What is the hookup and operating characteristic of a "Hall effect
switch"? The ones I am looking at are Sprague # UGN3075LT and #SS41.

43 responses total.

One pin gets +5v, the other com and the third is hi or low, depending
on the sense and presance of a magnet, for a digital device.
The device No's you list are not in my Sprague catalog.  Prob. too
old.  Give Reptron a call at 1-800-521-4978.  They handle Sprague and
should be able to send you the info.  (Sprague: (603) 224-1961)

Thanks, Klaus! Is it high or low in the absence of B? The cited units
are offered (cheap) by All Electronics. I considering building a water
velocity meter, based on a propeller with a magnet, and a ripple
counter (and a stopwatch). What does it mean (in the description) that
with "suitable output pull up, can be used directly with bipolar.....
logic circuits". Wouldn't it drive the ripple counter directly?

I would guess that the device is digital and has an open collector
output.  In order to drive TTL logic with it, you need not only tie
the output of the hall device to your logic but also to a 1.2K or so
resistor connected to +5V.  This is commonly refered to as a "pull-up
resistor".  If you use CMOS logic, you can increase the resistors 
value by an order of magnitude, thereby reducing power consumption.
(10 or 12K, maybe more?)
(I have thought or doing a similar thing for my wife's rowwing shell.
Just connect your propeller/magnet setup to a bicycle odometer/speed-
ometer/etc.  They are cheap, about $20 - $30, and may well be compatable
with a hall device (most use a simple magnetic reed switch, wouldn't
have to add the pull-up ;-) and are fully programable so far as the
the No. of rev's per mile/kilometer/nautical mile/ etc.)

Propellers are more "complicated" than wheels. I have assumed that I
would have to calibrate the device because its rotation rate will not
be strictly linear with flow velocity. However the odometer is a 
idea to consider - provides a readout, and *that* could be calibrated
to velocity. In regard to the Hall device: I'm going to need a circuit
sketch. Could you refer me to a source for "TTL circuits for dummies"?

If you give the propeller a lot of pitch, I would think it's rotation
rate vs speed would become fairly linear.

TTL for dummies?  How about Lancaster's book "TTL Cook Book" or "CMOS
Cook Book"?  Jung also does a good job with instructional books.  See
your local Purchase radio, Wedon'thaveitmeyers or Borders.  Radio Shack
has also surprised me with the quality content of there books over the
years.

I chose the prop with the greatest pitch available - 2.0 - with the same
thought in mind. Another problem I will have is, how do I calibrate
the meter? I can see that maybe I bit off more than I can chew with this
project.....

Calibration? Try this model:
  
Make about 3 tests wherein you pour/pump/squeeze/whatever a known quantity
of liquid through the measuring-unit and time it. Also (if possible) use
the thingie to count the number of rotations.
  
In each of the tests, chanzge the amount of liquid by a large amount and at the
susing integer multiples of the original amount. See if there is a good fit
or not between the linerity of the time, counts, etc. Then, the easist thing
to do would be to make a paper chart with the "prop count number" and the 
"volume of stuff having passed."
 
Then, just for kicks-n-grins, flow a lot of stuff through it and capture the
liquid output, read the prop-counts, and see if the results make sense.
  
Oh, measure the captured output, natch ...
 
How crude/fine of a measure do you need? There will *always* be a mis-measure
of some magnitude. If it's within whatever tolerance you can live with, quit
playing and get back to work!  <g>

I chose a somewhat large propeller - about 2 inches in diameter - to
minimize bearing drag. Therefore I would have to pour an *awful lot*
of liquid. Also, it shouldn't be in a tube - the unit will be used
in open flow, and tube walls would change the reading. I'm sort of stuck
with either having a "flume", or towing it on a boat. Klaus, if this
thing gets built, would your wife like to help calibrate it (via her
shell)?

It also sounds as if you'll be wanting to measure "an *awful lot* of
liquid." Also, if there is going to be "slipppage" in the measurement,
("the unit will be used in open flow") then you're measurements will
NOT have a lot of relevance to that flow! For it to work with any
degree of accuracy, +all+ the liquid has to be directed through the
propeller blades, with as little as posible "slipping by," un-measured.

Rane, what are you trying to do?  Measure the velocity of a boat
through water, or the amount of water passing a given point?
If you are trying to measure the velocity of an object through
water, and have good connections to the U of M, try getting some
time on the towing tank in the engineering building.  Otherwise
you can build a device which has 2 pinch rollers driven by a variable
speed motor.  Put your prop assembly on the end of about 500 ft.
of string and place it way out in a quiet lake.  Use the pinch
rollers to reel in the string, at some constant velocity, while
recording the reve/min. of your prop.  You could also use something
else to reel in your prop.

