Variable reluctance sensor

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Geoff James
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Variable reluctance sensor

Post by Geoff James »

Hi All , has anyone used a VR sensor for a pickup? If so----any pros and cons.
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Spuddo
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Mark Swanson
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Re: Variable reluctance sensor

Post by Mark Swanson »

Hello and welcome to the MIMForum. Sorry but we require the use of full names, first and last and don't allow "handles" or knicknames so please let us know your real name and we'll change that for you.
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Alan Peterson
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Re: Variable reluctance sensor

Post by Alan Peterson »

A VR sensor and a guitar pickup are pretty much the same thing; a moving metallic body cuts through lines of magnetic force, creating a proportional voltage in a coil (http://en.wikipedia.org/wiki/Variable_reluctance_sensor).

Do you actually have a purpose-specific sensor (ie, a rotational counter for industrial application) that you want to convert to a guitar pickup?
Alan Peterson
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Geoff James
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Re: Variable reluctance sensor

Post by Geoff James »

Hi Alan , thanks for your reply. I have been messing with one (industrial speed sensor) with view to using it for a one string instrument. Tried the one i had with a string , a=110hz and got about 450mv with a good pick , but thought there may be others out there that have used other types with more output.
The one i used had a chisel point pole and i believe a flat pole face will produce a greater o/p.
Since i intend to use one string , not 6 , it made sense to look at this option.
Regards
Geoff
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Greg Robinson
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Re: Variable reluctance sensor

Post by Greg Robinson »

Hi Geoff, have you private messaged someone on the staff with your full name? We'll update your registration once we get that info.

450mV is a HUGE output for a magnetic pickup. Most conventional pickups put out around 100mV with all 6 strings plucked hard, although there are "distortion" type pickups that put out up to a couple of volts. I don't think you need to be looking for anything with a higher output.
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Geoff James
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Re: Variable reluctance sensor

Post by Geoff James »

Hi Greg , i have pm Mark Swanson with full name.
Maybe i should check again-----maybe range switch in wrong position---DOH.
The reason i want high output is to drive passive ccts that will have their own losses.
I'm trying to avoid batteries. I'll press on experimenting----its the best fun you can have bending over a bench!!!!!!'
Thanks for you input.
Regards
Geoff
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Greg Robinson
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Re: Variable reluctance sensor

Post by Greg Robinson »

What kind of passive circuits? If it's anything like standard volume and tone controls, then, again, pickups with an output of 100mV do fine, so you don't need anything special. If you're planning on driving high loss networks like a Fender/Marshall/Vox (FMV) style tone stack, then you'd probably be better off buffering it anyway, as it's unlikely that you can effectively couple between stages anyway without.

The problem with high output passive pickups is that there is no free lunch. You can increase the output of a pickup in a few ways: moving it closer to the string, which can cause string damping and tuning issues, increasing the strength of the magnet, which causes the same problems, by focusing the magnetic flux lines, again causing the same problems, or by increasing the inductance of the coil by adding more turns to the coil, which raises the output impedance and shifts the reasonant peak (usually to undesirable areas), significantly altering its frequency response.
Another way is to increase the inductance with added ferromagnetic material in the core, but space limitations usually prevent this.
The standard designs work out reasonably well, and are a good trade-off between the various factors. There's only so far you can push those boundaries before it stops working acceptably.
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Mark Swanson
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Re: Variable reluctance sensor

Post by Mark Swanson »

Thanks, Geoff!
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Geoff James
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Re: Variable reluctance sensor

Post by Geoff James »

Hi Greg , the passive ccts i want to use are ring mods , freq triplers , diode clippers all on board and no active preamps---no batteries and all done with one string.
The vr sensor appealed to me because of cost and size---still yet to check output again.
All these ccts will cost output voltage , but possibly a stepup transformer can improve things.
Thanks again for your input.
Regards
Geoff
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Greg Robinson
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Re: Variable reluctance sensor

Post by Greg Robinson »

You're going to have problems trying to accomplish any of that with a passive pickup, not just because of low voltage, but also because of low current. Passive pickups are not good current sources. Say you use germanium diodes with a very low forward voltage, they still need forward current to conduct. The required forward current is very small, usually microamps, but still, I'm not sure you'll be able to get a high enough voltage and also current, particularly with the cascaded networks you're wanting to achieve.
If you end up trying it out though, I'd be interested to know what your results are like.
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David King
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Re: Variable reluctance sensor

Post by David King »

Passive implies that you are loading the string down so you will loose sustain. Impedance goes both ways.
Veronica Merryfield
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Re: Variable reluctance sensor

Post by Veronica Merryfield »

27 years ago I fitted 4 of these little sensors to my first bass. They still work just fine. Super high output. I have then about an inch and a half from the bridge to give a very bright output that I then post process to suit. They also tend to have to be close to the string which precludes them being mounted too far from the bridge since the string excursion would result in them being mounted too far away in order not to interfere with the string.

Schottky diodes may well be ok for what you have in mind, but you'll have to pick the Vf/If curve carefully, for passive clippers and a tripler, not sure what you would use for a ring mod nor what the second source would be for a mono-stringed instrument.
Geoff James
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Re: Variable reluctance sensor

Post by Geoff James »

Hi Veronica , thanks for your input. I've just bought a Red Lion sensor and that outputs between 200mv to 1.5v +. The PU is mounted about 100mm from the bridge an string clearance about 1.5 mm.
The whole idea of this little project was to use passive components to add some flavour to a single note.
I've tried a two diode (schottky) freq doubler with some success and will buid a ring mod to accept both the string output and the doubled output as inputs with the output hopefully giving me enough 5Th to be usable.
Regards
Geoff
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