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Difference between pages "Tower Pro MG90S Vs. Clones" and "Rainbow Fidget Spinner"

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(Created page with "The MG90S servo may be the most widely copied hobby servo today. Originated by the manufacturer Tower Pro, it is widely copied, cloned, and counterfeited. This article will e...")
 
(Created page with "==Introduction== The Vorpal Rainbow Fidget Spinner is a fun project that allows you to make a 3D printed fidget spinner that lights up in a rainbow of random colors when you s...")
 
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The MG90S servo may be the most widely copied hobby servo today. Originated by the manufacturer Tower Pro, it is widely copied, cloned, and counterfeited.
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==Introduction==
 +
The Vorpal Rainbow Fidget Spinner is a fun project that allows you to make a 3D printed fidget spinner that lights up in a rainbow of random colors when you spin it. There is no on/off switch for this project, rather it uses a clever centrifugal force switch that automatically engages when the project is spinning. Even fairly slow spinning speeds will light up the spinner. Because this project is open source, you can modify the spinner design, remix, and hack the project to your heart's content.
  
This article will explain the differences between genuine Tower Pro MG90S and commonly available clones and counterfeits.
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This project is open source and is licensed under the Creative Commons Share-Alike Attribution International 4.0 license.
  
==Definitions==
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A convenient kit including all the electronics is available from the Vorpal Robotics Store.
For the purposes of this article, we'll use the following definitions:
 
* Genuine Tower Pro MG90S: An MG90S servo actually manufactured by Tower Pro.
 
* Clone MG90S: A servo marked MG90S and generally compatible (same size, etc) with applications that use the genuine Tower Pro MG90s, however its label does not claim it was made by Tower Pro. In other words, the manufacturer is not pretending the servo was made by Tower Pro. This is completely legitimate.
 
* Counterfeit MG90S: A servo that is marked "Tower Pro MG90S" but was not actually manufactured by Tower Pro. In most countries this violates various intellectual property laws and is illegal. In some cases names very close to "Tower Pro" are used, such as "Tower Prop" or "Turbo Pro". A name that is purposely trying to fool the customer into thinking the servo is made by Tower Pro is also considered a counterfeit in this article.
 
  
==Tower Pro MG90S Characteristics==
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==Bill of Materials (BOM)==
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The following electrical and hardware items are required to build this project:
 +
* 5 x Random color LEDs, T3 size (3mm)
 +
* 1 x Rainbow Fidget PCB*.
 +
* 1 x Sorenson Centrifugal Force Switch
 +
* 1 x low profile CR2032 battery holder, half circle type
 +
* 1 x CR2032 button cell battery (not rechargeable)
 +
* 6 x Button head cap screws, M3x5mm
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* about 8 inches (200mm) of bare hookup wire, anywhere from 21 to 24 gauge is fine.
 +
* 1 x Skate Bearing (size 608)
  
The Tower Pro MG90S has these characteristics:
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<nowiki>*</nowiki> NOTE for self-sourcers: Instead of making the Rainbow Fidget PCB, you can substitute a piece of copper clad perfboard, 0.1" spacing, 6 holes by 2 holes, carefully cut out of a larger PCB with a hacksaw.
*It is a digital servo, meaning it has more sophisticated internal circuitry that generally provides more torque, more holding power, and faster updates in response to external forces. Digital servos can also generally take faster PWM signals, meaning it can receive updates from the microprocessor more quickly. However, digital servos can also be somewhat more subject to noise on the power rails, see discussion below.
 
*Very small deadband, 1 microsecond. This is somewhat technical, but the "deadband" is the amount of error the shaft position can have from the commanded position without the servo trying to adjust power to compensate. A deadband of 1 microsecond means the servo essentially will try to adjust the position if it is off by more than about 0.045 degrees. This is an extremely tight deadband and can cause issues, more discussion is below.
 
*High quality gears made from aircraft grade aluminum. Tower Pro gears rarely bind up and generally turn smoothly without any clicking or grinding noises.
 
*When running under load, you will typically hear a high pitched squeal as the digital servo circuitry compensates for loads on the shaft. However you will generally not hear clicks, pops and loud buzzing noises. (Exception, see "hunting" below).
 
  
Technical specs (from the Tower Pro official spec sheet):
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<gallery mode=packed>
* Weight: 13.4g
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File:RainbowFidget-PCB-Closeup.jpg|Close up view of the Fidget PCB
* Dimension: 22.8×12.2×28.5mm
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</gallery>
* Stall torque: 1.8kg/cm (4.8V); 2.2kg/cm (6.6V)
 
* Operating speed: 0.10sec/60degree (4.8V); 0.08sec/60degree (6.0V)
 
* Operating voltage: 4.8V~ 6.6V
 
* servo wire length: 25 cm
 
  
==Counterfeit MG90S Characteristics==
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The following 3D printed parts are needed for this project. You can find all of the STL files on Thingiverse by searching for Vorpal Rainbow Fidget Spinner. You can find the source 3D models on OnShape.com by searching for "Vorpal Rainbow Fidget Spinner".
  
