Self accelerating reed switch magnet spinner.

[Magluvin]

Dadhav did a circuit a few months ago that drives just about any kind of motor, and it triggers off of the drive coils. Im not sure this is similar yet.

http://www.youtube.com/watch?v=8b4xlCKn3LQ

Mags

[TinselKoala]

The diode adds a little "hill" of voltage drop, and seems to give a cleaner switching. I'm just going on intuition and visual observations, I haven't scoped anything yet. Since I'm using a single ended supply I didn't want the input voltage to the op amp to go below the zero rail. It might not be necessary.

The circuit switches as the magnet approaches TDC, and this is adjustable by spacing and the pot on the circuit.

I'm using the TL082CP version.

I've made another video showing the circuit switching a mosfet driving a car light bulb. I'm also using a different coil, a lot smaller and with no core.

Here's a neat thing: the coils I'm using have a tube in the middle where I can slide a core in and out. I can position the magnet in a fixed position wrt the coil, and the comparator will flip as I pull the core in and out of the coil. This means that linear pulse motors will be very easy to implement. I didn't show this in the video yet... a fellow's got to have some secrets!

http://youtu.be/RwRB1OPQZc0

[MileHigh]

TK:

I saw your second clip, cool.

For the diode, you can actually remove it and it will be better.  Then connect an upward pointing diode from ground to pin 3 and another upward pointing diode from pin 3 to +12 volts.  The far side of the coil looks like it's connected to a +6 volt biasing voltage with an 11 kohm output impedance.  So you have that voltage source going to the coil going to the comparator input.  That's exactly what you want because there is a danger that in a pulse motor, when the rotor is at full speed you won't know how much EMF will be generated.  It could be much higher than +12 volts and blow the op-amp input.  With the two diodes in place the output EMF of the coil will be clamped to +12.6 and -0.6 volts and thus protect the input of your op-amp.

For what it's worth:  Having the diode in series like that for a pure voltage signal is like putting part of the circuit into the Twilight Zone.  When the diode is not conducting current, it very much resembles a nearly open switch.  There is a semiconductor band gap and that more or less acts as a quasi open circuit with potentially a very high impedance.  The fact that an op-amp input is super high impedance kind of saves the day and even though the voltage signal from the pickup coil is in a kind of limbo.  It's in a way ironic because the diode needs current flow to "show" a reliable output voltage, but the op-amp input does not draw any input current, perhaps nanoamperes.  But the op-amp input is so sensitive that it works anyways, a kind of "double-cross."  Does that make sense to you?

Here's a neat thing: the coils I'm using have a tube in the middle where I can slide a core in and out. I can position the magnet in a fixed position wrt the coil, and the comparator will flip as I pull the core in and out of the coil.

That's somewhat similar to how an electric guitar pickup works!

MileHigh

[MileHigh]

TK:

Just one other comment that's pretty straightforward.  Leave the pickup coil untouched and then play with your 10-turn pot.  You will find the threshold point where the LED goes on.  Then just back up a bit from the threshold point, perhaps just 1/4 turn so that the LED is off.

If you do that then you will be set up for maximum sensitivity and it might be surprisingly easy to make the LED flash on.

MileHigh

[TinselKoala]

Success!

And both the sense coil and the drive coil are _coreless_.

For some strange reason I had to put a capacitor across the drain-source of the mosfet to get it to switch properly. But there it is.

Video coming shortly.

@MH: thanks! But I got it running great before I saw your last two posts. Good common practice right out of the cookbook on the diodes you suggest, maybe I'll try it if this chip doesn't hold up. Since the sense coil is not being switched abruptly I'm not too worried about its spikes, but caution is sometimes good. Still... it works great.

http://youtu.be/C9GCunnhQ_4