Reflective photosensor - 2 inch range?

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MartinM57

Moderator
I want to make a cheap and tacky one-off device (PICAXE-based) to measure fan rpm - yes, it's a fan on a car, about 15" diameter, 13 blades (who designs a fan with 13 blades?), 200 to 4000 rpm...and no it doesn't have a speed directly proportional to engine rpm.

So I'm looking at (IR?) reflective photosensors, but the ones I find have a range of about 0.2" - but I'd rather have more like 2" so as not to lose my fingers/the device in the whirling blades.

Any ideas (or alternative solutions)?
 
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Dippy

Moderator
Go on Martin you could make one.

A narrow-beam IRLED and fast Ir-photo-tr or photo-di in a binocular housing.
Sell that 'scope and buy a Bridgeport, then you could knock one up in 10 minutes.

I won't mention lasers or else every schoolboy on the planet will go blind.
Oh, too late.
http://www.farnell.com/datasheets/323568.pdf
 
D

Dippy

Moderator
A Myford what? Oh not one of those awful mills they did in the 90's made you know where and badged Myford ? :) (I think the brochure used to say "Pacific rim"). Just kidding, they're great... but not as good as Bridgeport.

That's a coincidence; that's the same type of photodiode I used in my old Reflective-IR people counter.
Chuck it into an op-amp and bingo.

As I don't know the physical setup I can't comment on design, but don't forget to protect diode from direct IR.

I must admit, I'm surprised you can't get a ready made Photo-switch type thing with a 2 inch range.
I haven't looked hard as that Ec's job.
 
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MartinM57

Moderator
My mistake - I'd forgot that Bridgeport only made mills - I have a Myford 7 lathe (old and very British - you'd approve :D).

I can find a parametric search that includes range.

I don't have an idea on physical setup either - probably cardboard tubes and hot-melt glue as it's a cheap'n'tacky job (well, it will be tacky if the hot-melt glue gets too near the engine).
 
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SAborn

Senior Member
Maybe not the answer you are looking for but have you had a look on fleabay for a hand held optical taco out of cheap land China.

They really are not that dear (about the price of 2 picaxes) and would be handy for lathe rpm to.
I know a few others who have bought them and find them very handy.
 
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Dippy

Moderator
What about the imbalance due to a weighty magnet?
What make of glue will you use?
When it flies off at 8000RPM and goes through the radiator will you smile?

Optical tachos are very practical.;)
If you are measuring the blades how do you tell the Chinese tacho how many blades you have? :)
I thought all those tachos were really for pointing at your shaft.
 
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SAborn

Senior Member
The tacos i have used have some reflective stickers you place on a single blade or the shaft etc and read the sticker passing the taco.

If not read every blade and divide the rpm by 13.

The other way is place a colour sticker on a blade and adjust the frequency of a flashing light till the colour sticker becomes stationary or moved backwards then read the frequency.
This method dont work so well if the speed is always ranging up and down.
 
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Dippy

Moderator
Oh, a real cheapy then. Well, if it works...

Slight aside:
1. If Martin needs the real-time data for some other procedure can he get the RPM data out of one those cheap tachos? I.e. is there an interface?
2. What's the refresh/update rate?

Oh, the old strobe method, yeah, not really for this app I suspect.
 
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Jeremy Leach

Senior Member
How close together do a hall effect sensor and its magnet typically have to be?
Click to expand...
I don't know :). Good point about the imbalance due to weight of magnet, I was just throwing it in as an alternative.

I wonder how the motor is driven? Insert low value series resistor and measure volatge drop?
 
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Dippy

Moderator
Oh Ec! Well, they DO make you blind - post 2. :rolleyes:
I won't mention the L word either, but it is the easiest.
 
E

eclectic

Moderator
Dippy said:
Oh Ec! Well, they DO make you blind - post 2. :rolleyes:
I won't mention the L word either, but it is the easiest.
Click to expand...
Sorry for my causing confusion.
The post was in line with Martin's original request:
"I want to make a cheap and tacky one-off device "

Sticky - tacky?
(Leaves the stage, booed off :))

e
 
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MartinM57

Moderator
Don't go EC, I need you to search stuff for me :D

The unit won't be "in the car" - it's a diagnostic tool used once in a blue moon in the garage and maybe passed around like-minded adults (oo-er) for fault finding.

I was going to put two magnets on if using hall effect, but with 13 blades that wouldn't balance :(

It's not driven by a (electric) motor - it's driven from the engine by a viscous coupling that slips as the revs get higher, and ultimately limits the fan rpm. At low revs the coupling doesn't slip at all/only slips a bit. The concern is detecting the two failure modes - goes really loose and the fan speed is always low or tends to lock up and keeps fan at engine rpm all the time. Static tests (spinning the fan by hand) gives an indication of the state of the coupling but doesn't really tell you how it reacts at engine speeds....

