Hall Effect Questions

bdickens

Need some electronics help. I am building a tachometer using the hall effect sensor from parallax (Melexis 90217 Hall-Effect Sensor) and the magnet (3/8 Diameter x 1/8 Thick Magnet).

First, does anyone have a simple circuit for using these ? I'm finding some pretty complex ones and all I need is 1 pulse per revolution. The one on the data sheet has some unusual components (at least according to digi-key)

I see that a fair number indicate that I need two magnets. Is that true ?

Finally, how do I identify the "south" side of the magnet ?

I'm replacing my reed switch which was dirt simple.

Thanks

kwinn

The Melexis 90217 Hall-Effect Sensor is meant for sensing teeth on gears, and it does so by sensing changes in the flux density of the magnet that is placed on the flat back of the chip. The magnetic flux increases when a tooth is close to the face of the chip and decreases in the valley between teeth. Where and how do you plan to mount the magnet and sensor (a diagram or picture would help) ?

The data sheet shows circuits for minimum protection and automotive use. These are fairly standard components. The output signal levels will be approximately the same as the power supply.

You can determine which side is the south pole by suspending the magnet on a thread and allowing it to align with the earths magnetic field. If you have a magnet that has north and south poles marked you can use that. Opposite poles attract.

Roger Lee

I made a project this past week using the Melexis 90217.
BTW: I'm using the propeller.

Take a look at page 5 of the DATA SHEET.
data sheet used is 3901090217 Rev. 009 Jun/04
from parallax hall effects page.

Vdd to 5 Volt
R1 5K ohm
C1 10nF (103 ceramic disk cap.)

10K ohm resistor between output of sensor and input pin of Propeller.

Every time the magnet passes within @3/4 inch. pin goes high.
So far I've run it up to 850 RPM, works great

Roger

bdickens

Kwinn : Forehead slap. Suspending the magnet was pretty obvious. Sorry for what is an amazingly dumb question.

Because it was replacing a reed switch and a counter, it really is just a magnet, embedded in a fiberglass/ply disk that spun over the switch. But at higher speeds (it gets to 3600rpm) the reed switch was overmatched.

I was having some trouble navigating mouser but found the components. So I should be set once those come in.

Roger: Thanks. That was a big help. I am also using a propeller but it's through the 1-wire counter. But the rest was dead on. Thanks

Prophead100

North South is easy to figure out. Either (a) use another marked magnet or (b) Use the "feedback" option on the counter to energize an LED each time the sensor is activated. You set bit 26 on the CTR register to %1 with an edge detect and bits 14..9 to define the output pin. You will see the sensor "stick" on in a test mode if you install it backwards. If it is correct then it will just flash.

As an FYI, I attached some code that runs with the motorminder object (modified as noted above) to convert the sensor into RPM including low speeds where you need to track long periods between pulses.