If I were doing it, I'd build my own prop, with about a 6 in. pitch,
and simply calculate the velocity knowing that that the prop is making
one rev. for every 6 inches it travels.  At least I think this would be
fine for low speeds like a shell or sail boat.  I would guess that a 2
in. pitch would be too tight and that the rev's vs velocity would not
be linear over a wide range, though it would depend on the drag of the
rest of the prop.

I don't think my wife can row a steady enough pace.  How about a 
motor boat on a quiet lake?  Motor at a constant speed between two
known (distance-wise) points?

That 2.0 is the pitch ratio. The prop is 2 inches, so the pitch is 4
inches. I want the device to measure the flow rate of small streams.
I can measure the depth/width cross section, and a few velocity points.
Well, I do have good connections at UM (I work there), but hesitate
to use all that high tech for my low tech gadget - they would say,
why don't you just buy one, instead of spending all this time making
one? (And they would have a point: a mechanical one costs $230, though
props with electronic counters start at $1300.) I do like your calibration
system, Klaus - of course, need some long wires to start and stop the
counter - maybe add radio control? For calibrating with a boat - I
thought that the boat could glide between two marks, at different initial
velocities. A measurement of the distance and the time would give the
average velocity, and of course the revolutions and time give the average
velocity too. Well, I haven't given this up, yet. I have both a canoe
and a sailboat myself: this will be a stimulation to get them in the
water more often. 

Ok, so it's a whole lot of water, that changes things ........
  
I'd suggest strongly that the freely-wheeling propeller be mounted inside
a tube - I'm guessing that it will be operating as an airplane propeller
operates. And would ask whether or not you considered working a 
paddle wheel at 90 degrees to the water flow, either on the surface or
submerged (or both?) turning some other type of indicator/counting/generating
device? 
 
I don't know how large/small/wide/deep these streams are nor how
complex or $$ cheap your parameters are? Of course this whole thing
might be simply pure fun with science as well as having a practical
value for some research.

And there is the remaining question of accuracy. Do you have any sort
of opportunity to look at one or another ofthe commercial devices, maybe
even with their accuracy parameters, construction, etc.?
.

A shroud (tube) isn't used in the commercial systems. The mechanical
ones use *very* high pitch props (to overcome friction), while the
electronic ones have more normal pitches (I have a catalog in which I
can look at the designs). The paddle-wheel type is used in pipe flow
applications; of course, for making submerged measurements (the "standard"
is to measure at 60% of the total depth, as there is a vertical velocity
profile in a stream), a prop is needed. The commerical devices claim
1% accuracy, but of course they have been calibrated. I cannot tell
whether the electronic units use *nonlinear* calibrations. 

The purpose of the device is to monitor the resurgence flow from a
spring in an area known as the Fiborn Karst Preserve, owned and managed
by the Michigan Karst Conservancy, a non-profit corporation. I am the
chair of the science subcommittee of the preserve. Lots of different
science is being done there; I am working on hydrology and geochemistry.
Its part of a serious avocation, but also legitimate research. Well,
I'm still gathering parts. 

Wow - this is getting interesting - and "neat."
  
Maybe if you could arrange a demonstration you could ask
enough questions, and make enough observations to quasi-duplicate
one of the commercial devices. or is there some sort of "store"
that sells the flow units?

Whatever you do, it also sounds like a lot of real fun.

There are probably numerous suppliers. My catalog is from Forestry
Suppliers, Inc (Jackson, MS) who, incidentally, is a great source for
all types of professional outdoors technology (but not for lightweight
hiking and camping, etc). It is a lot of fun, though I expect the trip
this June will be overrun by black flies! 

Could you borrow one of these calibrated units from someone to calibrate
the one you build?   

That is a most sensible suggestion. I may know someone that has access
to one.

Ahhaa! Synthesis and synergy - love it!  Still need to haul a boat for a way,
but at least there will be an accurate comparison. Any ideas on how to
effect the testing/comparing?   
  
Oh - got one - use a sail boaT or a speed boat - since the velocities of
each measuring device through the water would be the same, the actual
rate of the boat travel through the water .... Won't Matter!  ??eureka??
 
Anybody got a speed boat for rcurl to play with?

You could also measure the rate at which a length of string is unreeled
from the boat to a fixed point on the shore.  Since your prop has a 4"
pitch, you could make a pinch roller with a 4" circumfrance, also set up
with a hall device.  If the prop behaves as an ideal one, there should
be a one to one corelation to the "tick" marks from both devices.

If you can't borrow one, have a sales rep. loan you one to "try out"
for a while ;-)

  <<<but it's more FUN to borrow a speed boat .... >>>

In sailing circles speed boats are called "stinkpots". Also, that's faster
than most streams flow (unless you go over the waterfall). 

