Re: Hall Effect Sensors


"Chris" <cfoley1064@xxxxxxxxx> wrote in message
news:1176166121.015647.105980@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Apr 9, 6:32 pm, "Michael"
<QmQiQcQhQaQeQlQhQiQm...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
"Chris" <cfoley1...@xxxxxxxxx> wrote in message

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On Apr 8, 6:27 pm, "Michael"
<QmQiQcQhQaQeQlQhQiQm...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
"Chris" <cfoley1...@xxxxxxxxx> wrote in message

news:1176073372.369685.194980@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

On Apr 8, 9:36 am, "Michael"
<QmQiQcQhQaQeQlQhQiQm...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Hi,

I'd like to interface Hall Effect sensor to a uC and have picked
out
the
A1301EUA-T
(datasheet:http://www.allegromicro.com/en/Products/Part_Numbers/1301/1301.pdf)
with no
particular reason - just seems easy to get hold off.

Now based on the graph at the bottom left on page 5, am I right in
thinking:

Output from HES fed through a non-inverting amp. with gain of 250
would
produce a voltage of 625mV at 1G when the supply voltage is 5.0V?
It
just
seems a bit too easy (I'm guessing changes in temperature etc are
going
to
have a significant effect but right now just trying to get hold of
the
basics.)

If I told you the application was going to be a motor tachometer -
is
it
'better' to feed the (amplified) voltage through a comparator and
so
convert
the analog signal into digital signal (when above a theshold go
high,
below
or at theshold go low type setup) or feed it into a ADC pin and
work
out
the
digital signal using software? I'm worried that if I tied that to a
ADC
pin
then it could go over the pin's max voltage (5V) so would I need
something
along the lines of a zener diode to dump any 'extra' voltage over
say
4.5V
to be on the safe side?

I've only just started learning electronics so don't know if the
circuit's
above are 'suitable' and so would appreciate any help. ( It's not a
homework
assignment :-) )

Apologies if that's a few too many questions.

Thanks in advance,

Michael

Hi, Michael. In fact, it's not so simple, and unfortunately, I
think
you've taken a wrong turn here.

The nominal output of the sensor you're looking at is around 1/2 Vcc
(around 2.5V for a 5V supply). Now apart from any drift caused by
variations in supply voltage, you'll have to apply the sensor output
voltage to an op amp to subtract out the 1/2Vcc.

The data sheet says this quiescent output voltage is stable over
time
and temp, but considering you're talking about such a small sensing
voltage to begin with, you are heading for problems here.

The good news is that there are hall effect sensors with digital
outputs that react to the presence of a magnet quite nicely, and
will
have fast enough reaction times for a tachometer. You can forget
the
analog stuff, hang a pullup resistor at the output of the sensor,
and
just count the digital pulses. This is how it's done.

If you'd like further information or need help in sensor selection,
please post back with more info.

Cheers
Chris

Hi,

That sounds perfect - any keywords I should be using?

Michael- Hide quoted text -

- Show quoted text -

Hi, Michael. Since you've found a UK distributor for Allegro, I'd
suggest just going to the Allegro Unipolar Hall-Effect Digital
Switches page, and taking your pick. As I said, just have a magnet go
past the IC, use a 1K pullup, and just read the digital output with
your uC. If it's a very fast application and you don't have a
hardware counter on board, you might want to use a flip-flop to make a
divide-by-2 counter, then you can just see every transistion as 1
revolution.

http://www.allegromicro.com/en/Products/Categories/Sensors/unipolar.asp

You could do worse than choose one of the A1101-1104 and A1106 family:

http://www.allegromicro.com/en/Products/Part_Numbers/1101/1101.pdf

For all you want to know about digital unipolar hall effect sensors,
look at the appnote listed on p. 8, the Hall Effect IC Applications
Guide:

http://www.allegromicro.com/en/Products/Design/an/an27701.pdf

Cheers
Chris

Thanks Chris, that's perfect.

Is there an IC with a single flip-flop on or are they abit like socks?

Michael- Hide quoted text -

- Show quoted text -

Hi, Michael. For a bit more cost, you can get a bipolar hall effect
sensor, that will turn on in the presence of the south end of a
magnet, and remain on until in the presence of a north. If you can
arrange two magnets of opposite polarity at 0 degrees and 180 degrees,
you'll have the whole solution.

http://www.allegromicro.com/en/Products/Categories/Sensors/bipolar.asp

Cheers
Chris


Hi,

Yeah I noticed those in the app notes I think they called them latching hall
effect switches.

I take it these also need a pullup resistor?

Now I guess I have to choose one....

Cheers,

Michael


.

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