While it is difficult to be specific due to the large number of counterfeiters, generally speaking this is what we have found after examining thousands of counterfeits:
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* 1 x Circuit carrier
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* 1 x Bottom Plate
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* 1 x Top Plate
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* 2 x Endcap
  
* Most counterfeits claim to be digital servos but they are actually analog servos.
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The following tools and supplies are needed:
* Even as analog servos, many of them are defective. For example, there are sometimes batches of counterfeit servos where a large percentage of the servos will exhibit a shaft "drift" when they heat up. In other words, the shaft may drift (usually counterclockwise) anywhere from 2 to 90 degrees as the servo gets warm through use. This drift will revert when the servo cools. If the servo only exhibits a small (2 to 4 degree) drift, then it is usable for many tasks. But clearly, if the drift is as much as 10, 20 or even 90 degrees as we have seen sometimes, the servo is useless for most purposes.
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* Hex key that fits the screws, this would be a 2mm hex key for 3mm button head screws. A 5/64 inch hex key will also work because that's very close to 2mm.
* Gears are metal, but they are of very low quality. About 10 to 15% of all the gearboxes are bad right out of the box and will either fail immediately or within a very short period after use. Even gears that work are of obvious low quality. For example you can see right through the plastic case that there are striations, misshapen teeth, etc. If you carefully turn the shaft by hand, you will frequently feel rough spots, clicks, and pops as bad teeth engage and disengage.
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* Soldering iron, solder (lead free is recommended), rosin flux paste or liquid, wire cutters
* When running under load, you will frequently hear a lot of chatter, jitter, and noise even when the loads are steady and not changing (such as when the Vorpal Hexapod is standing still).
 
  
The technical specs given by vendors who sell counterfeits will match the real Tower Pro specs, however, these specs mean nothing because these are not genuine servos!
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==Assembly Steps==
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===Solder LEDs and Wires to Printed Circuit Board===
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The printed circuit board (PCB) has a side with copper and a side without copper. The LEDs should be inserted into the holes from the non-copper side. Each LED has a short lead and a long lead. The long lead of each LED should be inserted such that they come out through the side of the board marked with a + sign. The two holes that are farther apart, near the + and - symbols, are not for LEDs but rather for pieces of wire. Place the end of a 3 inch piece of wire in each hole.
  
Typically the clones will have less than half the torque, be much more sluggish when responding, and will consume more power than the genuine Tower Pro MG90S servos.
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To hold all the LEDs straight while soldering, you can use the circuit carrier 3D printed part. Use the side that doesn't have the deep grooves to hold the LEDs in place.
  
== Clones vs. Counterfeits ==
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Once they are set down, double check to make sure all of the long leads are on the + side of the board. Apply a little rosin flux then solder all the connections. It's very important not to bridge the solder between the + and - sides of the board, which are separated by a gap in the copper.
  
Many clones are identical to counterfeits (other than the label) had have all of the same problems.
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You may now test the LEDs by taking the CR2032 battery and touching the wire on the + side of the circuit to the battery side marked +, then touch the other wire on the other side. All LEDs should light up. If some or all of them do not light up, you may have reversed the short vs. long legs of the LED, or the solder joints may not be good. Carefully check the joints to rule out soldering. To check for reversed wiring, connect the wires to the battery the opposite way and see if the LEDs that fail to light now light up.  If they do, you did not orient the LEDs properly and you would need to desolder them and flip them around.
  
However, there are some manufacturers who put their well known brand name on clones, and they do a good job of ensuring quality.
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===Solder the Centrifugal Force (CF) Switch===
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The centrifugal force switch will turn on the lights only when the spinner rotates at a certain minimum angular velocity.
  
One of these "good clones" is the Turnigy MG90S, sold by Hobby King.  This MG90S clone is very good quality. It really is digital and has good torque and speed. Personally, we prefer the Tower Pro MG90S gearboxes though, because from our testing the real Tower Pro MG90s have a little smoother gearing and don't have any noticeable binding when turning by hand. But if you use the Turnigy MG90S you'll get a good servo at a nice price.
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One contact is the small wire coming off one side, the other contact is the drum shaped body of the switch. The switch is somewhat more difficult to solder than the LEDs and plain wires. Rosin flux should be applied and you may need to hold the soldering iron on the metal for a longer period to make everything stick. Don't worry, the solder will start to flow. The switch is not a sensitive electronic component so it can take more heat than the LEDs or other circuitry.
  
