Sunday, August 28, 2011

Guitar Pickup Winder






This projected started in June of 2011.  Remember the Google Doodle commemorating Les Paul's 96th birthday?  Well, that got me thinking about how electric guitar pickups work.  I had a general idea about how they work, postulating they are essentially variable reluctance sensors,  but doing a bit of research, I kind of got bitten by the electric guitar bug and dove right into the research of solid body electric guitars as a whole.

I have been primarily focused on guitar pickups and passive electronics at the moment.  In short order however, I would be requiring a test bed for these pickups.  That is why I made the decision from the very beginning that I would not only try my hand at building electric guitar pickups, but a complete scratch built guitar; wood has been harvested.  Later posts will focus on other topics about solid body electric guitar building.  Here, I will try to stay on the topic of an electric guitar pickups, but more specifically the pickup winder I built to wind my own pickups.

After announcing my new obsession to anyone who would listen, an electric guitar was given to me to experiment with.  It is a smaller than average guitar, possibly for a child, but it will do just fine to test my pickups with a full guitar still being built down the road.  Below is a picture of the pickguard with the old electronics installed.




Since I was going to be using an microcontroller to count the turns and stop the machine when the predetermined number of turns has been reached, I could make this winder do more stuff too.  So, I made this pickup winder into more of a pickup winding station.  Here is a feature list:

-Pickup Winding with turn counting, RPM, ETA and auto stop
-Stroboscopic guitar tuner
-Ohm meter for coil resistance measurement
-Gaussmeter for measuring polarity and magnetic flux density of pole pieces

I decided to use an Arduino to control my guitar pickup winder.  I figured it would be a good way to put it into use.  I bought the thing years ago and have only used it a couple of times.  It really just gathers (figurative) dust as was the graphic LCD I used here too.  I was glad to put some inventory to use in this project.

The code is ~850 lines so probably a bit much for inline posting with syntax highlighter.  Instead, you can download the arduino "sketch" (code) below.  Also, there is a link to a collection of free PDF's I came across while doing research for this project.  There is some good information in there about guitar pickups.  I also put a schematic for wiring my guitar in there too as I found the pictorial "schematics" confusing.

Files:

You can download the pickup winder schematic here.
You can download the v1.5 Arduino code here.
You can download the guitar electronics schematic plus PDF collection here.

The need for a pickup winder is obvious when you take a look at the construction of an electric guitar pickup.  Essentially, there is a bobbin made from AlNiCo rod magnets and insulating material.  Around this, there are ~6,000 to ~10,000 turns of 42 ga wire.  Depending on the pickup, the number of turns of wire and the wire gauge will differ, but this approximation will illustrate the need for a machine that can turn the bobbin and reliably count the completed turns.

Here are some pictures showing the complete bobbins and  the Jarrah wood spacers and tools for assembly.  Everything is sitting on my tree stump anvil that I collected while sawyering in the above youtube video.  The magnets are appropriately called "pole pieces" and this configuration is called "staggered".  The height of the pole piece corrects for volume as the fretboard is radiused.







Also, 42 ga wire is tiny and fragile!  Here is a microscopy photo showing the copper 42 ga wire on the right next to a hair from my head.  




O Kapton! My Kapton!  I wrapped the bobbins with Kapton tape to prevent possible abrasion from wearing thru the wire insulation and shorting out the pickup.  I also super glued the inside joint where the alnico 5 rod magnets meet the flatwork to discourage pressing them thru and abrading the coil wire.




I looked up some commercially available pickup winders to get an idea of what I was trying to make.  The speed ranges were all around 1000 RPM and variable.  Some of the commercial winders have a thru axle offering "reverse" winding capabilities.  I decided I can achieve "reverse" winding by mounting the pickup to wind "upside down" and the direction of wind will be opposite.  This way I do not have to worry about motor direction control.

I found a variable speed motor in the form of a sewing machine motor in one of my many junk boxes.  I made an infrared non-contact tachometer to measure the no load speed of this motor and was rather surprised to find that it turned at 10,000 RPM.  Clearly this is way too fast even with the variable speed throttle pedal.

I did not give up on sewing machine motors, though and went to the ReUse center and found an old Remington sewing machine for $5.  This machine was too sweet looking to hack apart so I decided to keep it as a sewing machine after a bit of tuning it up and make up an adapter to make it work for pickup winding too.  Also, leaving the sewing machine in tact gives me the benefits of mechanical gear reduction, friction to help slow it down, a flywheel and a face plate area to mount an adapter for putting the pickup bobbins.  It's a win-win situation.

