The A3144 output is open-collector, so it needs a pull-up resistor to read HIGH. It pulls LOW when the right magnet pole is near.
What the message means
An open-collector output can only pull the line down. Something must pull it up when the sensor is off.
Fix it in this order
- Connect a pull-up resistor of about 10 kohm from the output to the supply, or use INPUT_PULLUP.
- Connect power and ground to the right legs. Check the datasheet pin order for your part.
- Turn the magnet over. The sensor responds to one pole facing it.
- Read the state: it is LOW when the magnet is detected.
- Keep the magnet close enough, at a few millimetres.
- For speed counting use an interrupt on the falling edge.
If you see this, do this
| You see | Do this |
|---|---|
| Always HIGH | Wrong magnet pole or too far. |
| Floating values | No pull-up. |
Related
pull-up input, encoder counts.
Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.