Hall Effect Sensors

Hi,

I'd like to interface Hall Effect sensor to a uC and have picked out the A1301EUA-T (datasheet:

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) 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

Reply to
Michael
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I wouldn't worry to much about going over the voltage of the ADC pin how every, I would be concerned about the response time of the ADC when using it as a tech (HI speed input).. you would be better to use a simple Op-amp with hysteresis, or I think the input's of the PIC already have some hysteresis in them, so you might be just fine. I would look into assigning an input to one of the counters in the micro. (High Speed COunter).

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Reply to
Jamie

(datasheet:

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with no

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

Reply to
Chris

(datasheet:

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Hi,

That sounds perfect - any keywords I should be using?

Michael

Reply to
Michael

(datasheet:

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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.

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

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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:

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Cheers Chris

Reply to
Chris

If it's to be a tachometer on a gasoline engine the best way is a 1M resistor from the switched terminal on the low voltage side of the spark coil.

If you want to use a hall sensor it may be easier to use one with a digital output eg: take the phase sensor from an old computer fan.

It's better than too few.

Bye. Jasen

Reply to
jasen

(datasheet:

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Thanks Chris, that's perfect.

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

Michael

Reply to
Michael

(datasheet:

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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.

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Cheers Chris

Reply to
Chris

(datasheet:

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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

Reply to
Michael

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