Here is a drawing of the adapter jig.  It mounts in the flywheel on the end of the sewing machine with three rare earth magnets as seen in the picture below.  A ferrous piece of material, such as steel is screwed on to the aluminum jig to allow the pickup bobbin to be mounted with it's Alnico 5 magnets.





I found that the pickup bobbin mounted magnetically to the steel bar did not have a high enough coefficient of static friction; it would rotate with a small amount of torque applied yet it had quite a strong magnetic attraction to the steel bar.  I tried a piece of duct tape, sticky side towards the steel plate, and it helped, but the real solution was gluing a couple pieces of sand paper on the outer edges of the steel bar, atop the duct tape as shown below.  This allows for secure winding.




Below you can see how I mounted the pickup bobbin in "standard" orientation.  This allows what is known as clockwise winding.  The picture also shows how I used a straight edge to line up the wire guide stops.  Those wire guide stops are mounted to a 3/8" diameter rod from the hardware store.  The rod is held in place by a magnetic base dial indicator stand.  All sewing machines that I am aware of have a flywheel on the side that rotates the same direction.  Using the picture below for reference, the top of the flywheel moves to the left.  This is anti-clockwise as you look at the front end of it.




In the following pictures you will see how I mounted the bobbin to the faceplate to allow "reverse" or anti-clockwise winding.  I pressed some rare earth magnets into a piece of Jarrah wood.  The pickup bobbin is charged magnetically so it's alnico 5 rod magnets are attracted to the opposite poles on the rare earth magnets.  The rare earth magnets hold the Jarrah wood spacer and bobbin to the steel bar on the faceplate. 





Did you happen to notice a piece of black electrical tape across the flywheel in any of the pictures above?  That is for the infrared non-contact tachometer.  You can check the schematic for more details on that.  Since I am measuring rotation events over time (RPM), it is a trivial matter to count the total number of events (rotations) or coil winds.  I am also able to calculate estimated time to completion based on current RPM and number of winds left.  The software opens a relay when the number of winds is greater than or equal to the desired number of winds.  This cuts the power to the sewing machine so it stops when you have the correct number of winds.  Of course, inertia adds a few winds, but you can compensate for that by lowering the number of winds desired or just unwinding a few turns when you see how many over the machine went. It seems to overshoot by 10-15 turns due to inertia so I just set the desired turns number lower.  That is a pretty complete description of the coil winder operation.  There isn't much to it, but there is a lot of information on the internet and in books about coil winding that are worth a read. 

You can see a picture of the non-contact IR tach sensor below.  It has 2 magnets to attach to the cast iron sewing machine body.




Even with the gear reduction offered by keeping the sewing machine in tact and the added friction, I found the flywheel where my adapter jig is mounted still rotated too fast.  I found it necessary to slow the motor down further and I did this by adding a "resistor" for the AC hot line of the sewing machine motor.  I used a 100-watt light bulb for this purpose and now the final speed at the flywheel is a manageable maximum of ~1000 RPM.  The throttle pedal still controls speed and the light bulb is inserted in the circuit with standard 110ac 2 prong plugs so it can be removed for high speed sewing.    Below is a picture of the dimly glowing bulb acting as a resistor.




Here is a short demo video.




Stroboscopic tuner:




The stroboscopic guitar tuner works by flashing an LED at the resonant frequency of an in-tune guitar string.  I made a guitar pick with an led on it to pluck a guitar string.  An LED plectrum if you will.  The pick was etched using the sharpie and ferric chloride method.  It says "LED Guitar Tuner" on it.  You aim the led at the string while it vibrates and turn the tuning machines of the guitar until the string appears to be still.  Below is a picture of the LED pick and a video of the tuning in progress.  It is a little difficult to see in the video but in person, the strobe tuner is quite effective.  The video is taken after the pickups I made were installed in this guitar.  The sound you are hearing is coming from an LM386 audio amp I made for a different project.






