Pairing a 9 G TMR sensor with an 8 mm × 5 mm ceramic cylinder reaches a 22 mm activation gap at roughly 20% of the cost of an NdFeB N42 baseline, while the same NdFeB part in a smaller diameter still hits the baseline 23 mm gap at about 35% of the baseline magnet cost [S3]. For reference, 30–40 G is a common activation threshold for general-purpose Hall effect sensors, which is why the higher-sensitivity part opens the door to weaker, cheaper magnet grades [S3].
Sizing therefore breaks into two coupled decisions: which sensing element (Hall, TMR, reed, variable reluctance) and which magnet (NdFeB, SmCo, AlNiCo, ceramic) [S1][S2]. Treat them as a system, not two line items, because the magnet typically dominates both the bill of materials and the field strength the sensor actually sees [S3].
Operating Principle and Air-Gap Geometry
AS5000-series magnetic position ICs use integrated lateral Hall elements in standard CMOS, sensitive only to the field component vertical to the IC surface, with the Hall array on a typ. 2.2 mm diameter circle for on-axis rotation [S1]. On-axis angle measurement needs a diametrically magnetized magnet aligned on the rotation axis, while linear and off-axis angle work uses a multipole strip or ring whose pole-pair length matches the Hall array spacing [S1].
For pure proximity (no position decoding), the field falls off steeply with distance, so air gap is the single biggest sizing variable. A cylindrical magnet 8 mm in diameter and 5 mm thick paired with a 30 G Hall sensor activates at 23 mm; dropping to a 9 G TMR extends usable margin or allows weaker magnet material [S3]. Always size against the worst-case gap including housing wall, target deflection, and thermal drift of the magnet, not the nominal CAD dimension.
Sensor Family Comparison on Decision Criteria
Reed switches are passive, glass-sealed, and work without supply, but mechanical contact fatigue starts to bite above roughly 100 million cycles and vibration can cause contact jitter [S4]. Hall effect sensors are solid-state, need a stable 10–30 V DC supply, and can be disturbed by strong external fields from large motors or welders [S4]. TMR and other magnetoresistive parts push activation thresholds down into the single-digit Gauss range, which is the lever that lets you swap NdFeB for ceramic or shrink the magnet diameter [S3]. Variable reluctance sensors need the target itself to be ferromagnetic and moving, and output a speed-proportional sine rather than a clean digital level [S4].
Quick decision table (criteria: cycle life, supply, external field immunity, output type): Reed, under 100 M cycles, no supply needed, moderate, dry contact; Hall, effectively unlimited, 10–30 V DC, vulnerable near large motors/welders, NPN/PNP digital; TMR, effectively unlimited, per datasheet, better immunity with shielded packages, digital/analog; Variable reluctance, high but target-limited, none, good, analog sine [S3][S4].
Selection Criteria: Eight Questions to Lock Down
Start by defining the target event: passage or part presence, liquid level, or a safety/lock function, because the same sensor family will not serve all three [S2]. Then constrain the mechanical envelope: screw, PCB-mount, or tubular form factor, since the magnet has to fit on the moving part as well [S2].
Pick the contact logic: NO (form A) when the output must pull low in the active state, NC (form B) when the fail-safe path is the de-energized state [S2]. Confirm the electrical interface, switching voltage and current, leakage budget, and whether the downstream stage needs PNP, NPN, push-pull, or a 4–20 mA / IO-Link analog output [S4]. Finally, define environment: oil mist, metal dust, washdown, temperature range, and any nearby motors, welding gear, or soft-iron brackets that will distort the field [S4].
Magnet Material Trade-Offs
NdFeB (notably N42) is the strongest commercial grade and the default for tight gaps, but it is also the most expensive and corrodes without plating [S3]. SmCo holds up at higher temperatures and in mildly corrosive atmospheres at a price premium over NdFeB, and is the usual pick when the sensor sits near a hot motor or hydraulic block. AlNiCo gives a stable field with positive temperature coefficient, useful in older instrumentation, but is much weaker per unit volume.
Ceramic (ferrite) is roughly an order of magnitude weaker than NdFeB, but it is also a fraction of the cost and chemically inert, which is why a 9 G TMR plus ceramic cylinder can match a 30 G Hall plus NdFeB on activation distance while saving around 80% on magnet cost [S3]. Magnet cost scales with physical volume, magnetic strength, and material composition, so the cheapest way to a given gap is almost always a smaller, weaker magnet plus a more sensitive sensor, not the other way round [S3].
Where Each Family Fits and Where It Should Be Rejected
Specify reed switches for cylinder piston position, door and window alarms, liquid level sensing, treadmill speed, and elevator floor positioning, where the load is light, the cycle count is moderate, and the wiring is passive [S4]. Specify Hall effect parts for motor speed and direction, gear tooth detection, robotic joint feedback, and high-speed counting, where vibration, shock, and millions of cycles per shift rule out mechanical contacts [S4].
Reject reed switches in any application that crosses 100 million cycles, in high-vibration fixtures, or where deterministic bounce-free switching is required, and switch to a Hall or TMR part instead [S4]. Reject Hall sensors when the only available supply is below 10 V DC or when the sensor cannot be kept clear of strong external fields, and look at variable reluctance or a shielded magnetoresistive package. For absolute rotary position with sub-degree accuracy, an on-axis diametric magnet with a lateral-Hall AS5000-class IC is the textbook topology, while a multipole ring or strip is the right call when the shaft end is already taken by a brake, clutch, or joint [S1].
Verification, Standards, and Sourcing Signals
Verify the sizing at three points: magnet vendor curves for B(r) at the operating temperature, sensor datasheet minimum and maximum operate/release thresholds including hysteresis, and a bench test at the worst-case mechanical stack-up. For industrial panels, watch for the same CE/UL and RoHS/REACH conformity you would require on any control component, plus IP67 or IP69K ratings for washdown or outdoor enclosures. [S3]
Trackable signals to watch over the next sourcing cycle: vendors publishing matched magnet+sensor reference designs at sub-15 mm gaps, and IO-Link or IO-Link Wireless magnetic sensor launches that move the wiring decision off the traditional PNP/NPN binary. For a related read on a different non-contact family, see capacitive sensor sizing guidance; for the magnetic-supplier landscape, see capacitive sensor suppliers and manufacturers; and for adjacent industrial instrumentation see DC electronic loads for oil and gas.
The underlying component specifications are covered under magnetic sensor, linear guide, and crossed roller guide.