But on the other hand, I might just splash half the cheque I got from ERNIE on Friday on http://cgi.ebay.co.uk/Laser-optical-tachometer-test-motor-small-UK-/290415464815
 
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Andrew Cowan

Senior Member
I've got one of these. I've used it for lots of different measurements, and it is very easy to use (inludes MAX, MIN, last readings etc). It uses a 5mm LED with a lens, then some sort of photodetector setup. I haven't taken it apart to look inside yet.

The accuracy seems good - it agrees perfectly with the digital tachometer on my lathe. (Oh, and my lathe is a top quality brand - a Big Dog).

A
 
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Dippy

Moderator
Oh no not laser again.

There is a small chance that this tacho may slip out of your hand and being ChCh electronics the laser might switch on. The beam may be refracted through a raindrop, bounce off a bald man's head and go into the eye of an innocent child at home.
The child, now permanently blind, may stagger, slip and grab a mains cable. The cable pulls the plug out of the wall socket and cause the father to trip over it, and roll out of the front door . The momentum is such that he rolls across the lawn and gets run over by his wife just turing into the drive. She is unable to stop as she is driving a Tyota.

Thus we have serious injury caused by laser, mains electricity and the car all in the space of 30 seconds.

Oh Martin, how could you promote such irresponsible activity? :rolleyes:
 
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Andrew Cowan

Senior Member
Good news Dippy!

I bet that the one Martin linked to is the same as mine - it is a bright LED focussed to a 5 degree beam angle. The LASER term is simply overenthusiastic advertising.

That's the good thing about ChCh products - it might say they are dangerous, but they're not :D.

A
 
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Dippy

Moderator
Oh that IS good news. I can stop clenching now.

"overenthusiastic" - love it.
The next time I get caught telling lies I'll just say "Sorry, your Lordship, I was just being overenthusiastic!". ;)

The last time I was in court I told the Judge I was a firework - so he let me off.
 
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MartinM57

Moderator
Based on Andrew's personal recommendation I'm going to splash the cash on his link - just don't tell the current Mrs MartinM57 that I've been in her handbag for the extra funds...

...I'll report back (in 11-23 days - the quoted delivery time - good job I'm not in a rush....;)
 
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mega

New Member
Dippy said:
Oh no not laser again.

There is a small chance that this tacho may slip out of your hand and being ChCh electronics the laser might switch on. The beam may be refracted through a raindrop, bounce off a bald man's head and go into the eye of an innocent child at home.
The child, now permanently blind, may stagger, slip and grab a mains cable. The cable pulls the plug out of the wall socket and cause the father to trip over it, and roll out of the front door . The momentum is such that he rolls across the lawn and gets run over by his wife just turing into the drive. She is unable to stop as she is driving a Tyota.

Thus we have serious injury caused by laser, mains electricity and the car all in the space of 30 seconds.

Oh Martin, how could you promote such irresponsible activity? :rolleyes:
Click to expand...


Ahhh, lovely good old slapstick British humour, reminds me of "The Plank" and other Eric Sykes stuff, don't stop there Dippy, you may have missed your true calling in life, although I would have expanded on the TOYota by calling it a "LandBarge" (4wd usually driven by a "Soccer Mum" texting on her phone whilst driving at the same time).
 
techElder

techElder

Well-known member
Martin, you may have to paint a bit of white on one of the blades or do what I've done and put some of the 'aluminized' (that's AL - U - MIN - IZED) tape on one of the blades. Perhaps some reflective mylar tape would also work.
 
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Andrew Cowan

Senior Member
I bought a reel of self adhesive aluninium tape when I got my tacho - stick it on something black, and it gives a strong reading.

A
 
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Dippy

Moderator
Job done then.


Megareg, we get exactly the same where we live.
The same Mums , that make sure all their paper is recycled, ignore the school bus facility and waste gallons of fuel driving some spotty micro-dot to school.
They usually drive like morons, tailgating everyone because they are late and then clutter up the streets with the fumes from diesel and acne cream.

I didn't spell the name correctly on purpose. I didn't want to imply a duff manufacturer ;). As you know T*y*a have had bad press recently because of quite a few faulty items including brakes. That is always what happens when accountants have designers by the b@lls.

Purely coincidentally there used to be a TV Ad campaign in the UK ending with "The Car in Front is a Toyota" - so true, mainly because the throttle was stuck and the brakes didn't work and it couldn't slow down.
Naturally that just a nasty rumour... (Twilight Zone music..)
 
techElder

techElder

Well-known member
Now, Dippy, you know those problems were caused by users not understanding the range of the control systems in their cars.

