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Tuesday, 2 April 2013

Is there a gauss in the house? Pt 1

My old Mk1 gaussmeter!

Why, you might ask, might an ordinary guitarist need a gauss-meter (or magnetometer)?  Er … well you can probably see why it might be useful to me as a pickup maker, dealing with magnets all day … but why might you want to make one? Well aside from curiosity and stuff like that … if you mess about changing guitar pickups etc and more importantly, occasionally swapping humbucker or P90 magnets … a tool that not only lets you tell the polarity of a pickup, but also its flux strength is a pretty cool thing to have.  You can even 'age' magnets yourself once you can measure before and after strength, and ...the clincher ... is when you discover that a DIY meter need only four components, and requires only that you can solder reasonably well to make it! You will need a digital multimeter to read the info your gauss meter is sending out, but many of us have one of those anyway, and a cheapo e bay one for six or seven quid is fine.

Reality check? Okay, so a cheap compass will work for checking pickup polarity ... and a Schatten polarity tester - from StewMac in the US - will do an even clearer job. But with postage, the Schatten (which is only a little magnet in a tube with one pole marked nort and the other south) will cost more than building a gaussmeter ... and one of these is WAY cooler and should cost less than a tenner ... fifteen to twenty quid if you have to buy a cheap multi meter!
Firstly the major component is a ‘hall effect device’. This little gismo is used in many applications, and has the neat property that when you feed it a nice stable voltage of say, five volts … a nice stable 2.5 volts comes out the other end.  ‘BORING’ you may cry … well yes, until you put the hall sensor in a magnetic field … then if the pole is ‘south’ the voltage goes up proportionally with the strength of the magnet … and if north, it goes down … again in proportion to the strength of the field. 
In theory we only need to feed that 5 volts in and measure the DC voltage ‘out’ with a digital multi meter  to have a crude, functioning gauss meter. In practise it’s only slightly more complex than that: if you intend to power the hall sensor from a 9v battery then you’ll need a ‘voltage regulator’ to deliver that 5 volts. Many people leave it at those two components … but I was taught that it’s a good plan to ‘filter’ the DC from the battery to the power supply and also filter the power supplies’ output to the hall sensor … just to provide nice squeaky clean DC where it’s needed.  That simply means a capacitor before the power supply and one after … simples!
I mounted everything on a small ‘perf board’ I had knocking about … they are only a couple of quid on e bay.
So here’s the component list , and we’ll get to the circuit diagram and hooking it up in part 2.
1x LM7805 voltage regulator
1x 0.33uF ceramic capacitor
1x0.1uF  ceramic capacitor
And last but most important … an Allegro A1302 Hall effect sensor
E bay is good for all of these … a chunk of ‘perf board’  to hook it all up on would be cool … and if you are feeling like being posh … a pair of banana sockets to attach the meter and a box to put everything in (you could use an old pedal enclosure).

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