==How to Tell Genuine Tower Pro MG90S from Counterfeits==
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Twist the end of the wire coming from the - side of the PCB around the stiff wire coming off the CF switch. Don't get too close to the body of the switch, because if you bridge the wire and the body then the switch will always be "on".  Apply rosin flux and solder.
  
If the label is the same, how are you supposed to tell whether you are getting real Tower Pro MG90S or counterfeits?
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Insert the CF switch in the 3D printed circuit carrier. The wire side should be facing away from the center of the Fidget Spinner as shown. Route the wire from the - side of the PCB to the CF switch in the groove provided for this purpose.
  
There are some definite red flags to look for:
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Lay the end of a 3 inch long piece of wire on the body of the CF switch, and apply some rosin flux, and solder. After removing the soldering iron don't move the CF switch for a few seconds to let the solder set.
  
* If the price is less than about $4.50 USD, it is a probably counterfeit. If the price is under $2.50 USD, it's almost certain to be a counterfeit unless it is some kind of "clearance sale" where the seller is willing to lose money. Genuine Tower Pro MG90S have a wholesale cost well above $2.50 even in massive quantity purchases (thousands or tens of thousands).
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===Solder the Battery Holder===
* If the vendor covers up the name on the label in the catalog picture, it's definitely counterfeit.  This means Tower Pro went after them and forced them to stop using the Tower Pro name. If you buy such a "label covered in the picture" servo and it says "Tower Pro" on the label when you receive them, it's almost guaranteed not to be genuine Tower Pro.
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The battery holder has a + one one side and three terminals. Place the battery holder into the circuit carrier as shown in the picture below, with the + side on the same side of the carrier as the PCB.
  
There are also some physical characteristics to look for that are pretty obvious and can be used even if all you can see is the label. Again, we have to caution that there are many different counterfeits and clones, we're only showing one of the most common ones here.
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Route the wire attached to the body of the CF switch to the center terminal of the battery holder and wrap it around the terminal. Needle nose pliers might make it easier to get a tight wrap. Apply some rosin flux and solder. It may be easier to solder from the back side of the circuit carrier (the side where the LEDs are poking through the plastic). Trim off excess wire with angle cutters.
  
A few characteristics of the label printing style are a dead give-away as shown here:
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===Test the Circuit===
[[File:MG90S-SideViewComparisonDiagram.JPG|left|600px|Side view comparison of labels]]
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You can now test the circuit without even spinning by touching a piece of scrap wire across the CF switch. Join the wire stem to the body using the scrap of wire and the LEDs should all light up.
  
<br clear=all>
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If some or all of the LEDs do not light up, trace the wiring and check solder connections to make sure everything is solid.
From the top view, the genuine Tower Pro MG90S has deep "dimples" around the bracket, and the washer under the plastic where the shaft comes out of the housing is dark in color. By contrast, fake MG90S have little or no dimple marks and have a light copper colored washer under the shaft. Copper is a cheaper metal than the high quality aluminum used in the real Tower Pro servo.
 
[[File:MG90S-TopViewComparisonDiagram.JPG|left|600px|Top view comparison]]
 
  
<br clear=all>
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===Insert the Skate Bearing And Balance Weights (Nuts)===
  
== Using Genuine Tower Pro MG90S with Vorpal The Hexapod ==
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===Screw the Housing Together===
While the genuine MG90S are superior in just about every way imaginable, there are a couple of things you need to know when using them with Vorpal The Hexapod.
 
  
The information below also applies to "good clones" that are digital like the Turnigy MG90S.
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===Have Fun!===
  
===Servo "Hunting"===
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<gallery mode=packed>
Digital servos have far more torque and are far faster than analog servos. They also have a very narrow "deadband" meaning they generally have much less error when reaching a commanded position. This is all great news! But there is one side effect of that extra power and narrow deadband that is undesirable: hunting.
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File:Rainbow-Fidget-Exploded-View-With-Lines.JPG|Exploded view of assembly for 3D printed structures.
 
