Measure car engine speed RPM via the cigarette lighter----make money online

hey everyone check it out here's the

newest addition to my lab I'm extremely

grateful to Tektronix for sending me

this oscilloscope and I plan to make a

bunch of videos showing off what you can

do with it so to start I thought we'd do

something kind of simple for an

oscilloscope but it does make use of

some of the features there and I learned

something interesting along the way I

recently had my car smogged

and the attendant measured engine rpm by

putting a plug into the cigarette

lighter so I thought I'd try it myself

and see how reliable that signal is so

to set up the scope I put it into the

default mode just so you can see

everything that I'm going to change here

we're going to make some assumptions

about what kind of a signal that we're

going to get out of the cigarette plug

the idea is that when the ignition coil

fires in the car that's going to produce

a little spike an inductive spike in the

waveform and by measuring those spikes

we can figure out how often the ignition

coil is firing and how fast the motor is

turning so I'm going to turn this down

and we can make a few more assumptions

knowing a bit about the car itself so if

it's a six-cylinder car and the engine

is turning about 750 rpm at idle then

it's and it's a four-stroke engine so

it's going to so each cylinder is going

to fire once every other engine rotation

and any of six cylinders so that's three

firings per revolution 750 rpm divided

by 16 let's just say about 600 it's

about 10 revolutions per second or a

little bit more times 3 gives us about

30 Hertz so at idle we would expect to

see these inductive spikes at

approximately 30 Hertz so just to get us

in the right ballpark I'm going to set

up the scope on the bench here and then

we'll take it out to the car and I'm

going to put my finger on the probe just

to get a sort of a dirty looking 60

Hertz

signal this is just picking up power

line noise in the lab here so I'm going

to turn off persistence and I'm going to

set the trigger level here and push the

signal up here so one sort of the first

attempt if we

had a nice clean periodic signal like

this what we could do is just measure

the period of course and that would give

us engine rpm however if the signal

we're going to get from the car is much

more unclear the inductive spikes are

very noisy I mean very ragged and

there's lots of other things that are

superimposed on top so to make our job a

little bit easier I'm going to use an

FFT so I select the math function and

then FFT and I'm going to make the time

base fairly long so that we can get good

resolution down at the bottom of the FFT

we're interested in signals around 30

Hertz or there abouts and so we need a

long time acquisition here to get those

low frequencies one thing I like about

the tech that I haven't seen on many

other scopes is the numeric entry so

down here we can change the center point

and spatial resolution or frequency

resolution of the FFT and you could use

the knob to do this but if you already

know what you want like we already

already know that I want maybe about 50

Hertz for the frequency per division and

if we want to change the centre points

since we're kind of looking way off in

the middle of the FFT we could set the

centre point to be something like a

hundred Hertz and then we've already

dialed this thing in without dealing

with the wheels here I thought that was

a nice feature that doesn't show up too

often

so I'm going to move this down just to

make it a little easier to see and we

can see we've got a nice peak here so we

could measure this with a cursor we

could turn on cursors and if I hold down

the cursors button it gives me the the

menu here so I'll say bring on screen

and then use a this guy and it's telling

us the a cursor is right about here and

the a cursor is sixty point five Hertz

so that all makes sense however it would

be nice if we could track this peak and

so I came up with a little hack here

that I don't know if tech was really

thinking you'd be used this way but I

found it to work so if we turn cursors

off we can use this wave inspector

search function so I'll turn search on

turn search on and what we're going to

be looking for is a slope or an edge and

I'm actually going to select both rising

and falling edges and the source is

going to be the math function the FFT

that we just set up and then we can set

a threshold so it's giving us this red

line here and as I dial the threshold up

and down you can see what's happening

it's actually putting little markers up

here telling us that it's found in a

tent so I'll set it high so that we only

get the markers on this first big peak

and the cool function the interesting

thing about this we go back to the

search menu we can pull up a table of

all the events that the search is found

and this is real time so if I take my

finger off the probe after it processes

the next set of stuff there it goes away

and if I put my finger back the data

comes back and this makes sense so it's

showing us the rising and the falling

edge of the of that first peak so this

will work what with in theory if we go

