Hall effect sensor breakout: SI7201-B-06 sample rate insufficient for fast magnet detection

TECHNICAL SUPPORT Sensors & Modules
1

Customer:

I purchased a Hall effect sensor breakout with digital output and connected it with a 30K pull-up resistor and 5 volt power. I placed a magnet on a 3" diameter disc attached to a DC motor running at about 30 RPM, producing a pulse approximately every 1/2 second. On an oscilloscope, the output pulled low as the magnet passed the sensor for about 400mS as expected. However, detection is inconsistent: sometimes the sensor fails to detect the magnet, sometimes the low pulse is only about 200mS. The pattern is random - the sensor might detect 10 times, skip one, detect 16 times, skip one, or skip several detections before detecting once or twice. Moving the magnet distance from 4mm to 20mm made no difference. At 25mm the sensor almost never detects, but I plan to operate at 4-6mm. Wire runs are only about 1" long. I’m working on an ESD mat with a grounded wrist strap. Why is detection random?

Originally submitted via email support on 7 February 2026. Migrated to the forum for community visibility.

2

Soldered Support:

Could you provide information about the magnet and its magnetic field strength? This will help determine if it exceeds the threshold of the onboard SI7201-B-06 sensor.

3

Customer:

I don’t have detailed information about the magnet - it was just a paper holder for testing. When stationary, the SI7201 sensor detects the magnet from 4-20mm.

I found the problem: operator error. The SI7201 sensor has a sample rate of 5 Hz. For reading turntable rotation speed at 33 or 45 RPM, the 5mm magnet passes the sensor in 0.7 to 0.94mS every 1300-1800mS. That’s too fast for the SI7201 to detect reliably. The fact that the sensor detects it at all is remarkable. I need a sensor with a much higher sample rate.

4

Soldered Support:

Thank you for the detailed troubleshooting. The thorough approach you took beforehand is exactly what makes solving these issues straightforward. Best of luck with your project!