I have found in practice that the stroboscopic guitar tuner is much easier to use on the lower frequency strings.  Like the low-E at 82.41 Hz.  The video above is of the G string at 196 Hz and it is easy enough to see in person too.  I guess, as with anything, practice is helpful to getting it right.  You can use relative tone matching by ear for tuning the higher frequency strings.  There is nothing to say you cannot strobe tune all strings with the strobe tuner, however. 


Gaussmeter:

I think it will be useful to know the relative Gauss strength of the pickup pole pieces and knowing the polarity, north or south, will be paramount to success.  Many websites on pickup making suggest using a polarity tester for determining polarity but, using a continuous, ratiometric hall effect sensor I can measure the magnetic flux density of the pickup pole pieces at 1.3mV/G as well as the polarity.  This is an uncalibrated value, but it is sufficient for my purposes.  The sensor I used is an Allegro A1302 hall effect sensor.  In the gaussmeter mode, peak gauss values are recorded and instantaneous values are reported.




It is important to note that if the gaussmeter displays the polarity as "North", that means you have a "South" pole of a magnet facing it and your pickup is a "South" polarity pickup.  This is the same as how a compass would work.


Ohm Meter:

Not too much to say here.  There is a voltage divider circuit made with a 10k0 resistor and the guitar pickup under test.  The instantaneous ohmic value is reported and peak value stored and displayed.  I remember measuring the resistance of a coil right after winding.  It measured ~6k2.  After leaving it for some time it went down to 6k0.  I believe the friction from my hand when winding caused the temperature to rise enough to cause this.




I had originally planned to make this pickup winder have an automatic wire feed traversal so it could wind coils on its own.  I may get back to doing that at some point but for now I will "hand" wind them.  That is, feed the wire to the spinning bobbin by hand.  This seems to be the preferred method of winding of some musicians anyhow.  I left a PWM channel available to connect a servo motor to at some point for this purpose should I choose to implement it.

Once the pickups are wound and wire ends soldered to the eyelets they are was potted.  I wrap one round of kapton tape around the coil wire before potting in 20% beeswax and 80% by mass of gulf wax.  I put the jar in a bath of ~160 Deg F water to melt the wax.  I put the pickup in the wax and wait for it to come up to temperature.  You can see in the video below that the pole pieces are that last to warm.  The whole thing locally cools the was and it solidifies.  Within about a minute, depending on the relative thermal masses the pickup is up to temperature and all the wax around it is remelted.




Once they are up to temperature I apply a partial vacuum to the jar removing air and replacing it with wax in all the nooks and crannies of the coil winds.  Besides being good practice for durability of this tiny wire this also stops what guitar players refer to as microphonic pickups.




Here is a shot of the pickups wax potted and ready to be installed into the guitar.  The black wire is the ground.  I make the ground wire whichever eyelet is the coil finish so that the outer coils of the pickup can act as a shield also.




I wound my middle pickup to be opposite of my neck and bridge pickup.  It will be reverse-wound, reverse-polarity for hum cancellation in guitar switch positions 2 and 4.

As far as charging the pole pieces goes it's pretty easy.  Alnico is very coercive.  I bring the assembled and potted pickup into contact with one pole of a strong rare earth magnet.  I use a 3-inch diameter 400 lb pull magnet.  I suspect you can get away with a much smaller magnet, that is just what I had on hand.  So, if I want a "North" polarity pickup I bring the top of the pickup into contact with the "North" side of the charging magnet.  The approach and withdraw is axial.  Then I do the same on the other side bringing the bottom of the pickup close to the magnets' "South" pole.  This makes the top of the pickup "North" and thus attracts a compass "South" needle, thus a "North" polarity pickup.

I installed the new pickups I made into the old pickguard.  I had to whittle away some of the bass wood of the body cavity to fit an extra tone potentiometer.  As I stated earlier, I have a reverse wound, reverse polarity middle pickup here.  I wired them up according the the schematic I drew up and linked to above. I think this is the standard "Strat 2-tone wiring".   

So, in the end, everything seems to have worked.  It makes noise anyway.  I am waiting for someone who knows how to play a guitar to come over and test it out, but I think I am off to a good start.  If you have any suggestions on how to improve my pickups let me know.  Or, if you have any tips on scratch building guitar bodies/necks that would be appreciated too.  I still need to make a better case for the Pickup winder electronics.  Probably a wooden box to set the plexiglass in to keep fingers and 110 vac separate.

A final "finished product" picture... for now anyway.  Also, the missing string has been replaced already. 







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