to the car and plug this thing in we

will have a spike at the frequency

caused by the inductive kicks from the

ignition coil and then we can use this

automatic tracker to tell us what the

Freak

see is I was looking for a search type

that would tell me the center location

of this of this peak but I couldn't

figure that out and for this thing you

know we're not really looking for super

high accuracy stuff so I figured we

could just do rising and falling I

noticed that if we change the threshold

carefully enough you can actually get it

to just do a single event because

there's not enough resolution and so if

the peak stays at about the same

amplitude you can get in there with a

really high threshold and then if we

check the table it's only going to give

us one point even though it's finding

rising and falling edges and it's pretty

much right on the money

so to plug this into the car I just made

this super simple adapter it's just a

cigarette lighter going straight into

the scope and I used AC coupling so the

way I look at it is if you want to

manage your engine rpm you might as well

do it in style so I used my DeLorean

it's the test subject for this

experiment and plugged it into the

console mounted cigarette lighter and

collected a waveform that looked like

this so what I did here was collected a

long record length of data from the car

and stored it as reference 1 just so we

can look at it here in the shop and as

you can see there are inductive kicks in

the signal here and they're actually

quite big so if I r1 is actually 1 volt

per division and so you can see the

spike here is easily a volt it's

actually a little bit more but just from

inspecting it you can see there's

something strange going on here the

spikes are not evenly spaced which is a

bit of a surprise to me at first I

thought there must have been something

wrong with my setup I mean how could an

engine have uneven ignition timing but I

learned something unusual the engine in

a Dorian is a v6 but the spacing like

the angle between the cylinders is 90

degrees and due to manufacturing

difficulties they decided to not fix the

cam sure not fix the crankshaft so that

the cylinders would fire evenly so the

motor actually fires in an uneven way or

it has this kind of gala P sort of

loping pattern to

you can search I'll put some links in

the description there but you can see

that some v6 engines that are spaced 90

degrees

have this strange odd firing sequence

you can see here why this is the case so

in a v6 engine it's very common to have

two cylinders share the same journal on

the crankshaft and if they do this

the journals have to be spaced at 120

degrees so that they're even for

rotation but since the cylinders are

spaced at 90 degrees there's going to be

this strange sort of beat frequency

between 120 degrees of the shaft and the

90 degrees spacing of the cylinders you

can fix this by changing the point where

these pistons connect to the crankshaft

but if you don't fix it mechanically you

basically have to live with this kind of

uneven firing sequence so if we turn

cursors back on I will hold down the

cursors button again and change the

selected waveform to be r1 and if we

want to measure one of the short periods

put the a cursor there and the B cursor

here just to get kind of an approximate

reading we're kind of at about 20

milliseconds Delta and the longer

interval is more like 32 milliseconds

and then if we switch the cursors to the

FFT okay so the a cursor at this first

peak this is this is garbage that's too

low frequency we're at 38 Hertz for that

first peak and about 50 7.5 Hertz for

the second peak and what we're really

interested in is the average because we

don't care about we were trying to

ignore this kind of low key 'no stew the

engine so if we take the average of the

two periods that we have and divide by

three and multiply by 60 we get 689 rpm

which is pretty reasonable that's about

the idle speed of the car so back at the

car you can see that with the table

pulled up we can have

those two peaks highlighted for us and

we step on the gas pedal and raise the

engine rpm you can see that the table

updates in real-time and then we can

pull the new values out and in this case

it gets up to about 60 and 120 Hertz and

if we take the average of all that we

get about 1800 rpm so I thought that was

an interesting little experiment if you

wanted to implement this in a

microcontroller it would be very easy to

AC couple this signal and feed it into a

comparator and basically just trigger

every time you have one of these high

spikes in this case all the other this

is like alternator noise and other noise

from the electronics on the car the

engine control electronics but those

ignition spikes are very high compared

pretty much to everything else so it'd

be easy to pick them out with a

comparator you could trigger an

interrupt and just measure the time

interval although interestingly enough

since some engines are odd fire and

you'd have to take into account the P

interval varies every other time so you

would have to take an average over eight

samples or something like that okay see

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