Weak Magnetic Field Sensing

What exactly are you trying to accomplish? Do you really need resistance as the output? Most important, do you need to detect a stationary magnet, or only a moving one?

If you want to detect a moving magnet, all you need is a big coil of wire and a simple circuit to detect the voltage induced in it by the magnet's motion. However, when the magnet stops, the output stops as well.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Christopher,

I can see what you are getting at.. maybe I am a different species?. Yes this conversion is possible, but might require some cumbersome electronics.

The true hall effect sensor you speak of outputs a voltage proportional to both the applied field and transverse bias current. In the 'easy' sense what you'd like to do is unreasonable, but it can be acheived in effect, approximately, with additional circuitry... with some likely requirement of isolating the hall effect bias current voltage source from the rest of your circuits. I think you can forget about using the hall device directly to emulate some kind of resistor. However it's output voltage can be amplified and sent through a transconductor of some kind which approximates a resistor, or more readily an analog to digital converter driving a digital potentiometer.. with some added resistors, or offset scheme to get your range expressed in 64, 100, 256 ohmic range steps.

LakeShore has hall effect sensors of the type you are interested in. I suspect Digikey or Allied also carries more than one proportional hall effect device, and one that is easy to use... though most are as you say.

There are lots of way to skin a cat (no cats were harmed during testing) . Since hall devices are linear with respect to applied field and current, the output will not vary linearly with distance, as the field strength of the magnetic rod will not correspond linearly over that range (but will appear close to linear over very very small incremental changes in distance between rod and sensor).

There are magneto-resistive sensing elements out there, maybe there is some way you could use a Honeywell device.. still I would wonder why you couldnt use the changing voltage to effect whatever desired action somehow.

You could set up 10, 0/1 devices over the 8-inch range, each switching in a new resistor with some clever way of latching the present state to hold R-value in the dead-zones between sensors. It's crude, but sort of linear.

You can do this to some effect with a few components.. as you said, not necessarily linear, but over some range.

I'm not sure how to set it up, but I've heard that if you shred the constitution, torture people for corporate profits, and (mile-long list truncated), resistance changes proportional to that... but I think it usually means boycotting beer for an hour at best, which isn't really resistance, or ohmically useful either.

Anna

Hello Bob and Hello Anna,

You ask:

Find the best method using any type of sensor or effect for a non-tactile, hand controlled volume control.

I am attempting to turn my hand into a human potentiometer with at least an 8" sweep range. I thought holding a magnet could give me a usable effect or force to do this.

I want to explore a new volume control method of a special musical instrument called a "Theremin". The normal theremin volume method uses a antenna and detects the shift in frequency of two perfectly balanced oscillators. It has very wide physical sensing area of more than one a foot diameter. Placing my hand out 12" then moving it next to the control antenna quiets the volume without touching it.

Yes, I can do the above technique, along with an optical method and a mechanical balance of two magnets opposing one another method which works but is not responsive enough and is sloppy.

I want to use a different approach that will have a working width in free space of about a 4" to 6" diameter and have detectable downward sweep range starting at least 8" to the outside, then have a controlled measurable output as my hand magnet moves down to the control sensor pickup.

This will allow me to have more than one (maybe four) sensor/detector spread out in the original one foot diameter or musical volume control panel area of which my hand approaches.

Non of the above values or measurements are critical and could be modified to adapt to the approach.

I have a webpage I show hobbyists/musicians how to build my theremin design and this volume control method would be a great enhancement.

"The theremin is the only musical instrument played without touching anything." It has can sound like a musical saw or an angels voice.

This is Clara Rockmore Circa 1940 .mp3 777kb

Thank you, your ideas are greatly appreciated.

• * * Christopher

Temecula CA.USA

Chris:

Voltage induced in a coil by a moving magnet would have a velocity response, instead of the position response you seek. It might be worth investigating, but it would have a bit of a learning curve. However, since the Theremin (or any instrument, for that matter) has a learning curve, this doesn't need to be a show-stopper. It might turn out that rate is easier to control than position. You might want some sort of gestural interpretation to activate peak hold, so you don't need to keep moving at a constant velocity to keep a constant volume.

I'd think that some sort of photo-optical arrangement would work pretty well for position sensing, but you'd probably need something like rows of LEDs and photodiodes to replace the Theremin antennae (or arrayed along the antennae). There could be separate zones for different controls, but I don't know how you could control more than a couple at once. (Assuming you have no more than the normal allotment of appendages!)

How about a photo method where instead of a magnet, you hold a disk of polarizing film, or a transparent color wheel. I'm thinking there might be some way to control multiple channels by using distance as well as orientation, but I confess this is right off the top of my head and not thought out at all.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis