Papa's Pulse Motor Gets a Transistor Upgrade Sensor Coil and Hits 700 RPMs

Hook

I finally got my pulse motor running smooth with a transistor upgrade sensor coil instead of the reed switches I’ve been fighting for a week. This 4-magnet rotor with three coils in series is quiet, simple, and hit right around 700 RPMs at 9 volts.

Watch this experiment

I spin it up by hand, dial the voltage, and you can hear how quiet it stays because there’s no mechanical trigger clicking. We watch the RPMs climb, test the transistor, and see what happens when we push it to 25 volts.

What you'll learn

You’ll see exactly how the sensor coil pulses the transistor base, why grounding matters, the coil series order, and what the motor does at different voltages. Plus I talk about the printed rotor grip and why I’m sticking with this setup for now.

From the bench

We’ve got a four magnet rotor on a four coil spread. One coil is the timing coil — I call it the sensor coil because that’s what it’s doing. The other three are the drive coils: 20 gauge, 16 gauge, and another 16 gauge. They’re oriented counterclockwise, the direction the rotor spins. I tried both orientations but this one felt sweet.

The whole thing is running off a simple transistor circuit. No generating coils, no pickup coils this time — just making it spin.

How it works / what we changed

Power comes in the black clip, goes through the first coil, yellow clip to the second coil, red clip to the third coil, then blue clip to the transistor collector. The sensor coil hooks to the base of the transistor and is also grounded. When the magnets pass the sensor coil it triggers the base, the transistor opens, and current flows collector to emitter to ground. That’s the whole loop.

I’ve been messing with reed switches a lot lately and this transistor upgrade sensor coil feels way cleaner — no mechanical noise at all. It’s basically acting like a virtual Hall effect sensor.

Upgrade sensor coil and

The upgrade sensor coil and transistor setup replaced my reed switch completely. It pulses the base silently and the motor runs so quiet you can barely hear it. At 9 volts it fires right up and stays stable. I checked the transistor at 5 volts — no bouncing, steady at 5.38 V — so we left it there.

Coil and coil series

I wired the coil and coil series so power flows through all three drive coils before hitting the collector. 20 gauge first, then two 16 gauge coils. The sensor coil is separate and only drives the base. This series arrangement is what lets the single transistor switch the whole load.

Coil series magnet rotor

The coil series magnet rotor combo is a four magnet rotor on a four coil spread. The series drive coils are oriented with the spin direction (counterclockwise) and the timing coil sits where it can read the passing magnets cleanly. I tried the mirror-image zigzag orientation too but stuck with this one for the test.

Series magnet rotor 700

With the series magnet rotor 700 target in mind we watched the RPMs climb. At 9 volts it was already doing well and I called out 700 RPMs as the realistic number once it settled. We never quite needed the full 36 volts I first mentioned — 700 was right in the sweet spot before amps started climbing too high.

Magnet rotor 700 rpms

The magnet rotor 700 rpms is what we actually saw once it leveled out. Super quiet, no trigger noise, just smooth spinning. At 25 volts it tried to go faster but the amps shot up and the transistor started losing its grip, so we backed down and stayed at 9 volts for the long run.

Rotor 700 rpms

Once the rotor 700 rpms settled in I was happy. The PLA printed hub has a little rubbery grip from the iron-infused filament and magnetic flux, but the 100 % aluminum rotor I printed feels smoother. Still, this one runs fine at 700 RPMs and the transistor upgrade sensor coil keeps everything stable.

Sensor coil

The sensor coil is the external timing coil that replaces any physical trigger. It sits there and pulses every time a magnet passes, feeding the transistor base. I call it a sensor coil because that’s exactly what it’s doing — virtually working as a Hall effect sensor. No clicking, no bounce, just clean pulses.

Builder checklist

• 4-magnet rotor on printed hub (PLA or aluminum)
• Three drive coils: one 20ga, two 16ga in series
• One separate sensor coil for base trigger
• Transistor wired collector to last coil, emitter to ground
• Sensor coil to base + ground
• Power supply adjustable 9 V to 25 V
• Multimeter to watch voltage and current
• Check transistor at 5 V for stable operation

Troubleshooting

If it won’t spin, make sure the sensor coil is grounded — it simply won’t work without that ground. If amps climb fast and it shuts down, the transistor is losing grip; back off the voltage. Rotor grip from PLA can slow things down — try the aluminum version if it feels too rubbery.

Safety

Keep an eye on current when you go above 9 volts. At 25 volts the amps rose quickly and the transistor got hot. Always test the transistor separately before running hard. Don’t leave it unattended at higher voltages.

FAQ

Why is the sensor coil grounded?

In this setup it just doesn’t work without the ground. Maybe an AC version wouldn’t need it but we’re staying simple.

Can I add diodes and lights like an SG?

Sure, I’ve got the parts lying around, but I didn’t hook any up this time. It runs fine without them.

Will iron-infused PLA rotors cause problems?

It adds some magnetic grip and the plastic feels rubbery. Conductivity is probably reduced too. I’m thinking about printing a pure wire-shape test later.

Why switch from reed switches?

I’ve been using them for a week and it was getting boring. The transistor upgrade sensor coil is quieter and feels more solid.

Related on this site

• How to Build a Pulse Motor: Parts, Coil Gap, First Spin
• Pulse Motor Rotor Design: Magnets, Materials and Balance
• Which Transistor for Your Pulse Motor? (2N3055, TIP31, and More)
• Using a Hall Effect Sensor in Your Pulse Motor Build
• How to Maximize RPM on Your Pulse Motor

Watch this experiment on YouTube

• ▶ Watch full on YouTube