49E linear Hall sensor
The 49E linear Hall sensor (Honeywell SS49E), a TO-92 whose output follows the magnetic field. Aliases: SS49E, KY-035, analog Hall sensor.
The A3144 is a switch: it trips on a south pole and ignores a north one. The 49E is its analog sibling. With no field its output sits at half the supply; a south pole at the printed face raises it and a north pole lowers it, in proportion to the field. It is what a gaussmeter, a contactless throttle or a magnet position sensor is built on.
Pins
Printed face toward you, left to right:
| Pin | What it does |
|---|---|
VCC |
Supply, 2.7 V to 6.5 V. |
GND |
Ground. |
OUT |
Half the supply with no field, 1.4 mV a gauss on 5 V. Read it with analogRead. |
While it runs
The slider is how far the magnet is from the printed face, 0 to 40 mm; at the far end there is no magnet. Click the magnet to turn it around. The readout on the body is the field at the sensor in gauss, with S or N for the pole.
What the model gets right
Null and sensitivity. Honeywell's sheet gives the null as 2.50 V typical at 0 G on 5 V and the sensitivity as 1.4 mV/G typical. Both are ratiometric, so on 3.3 V the null is 1.65 V and the slope 0.92 mV/G.
Both poles. South raises the output and north lowers it by the same amount.
The span. The output stops 0.95 V from either rail (the sheet's typical), which on 5 V is 0.95 V to 4.05 V, about 1100 G each way. A neodymium magnet against the face pins it at a rail.
The magnet. The same 10 mm by 3 mm N35 disc as the A3144, with the textbook field on its axis: about 230 G at 10 mm and 90 G at 15 mm. On 5 V the output reaches a rail closer than about 5.5 mm. The useful range of a 49E is a centimeter or two.
What it does not model
The null and sensitivity drifting with temperature, the sheet's spread between parts (null 2.25 V to 2.75 V, sensitivity 1.0 to 1.75 mV/G), noise, and fields that are not along the axis. The output's 100 ohm is an assumption; the sheet gives none.
Common mistakes
Expecting a digital output. digitalRead sees half the supply as whatever the pin
happens to call it. Use analogRead.
Assuming the null is exactly 512. The sheet allows 2.25 V to 2.75 V, so a real meter zeroes itself with no magnet near, as the gaussmeter example does.
See it in action
Pocket gaussmeter shows the field and the pole on a 16x2 LCD. Open it at /templates.