PS. I was snickering over those folks buying a "Tiotah" car with a 'fly-by-wire' throttle setup. That is until I looked at my Chevrolet truck ... same thing. I guess we're mostly all in the same boat.
 
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mega

New Member
Thanks Dippy, yes, it seems that there is not much difference between us, do you also have the spotty youth with a baseball cap driving some small Japanese sporty car with wings all over it and..... the exhaust pipe with a diameter of around 150mm or so???? I usually think of them as having a very important piece of their body inversly proportional to the diameter of the exhaust.....

I also hear what you're saying about T*y*ta, Have heard about the re-calls, and I have two T*y*ta's, a 2009 Aurion and 2000 v6 C*mry, (I won't have a 4wd, I don't live in the country and I can drive!), but no problems with them at all.

Hmmm... sorry about the thread hi-jack, but I just love a good laugh and I cracked up when I read Dippy's post... don't stop that humour Dippy.

I'll get back to my lurking now, always good to read the friendly banter here, not to forget the pearls of wisdom too.

Cheers
Reg
 
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Dippy

Moderator
Yes, our home-grown sebum factories usually have BB caps and/or hoodies.
Tatty old Beamers and Subarus are their favourite- obviously the 3000 (Chinese) Watt (5W RMS) ICE is mandatory as this is used as the mating call.
And the low-level foglights are obligatory.
I take it that Jobseekers Allowance is quite significant?

I am in the process of designing some auto servo driven mounts for UZIs on the front of my car to counter the insurgent foglights. An optional mount for an Ingram is available at extra cost.
 
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SAborn

Senior Member
mainly because the throttle was stuck and the brakes didn't work and it couldn't slow down.
Click to expand...
Now that excuse could get you out of a lot of speeding fines if you own a toy-oh-ta.

Yes i too enjoyed the giggles thus far of Dippys warped view of life.
 
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Piers

New Member
Texasclodhopper said:
PS. I was snickering over those folks buying a "Tiotah" car with a 'fly-by-wire' throttle setup. That is until I looked at my Chevrolet truck ... same thing. I guess we're mostly all in the same boat.
Click to expand...
Pretty much now, doesn't matter what it is, automotive, commercial, agricultural, construction plant... I used to be a John Deere tech, some of the tractors had eight or ten computers on them, depending on exact spec, all linked by canbus, all looking after their little bit of the tractor.

Piers
 
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Dippy

Moderator
Yup, if there's a task just stick in a micro. If it doesn't improve efficiency at least it'll impress the Great Unwashed.;)

I love the way that people have to talk digital-bull about everything.
A chap in my local pub has a Citroen and the electric windows have stopped working.
Apparently they aren't "broken" , they have "gone offline". It's pathetic :rolleyes:

My car A/C (Aircon , not Andrew Cowan) was playing up this summer.
If I placed a polystyrene cup over the top-of-dash temp sensor it was fine.
I took it to the garage but "Computer Says" it's OK.
How do I explain the software writer is a moron and get it changed to remove the polystyrene cup sub procedure?
 
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MartinM57

Moderator
MartinM57 said:
Based on Andrew's personal recommendation I'm going to splash the cash on his link - just don't tell the current Mrs MartinM57 that I've been in her handbag for the extra funds...

...I'll report back (in 11-23 days - the quoted delivery time - good job I'm not in a rush....;)
Click to expand...
China has delivered....

Says it's a laser (3mW 65-670nm it says on the label) and I can see the dot about 30m away on the fence in the garden - guess I won't be pointing it at the sky (or burglars) then. Not checked if it really is one (not sure how to TBH) or just a bright LED...

Does it work - yeah, really well. Here's a pic, perfectly focussed on my garage floor :rolleyes:, of it pointing straight at the fan blades (no reflective tape - three 8 inch strips provided with it - needed) at idle. 11566 rpm with 13 blades = 889rpm on the engine. Rev counter showed 900'ish.

So it's in the ballpark.

Is it traceable to international rotating things standards? Nope
Is it VFM? At about £12 delivered IIRC, yes
Will it last? Who knows
Is it cheap'n'tacky? Not really. It's fit for purpose and a lot easier than making, that's for sure
 

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Dippy

Moderator
I don't think you'd get an LED to be a little bright dot at 30m.

Oh do stop panicking, why worry about scum burglars?

And if you can aim it (hand held) into a burglar's pupil for long enough at 30m to do permananent damage I will happily do your prison sentence for you.;)

Ballpark?
I thought the idea was to have an accurate standard measuring device?:confused::)
 
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