A water meter's job description reads like a sensor designer's nightmare: run for ten years on a battery you cannot replace, wake on a magnetic field from any direction, ignore temperature swings, and cost less than the housing it lives in. Hall switches have historically been the answer, with compromises. The KTM1302 from CONNTEK (Kuntai Xin) uses TMR — tunnel magnetoresistance — technology to change the compromise, most visibly in the current budget: 160 nA at 3.0 V on its 50 Hz version.
The short answer
For battery-powered position and flow detection — water and gas meters, flow meters, valve position, electronic locks — the KTM1302 delivers omnipolar detection, nanowatt-class idle current, 1.8–5.5 V operation and an 8 kV HBM ESD rating in a SOT-23-3. The TMR core is what buys the sensitivity-per-nanoamp that Hall parts cannot match.
Verified key parameters
From the KTM1302 series datasheet (Kuntai Xin Microelectronics):
| Parameter | Value | Condition / Note |
|---|---|---|
| Sensing technology | TMR (tunnel magnetoresistance) + CMOS | Omnipolar |
| Idle current (50 Hz version) | 160 nA @ 3.0 V | |
| Idle current (1.6 kHz version) | 600 nA @ 3.0 V | |
| Continuous-work version | 1.9 µA @ 3.0 V | |
| Supply range | 1.8 – 5.5 V | |
| Threshold options (BOP/BRP) | ±45/±36, ±30/±21, ±18/±12, ±9/±6, ±7/±4 Gs | Five grades |
| Output | NMOS open-drain | |
| Package | SOT-23-3 (MSL1), TO-92S | |
| Temperature | −40 to +125 °C | |
| ESD | HBM 8 kV | |
| Compliance | RoHS |
Why 160 nA rewrites the battery maths
Consider a water meter with a lithium thionyl chloride cell — call it 1200 mAh usable. A Hall switch idling at 5 µA consumes 43.8 mAh per year; the meter's ten-year service life loses nearly 400 mAh to the switch alone, before the MCU wakes. The KTM1302 at 160 nA consumes 1.4 mAh per year — the switch disappears from the battery budget entirely, and the remaining capacity goes to the radio and the display, the loads that actually need it.
The three speed grades let you pay only for the response you use:
- 50 Hz / 160 nA — slow mechanical events: valve position, lid open/close, gear rotation in a meter register
- 1.6 kHz / 600 nA — moderate pulse counting: flow turbine pickup
- Continuous / 1.9 µA — fast event capture without duty-cycling
Omnipolar means simpler mechanics
A Hall latch or switch usually cares about pole orientation — south pole toward the face, or a bipolar latch pattern. In a meter, where a rotating magnet spins through all orientations, omnipolar detection (responding to either pole) removes the mechanical design constraint of placing and orienting the magnet. The five threshold grades from ±7 to ±45 Gs then let you match the magnet's field at the worst-case air gap rather than over-specifying it — and higher thresholds trade sensitivity for noise immunity in a metal-rich environment.
The TMR advantage in one table
| Property | TMR (KTM1302) | Typical Hall switch |
|---|---|---|
| Idle current | 160 nA | 2–10 µA |
| Sensitivity per µA spent | High | Low |
| Supply range | 1.8–5.5 V | 2.5–5.5 V typical |
| ESD robustness | 8 kV HBM | 2–6 kV typical |
| Pole response | Omnipolar | Pole-dependent (unless omnipolar type) |
TMR's resistance change per unit field is roughly an order of magnitude beyond Hall's — that physics is where the current budget and the sensitivity both come from.
Application notes that matter
- Decoupling is mandatory — the datasheet requires a capacitor across supply and ground to filter supply noise; follow the reference schematic
- NMOS open-drain output — needs a pull-up to your logic rail; plays well with 1.8 V to 5 V MCUs
- Two packages — SOT-23-3 for SMT boards, TO-92S for leaded assemblies common in meter retrofits
- Threshold selection — start from the worst-case air gap in your mechanical drawing, add margin, pick the grade
Frequently asked questions
What is the difference between the 50 Hz, 1.6 kHz and continuous versions?
Sampling rate versus current. The 50 Hz version polls the magnetic field fifty times a second at 160 nA; the 1.6 kHz version catches faster events at 600 nA; the continuous version never samples-and-sleeps and runs at 1.9 µA. Choose by the fastest event your application must not miss.
Can it detect both magnet poles?
Yes — the KTM1302 is omnipolar. Either pole approaching the sensing face triggers the output, which simplifies magnet placement in rotating or sliding mechanisms.
Will it work directly with a 3.3 V or 5 V MCU?
Yes. The 1.8–5.5 V supply range and NMOS open-drain output make it rail-agnostic; with a pull-up to the MCU's IO voltage, it interfaces directly.
Which threshold grade for a water meter register magnet?
That depends on your air gap. As a design anchor: the ±18 Gs grade suits mid-gap placements in compact registers; the ±7 Gs grade reaches weaker fields at larger gaps but with less noise margin. Send us the mechanical drawing's magnet specification and gap and we will recommend the grade.
Sourcing note
The KTM1302 series is stocked through Mandu with the full CONNTEK datasheet and threshold-grade selection support. Send your application, magnet spec and annual volume for samples and pricing.