorry I got confused with single-coil dual-winding/two cylinder bike engines, they fire on exhaust stroke. The Royal Enfield is a 4-stroke some models with dual-sparkplug, so I figure 2 revs/spark.

2000RPM/60=33.3Hz crankshaft speed/2 = 16.67Hz ignition frequency.
At 800RPM =6.67 Hz, at 8,000 RPM = 133.3Hz
At 800RPM =150msec, at 8,000 RPM = 15msec between pulses

I would use a timer on the Arduino to measure time between ignition pulses and convert to RPM.
You are using the LM2917 to drive an analog meter? See page 10 on app. note http://www.national.com/an/AN/AN-162.pdf

A capacitive ignition sensor can be made with a couple turns of wire wrapped around the spark-plug wire and then into the 555 circuit. I have never found a source for inductive pickups. You can make one with a split ferrite core (like EMI snap-on cores).

#### dc42

#11
##### Sep 14, 2011, 08:19 pm
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

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