By Papa Bale · April 6, 2026
The traditional pulse motor trigger uses a bifilar-wound coil — a second strand wound alongside the energizer that picks up a voltage spike as rotor magnets sweep past. It's elegant, passive, and requires no external power. But there's another option that many builders prefer for its simplicity and tunability: the Hall effect sensor.
A Hall effect sensor is a tiny solid-state device that detects magnetic fields. When a magnet passes within a few millimeters, the sensor's output switches — giving you a clean, reliable trigger signal for your transistor circuit. No coil winding required. No trigger-strand resistance to worry about. Just a small sensor, three wires, and a pull-up resistor.
At the silicon level, a Hall effect sensor works by detecting the voltage produced when a magnetic field deflects current-carrying electrons sideways inside the chip. This Hall voltage is then amplified and compared to a threshold — the output goes low (or high, depending on type) when a magnetic field is detected.
For pulse motor use, you'll typically encounter two variants:
For most beginner pulse motor builds with all magnets facing the same direction, a unipolar sensor is perfect. The A3144 is cheap (~$1), runs on 4.5–24V, and is widely available.
The A3144 and most common Hall sensors come in a 3-pin TO-92 package (looks like a small transistor). Pin functions:
When no magnet is present, pin 3 is pulled high by the resistor. When a magnet is detected, pin 3 pulls low. This active-low signal goes into your transistor base circuit through a 470Ω base resistor. The transistor switches on, current flows through the energizer coil, and the rotor magnet gets a push.
Note on polarity: Some Hall sensor output stages are active-low (output goes LOW when magnet detected) while others use different logic. Check your datasheet. If your transistor is NPN and the sensor output goes low on detection, you may need an inverting stage — or simply use a PNP transistor driven by the sensor.
Position is everything. The Hall sensor's active face (the flat side on most TO-92 packages) must face the rotor magnets at a gap of 2–5mm. Mount it in a small bracket or hot-glued to the stator frame, roughly aligned with where the leading edge of each magnet enters the coil field.
Unlike a trigger coil whose position is fixed by the winding geometry, a Hall sensor can be slid circumferentially (around the rotor path) to advance or retard timing. This is one of the biggest advantages over bifilar trigger coils — you can tune the firing moment without rewinding anything.
Start with the sensor positioned so it triggers just as the leading face of a rotor magnet enters the coil's sweet spot. Power up at a low voltage (6V) and give the rotor a gentle spin. If it self-sustains, great. Now use a multimeter to watch current draw while sliding the sensor incrementally. Lower current for the same rotation speed = better efficiency.
There's no universal answer — both approaches have merit:
For a first build, the Hall sensor route is often easier — fewer winding variables mean less to debug. Once you understand how the circuit works, moving to a trigger coil gives you the full Bedini experience and opens up more advanced back-EMF recovery configurations.
Check that the sensor is detecting properly: wave a magnet past it and verify the output switches with a multimeter. Ensure the sensor output is properly inverted or matched to your transistor's base logic. Try advancing the sensor position by 5–10mm.
This usually means the coil gap is too wide, or the sensor is triggering too late. Move the sensor slightly upstream (in the direction opposite rotor travel) to fire the coil earlier.
The sensor may be firing when the rotor magnet is directly in front of the coil rather than entering it. Advance the trigger timing forward so the pulse begins while the magnet is still approaching.
Papa Bale's channel covers sensor-triggered and coil-triggered pulse motor builds side by side. Watch to see the practical differences in real time.