SS49E Linear Hall-Effect Field Meter
Read magnetic field strength as an analog voltage — wave a magnet over the SS49E and watch the Arduino plot the field on the Serial Plotter.
Difficulty: Breadboard. Estimated build time: about 30 minutes. Estimated parts cost: about US$4.80. 4-line bill of materials. Compare supplier offers when available. A pocket-sized magnetic field meter built around a single SS49E linear Hall-effect sensor.
Identify the SS49E pins
The SS49E is a TO-92 (three-leg through-hole) sensor. With the flat face toward you and the legs pointing down, the pins are: pin 1 (left) is VCC, pin 2 (middle) is GND, pin 3 (right) is the analog OUTPUT. This is the Honeywell pinout — some Allegro and Asahi Kasei look-alikes swap GND and OUT, so always check the datasheet for the exact part you have. Get this wrong and the chip survives but the output stays stuck at one rail.
Wire to the Arduino
Plug the SS49E into the breadboard. Connect pin 1 to Arduino 5 V, pin 2 to GND, and pin 3 to Arduino analog input A0. Drop a 100 nF ceramic capacitor between VCC and GND right at the sensor's legs — the Hall element is a tiny analog amplifier and it picks up supply noise as if it were field. With no decoupling the meter will jitter even with no magnet present.
Confirm the quiescent output
Upload a sketch that does `Serial.begin(115200); Serial.println(analogRead(A0));` in loop() with a 100 ms delay. Open the Serial Plotter and you should see the value sitting near 512 (the 10-bit ADC midpoint, corresponding to roughly 2.5 V which is Vcc/2). If it sits at 0 or 1023, the sensor is saturated — either a strong magnet is nearby, or VCC/GND are swapped. If it drifts wildly with no magnet, you skipped the decoupling capacitor.
Wave a magnet and watch the trace
Hold a small neodymium magnet near the flat face of the SS49E. Bring the north pole close and the trace climbs above 512; bring the south pole close and it drops below. The closer you get, the further from 512 the reading moves. With a 5 mm disk at about 1 cm, expect swings of 100–300 ADC counts. Two things to notice: (1) the sensor only responds to field perpendicular to the flat face — a magnet held to the side gives almost no signal; (2) flipping the magnet end-for-end flips the sign, which is why you would pick a linear Hall over a digital switch.
Convert ADC counts to gauss (optional)
If you want real engineering units, compute `gauss = (adcReading - 512) * (5000.0 / 1024.0) / 1.4` where 5000/1024 converts ADC counts to millivolts and 1.4 is the SS49E's typical sensitivity in mV per gauss from the datasheet. The number will be approximate — part-to-part variation is large and the chip isn't factory-calibrated — but it puts you in the right ballpark. For comparison: Earth's field is about 0.5 gauss, a fridge magnet is around 100 gauss, and a strong N52 neodymium can hit several thousand gauss right at the surface. Now you have a tool that can plot all three.