I'm back on this project...amazing how time flies. This  time I am
looking *seriously* at Hall Effect switches...new ones from DigiKey
cost the same as "surplus", so I have my choice - but what do I need?
They come unipolar or bipolar, open collector or pull-up, and with
various high-low flux densities. Bipolar would need two magnets (possible...).
What I expect to drive is an "exercise computer" which uses just a contact
for input (it reports speed and distance - I'd calibrate as needed). 
The little RS neodymium magnets mentioned in another cf look ideal. Do
they have enough oomph 220 gauss unipolar HE? [I could use -120/+120
bipolars with two magnets if necessary.] Final question is....will one
of the HE switches itself act as a "contact", or would I need more
circuitry, or a relay? [A reed relay is a possibility if one of those
little magnets would operate it at a spacing of 5 mm or so.] 

Advice from those experienced in HE switches would be very much appreciated.

Hmmm...

The project I did with HE sensors was for controlling motors on a Leslie
rotary organ speaker.  I used unipolar sensors, since the bipolar sensors
require a *reversed* magnetic field to switch back to "off".  I used the Radio
Shack magnets since they were very light and therefore would not affect the
spin balance of the rotors.  I also tested optical sensors (decided that stage
lights might be a problem) and reed switches (too noisy for a recording
studio).  

The Hall Effect sensors have a 5V logic output which does not need to be
debounced.

<dave tiptoes quietly away from discussion in this foreign language>

I can put two, reversed, magnets on the propeller I use - that would balance
it too (not that balance matters at the speed that will occur), so could
use the more sensitive bipolar HEs. Scott, how far could you place the RS
magnets from your HEs? Also, can you give me the ID for those HEs? Any
practical information like this would help me. I guess I will have to
characterize the "tach" I propose to use, before I can figure out a driving
circuit.

OK, my dusty old notes say that I used a Microswitch SS44, a ~180 Gauss 
unipolar.  The Microswitch phone number is/was (815) 235-6600.  The 
current RS catalog lists the magnets at 2 for $1.49, no data on field 
strength.  I don't recall the exact range limits, but 5mm sounds safe.

Thanks, Scott - a data point!  The "exercise computer" I got ($5) for
measuring rpm runs on 3 volts. One sensor contact is on the negative side
of the 3 volt supply, and the other looks into *2 megohms* and +3 volts. I
ran it from a low impedance source by providing a 50% on square wave, 0 to
+3 volts (0 volts = closed contact and +3 volts = open contact). The
"computer" reads speed and distance (and time). At 100 Hz, the speed reads
12.1. It only reports one decimal place, so low frequencies are
inaccurate. The unit bogs down at 300 Hz and will not read higher
frequencies linearly. For a flow velocity of 1 fps, with a 2" prop with a
pitch ratio of 2, the speed is about 3 rps, or 3 Hz from an HET. For such
a low velocity I would use the distance scale. It changes 0.010 for 300
pulses, so would take some time to measure velocity to (say) 5% (600
pulses, or 200 secs). The "computer" is also sensitive to the signal
symmetry - 50% on gives the highest counts, and anything else give less
(but symmetrically - 40% on or 60% on give the same counts. I don't
understand how this works). Since it needs 50% on, it looks like I need a
*bipolar* HET with two magnets (or with 4, or 6, magnets, to increase the
counts).

The above "computer" is what I need to drive, so the HET circuit needs to
switch between 0 and +3 volts (or between a low resistance and ca. 5
megohms - which I don't think is practical). How would I design the
bipolar HET (OC or pullup) to do this? It might work with the 'low'
voltage higher than 0, but I have not determined how much higher it can
be. 

I suspect you can drive your counter with a HET directly.  I don't have
a data sheet in front of me, but I suspect a HET requires 5v for operation,
a common connection and an output connection.  Get one with an open collector
output.  An open collector output is like a NPN transistor with the emitter
tied to common, the base being driven by the magic inside the HET, and the
collector brought out for the output.  I assume that when you say that one
contact is on the negative side of the 3 volt supply, that it is tied to
the - side of the battery.  If so, this should become the common side of
your circuit.  I will also assume that the other contact is +3 volts
with respect to "common".  When the two contacts are shorted together, the
once +3 volt contact goes to 0 v with respect to common, right?  If so, the
unit has an internal pull-up resistor tied to +3 v.
 
This being the case, you can tie common, from your HET, to common of your
counter.  +5v (or whatever is needed to power the HET.  3 v from the 
counter if it'll go that low.) to the supply pin of the HET and the +3 v
contact from the counter to the open collector output from the HET.  The
transistor inside the HET will now act as  switch between the two contact
points.
 
