Misbehaving Hall sensor A1102EUAT = UGN3140

Hi everyone,

I've bought a hall-effect sensor (switch in this case as there is a small op-amp encased) and can't seem to get it working! But even weirder, it's working (partly) but when I reverse it's +5V and GND...

Let me explain a bit more, the sensor is this one : http://www.selectronic.fr/capteur-a1102euat-ugn3140.html I fed it with a +5V from the arduino, it's friend GND and I put a 10K pull-up resistor between it's output and +5V. All good so far but... when I probe it's output, it stays firmly around 4,4-4,5V and doesn't react to any magnet I can find.

Now, out of a mistake (I'm that bad), I put it's Vcc to the GND and GND to +5V and, surprise, it gives me a steady -4,5V BUT when I get close to a magnet, it switches to +5V!! So well, it's not exactly the behaviour I'd like to see, and not even the right polarity but it works in reverse... How come!!!????

And how can I get it to work right???

Thanks for the help!


For the UGN3140 : With the flat side towards you, so you see the text on it: the left pin is 5V and right pin is output. For the A1102EUA-T : With the flat side at the back ! The left pin is 5v, the right pin is the output. Are you telling me that you made the wrong connections by looking at the wrong datasheet ?

When you have the output connected to 5V or a reverse voltage, the sensor might be broken. I think 50% chance that the sensor is broken. The way you describe it is okay. Resistor of 10k to +5V and output to arduino input, or measure it with a multimeter.

Hi Caltoa,

Looking at both datasheets, I don't see any differences in the pins. Anyway, both sensors were manufactured by the same company and are given to be interchangeable so I don't see them making this kind of practical joke. :)

Also they give it a +-30V rating so it shouldn't be dead...

Well well, that leaves us where we were...

Can you give links to the datasheets that you were looking at ?

The UGN3140 is with the text to the front : http://www.radiomuseum.org/tubes/tube_ugn3140.html The A1102EUA-T is with the flat side on the back : http://www.sumzi.com/en/articles/16/6439.html

And yes, if you connect the output to 5V, it can be broken.

I think the sensor was not misbehaving, neither was I ( http://en.wikipedia.org/wiki/Ain%27t_Misbehavin%27_%28song%29 )

Well you're right!

But the mystery continues... Actually I was looking at the datasheet that the distributor (selectronic, french electronic shop) put on it's site. And now that I look at it, it points towards the UGN 3141-3144...

Thing is, on my sensor, it only says A(for allegro) 02E and on the back (engraved but almost invisible) 1212 800J. According to the datasheet it must then be a component which P/N ends in 02... should be A1102 EUAT then?

So, to sum it up, I was reading the "wrong" datasheet (UGN 3141 to 3142) which in the end seems to give the same pinout than the A1102EUAT that I have... In the end, no error on the pinouts (or so it seems), but what could be the problem?

Thanks for the help!


What are markings on the device ? Did you notice that the datasheet for the 3140 says the supply voltage is -35 to +28Vdc ? Have you considered putting a silicon diode with the cathode connected to the output and the anode to GND ?

Can you make a photo of it ? I would like to see it. Did you try both the North and the South with a magnet ? Do you have some more of them, at least a few that you have not used yet ? If everyone was connected wrong, they all might be broken.

If everyone was connected wrong, they all might be broken.

Very unlikely. In 30 yrs working with electronics , I've only encountered one or two defective new (straight out of the package) semi-conductor devices . From working in a semiconductor fab , I know the chips (or dies) are tested extensively before they ever cut into individual dies for packaging.

Nooooooo, I ment that if the open-collector output of the hall sensor was connected to the 5V, and the chip has enough power for a base current, the open-collector output could get too much current and the hall sensor would be broken.

With "they all", I ment the used ones.

Hi everyone,

I'll make a picture of it in a couple of hours when I get home.

Meanwhile, I just wanted to say, I don't think I fried the collector since it was connected to an arduino digital input which sinks not more than 20mA IIRC... And with a 10K pull-up but that makes a ridiculous current...


Here they are!


Looking at the second photo, IMG_2653.JPG, that is the front. If the pins are downward, the first pin on the left would be pin number "1" and is 5V.

Can you confirm that ?

Hi Caltoa!

Yes I do confirm that!

If you connect it to 5V (you can use a resistor of 180 ohm to 5V to protect it), and GND, and pullup resistor of 1k or 4k7 out the output, and use a multimeter, it must work.

Perhaps it is not correctly soldered ? Bad soldering can do crazy things. Or do you use breadboard ? those have sometimes bad contacts.

I have no idea what could be wrong, it should work the first time. Some components are fake, but I have not heard of fake hall switches yet.