Odd about the 50% duty cycle requirement.  All bike speedo's I've ever
seen have a small magnet on one spoke to sense a wheel revolution. What?
Perhaps a 5% duty cycle?

My function generator has an INV button, which inverts the signal. I
just press that, and the 0 level becomes +3 and the +3, 0. The %
duty cycle stays the same. At least I think so - I didn't put it on a
scope, but just timed it on a multimeter at 0.2 Hz. 

Well, I'll give that a try. Can a transistor really pull the collector to
common? Isn't there a *little* voltage? [I should learn to read transistor
characteristic curves - I used them a lot for *tubes*!]

A transistor will pull the collector to about .3 volts of the emitter.
The junction potential from base to emitter is about .7 volts.  .3 v
should be close enough to 0 for your circuit to consider it a 0.  The
best way to find out is to try it ;-)

Nothing ventured, nothing gained.....
I did a more accurate study of the calibration of the "exercise computer".
It has no brand name or model, and the only printed units on it is Kcal/Min.
for a bargraph (it is LCD). The distance display is x.xxx. The number
of pulses of a 50% square wave for one unit in the last place, 0.001, is
30.72 (averaged over tests at input frequencies from 5 to 640 Hz, where
it is very linear). The speed display is y.y, in distance/*hour*. Does
that number 30.72 (+/- .1) mean anything to anyone? I would have thought that
the distance display would be a direct counter output, but I don't se how
30.72 fits with that. 

Well, the mechinism of the exercise machine must have been such that
the counter received 30.72 pulses for every 0.001 units traveled.
 
The 50% duty cycle criteria is also strange.  How do you plan on achieving
that?

What's strange about 30.72 is how it can be accomplished. The LCD is
reporting in decimal, so somehow counts are manipulated to report a
decimal number with that funny ratio. 30.72 is close to centimeters/foot
(30.48). It is also exactly 1024(.03). The device does have a clock in it,
as the display will also report time. Could there be an actual computer in
there? I guess there must be, since it also displays speed (but not
exactly - the speeds calculated from distances are all one to three digits
in the last significant figure less than what is displayed - could there
be an analog component?). 

I get the 50% duty cycle by using a bipolar HES with symmetrically placed
magnets (with reversed polarity). I'm thinking of putting in 8 magnets to
increase the pulse frequency at the lowest flowrate (to increase ca. 3 Hz
up to 12 Hz), to increase accuracy. Getting them exactly symmetric to get
exactly 50% duty cycle might be difficult, but I intend to calibrate the
instrument with real flow, so it shouldn't matter. 

It has occurred to me that the device measures distance not by counting
pulses but by using the pulses to gate the clock and counting the clock
pulses. If somehow the gate was the shorter segment of the rectangular
pulse sequence for more or less than 50% duty cycle, it would account for
the decreasing speed/distance measured with any departure from a 50% duty
cycle at the same frequency.

OK: what's inside - an x'tal, a MC14017BCP chip, a cap, a resistor, and
something hidden under a blob of black glop with lots of traces to the LCD
display (the driver, I presume). 

Yep, your clock gating idea is very likely.  That way one can tailor the
linearity of the whole system... Interesting...  Yep, you can be sure
there is a little computer in there.  Most likely a 4 bit.

The MC14017 is a Motorola Decade Counter.  It has 10 outputs, clock, 
enable and reset inputs.  Good for divide by N counting.  The blob probably
covers the CPU and I'll bet the display drive function is handled by the
CPU.

The blob is not as high as the chip, round, and maybe ca. 7/16" diam. There
is no usual chip under there. 

Perhaps the 30.72 is a result of some horrible analog kludge, and the logic
is then calibrated to make a useful value?

Possible....the device comes from an exercise machine/treadmill/??, so
there are analog conversion of the mechanical and pickup components. However
I am an old hand at reverse engineering, and always look for meaning in
numbers and ratios (I was able to deduce that the speed is given in
distance/*hour*, although I originally calculated it as distance/second -
I noticed the "suspicious" ratio of 3600 to match the display). 

I picked up some RS rare-earth magnets. Says they have a field strength
of 10,000 gauss (!). That's nudge an HET. They are actually 0.20 inches
in diameter, although the catalog says 1/8 inch. I would have preferred
1/8 inch, but oh well....

Under the epoxy blob on your PC board is the die one usually finds in a
chip.  Common practice in many devices like that.  i.e. the PC board is
*part* of the chip.

So, without the epoxy, it would be a bare IC? How is it put on the board
and connected to the traces (which cldarly radiate from various sides of
it). What is the advantage of this?

- Backtalk version 1.3.30 - Copyright 1996-2006, Jan Wolter and Steve Weiss