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File:RainbowFidget-adding-flux-toPCB.JPG|
Let's consider a "hip" servo (the servos attached to the base) swinging a leg to a new position. Because the leg has weight and therefore inertia, what can happen is that the hip servo overshoots the desired position. The greater speed and torque of the digital servo makes this much more likely than with an analog servo. Once the servo overshoots, it needs to reverse power and try to bring the leg back to where it's supposed to be. Because of the very narrow deadband, however, it is almost impossible for the servo to reach the desired position without overshooting!
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File:RainbowFidget-CircuitInserted.JPG|
 
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File:RainbowFidget-CircuitInsertedLEDSide.JPG|
The result is a rapid quiver around the commanded position. This situation is called "hunting". The servo is searching, or hunting, for the desired angle, but it never finds it because the inertia of the leg keeps causing it to overshoot beyond the deadband range.
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File:RainbowFidget-PCB-soldered.JPG|
 
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File:RainbowFidget-Prep-Solder-Centrifugal.JPG|
Because hip servos have no intrinsic bias, the leg can resonate like that indefinitely. The knee servos, on the other hand, do not exhibit this kind of hunting behavior to any significant extent because gravity biases the movement in one direction and not the other. This causes the hunting resonance to break up before it even starts.
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File:RainbowFidget-SolderingPCBSetup.JPG|
 
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File:RainbowFidget-Trim-PCB.JPG|
===Solution to Hunting===
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</gallery>
The solution is actually quite simple: add a small O-ring to the shaft of the hip servos. All of the kits we provide that have genuine Tower Pro servos come with the right size O-ring. You just slip this on before you put the servo horn on the shaft.
 
 
 
The reason this works is that it provides a little bit of friction that damps out the resonance effect. This friction reduces the amount of overshoot enough to kill the resonance.
 
 
 
Note that without the servo horn screw in place, the leg may still exhibit "hunting".  The screw is needed to clamp down on the servo horn and press it against the O-ring.  You should only tighten the screw enough to stop the hunting behavior. We don't want to add so much friction that battery run-time is reduced.
 
 
 
Normally you only need the O-rings on the hip servos. Sometimes the front two legs will show some of the quiver associated with hunting when in F1 "fight" mode. If you want, you can add O-rings to the knee servos on the front two legs to damp out the hunting there, but its not really necessary. Some people think the knees quivering like that in F1 mode actually looks cool!
 
 
 
===Electrical Noise===
 
 
 
Both analog and digital servos are subject to oddities that occur due to electrical noise. Because Vorpal The Hexapod has 12 servos all sharing the same power supply, there can be some feedback in the signals going to each servo which causes this noise.
 
 
 
For analog servos, this will usually manifest as little clicks and twitches in the servos. For digital, that can happen, but sometimes a more serious situation occurs where the servo goes off to a weird angle for a moment.
 
 
 
The solution here is to make sure you're running the latest version of Vorpal The Hexapod's robot code, which is optimized for digital servos. This code runs the signals much more quickly out to the servo, at 120 Hertz rather than the 60 Hertz we used to use for analog servos. The faster signal doesn't stop the noise, but it causes new updates to go to the servo more quickly which allows the noisey signal to be overridden by a good signal in a shorter time.
 

Revision as of 12:17, 20 March 2018

Introduction

The Vorpal Rainbow Fidget Spinner is a fun project that allows you to make a 3D printed fidget spinner that lights up in a rainbow of random colors when you spin it. There is no on/off switch for this project, rather it uses a clever centrifugal force switch that automatically engages when the project is spinning. Even fairly slow spinning speeds will light up the spinner. Because this project is open source, you can modify the spinner design, remix, and hack the project to your heart's content.

This project is open source and is licensed under the Creative Commons Share-Alike Attribution International 4.0 license.

A convenient kit including all the electronics is available from the Vorpal Robotics Store.

Bill of Materials (BOM)

The following electrical and hardware items are required to build this project:

  • 5 x Random color LEDs, T3 size (3mm)
  • 1 x Rainbow Fidget PCB*.
  • 1 x Sorenson Centrifugal Force Switch
  • 1 x low profile CR2032 battery holder, half circle type
  • 1 x CR2032 button cell battery (not rechargeable)
  • 6 x Button head cap screws, M3x5mm
  • about 8 inches (200mm) of bare hookup wire, anywhere from 21 to 24 gauge is fine.
  • 1 x Skate Bearing (size 608)

* NOTE for self-sourcers: Instead of making the Rainbow Fidget PCB, you can substitute a piece of copper clad perfboard, 0.1" spacing, 6 holes by 2 holes, carefully cut out of a larger PCB with a hacksaw.

The following 3D printed parts are needed for this project. You can find all of the STL files on Thingiverse by searching for Vorpal Rainbow Fidget Spinner. You can find the source 3D models on OnShape.com by searching for "Vorpal Rainbow Fidget Spinner".

  • 1 x Circuit carrier
  • 1 x Bottom Plate
  • 1 x Top Plate
  • 2 x Endcap

The following tools and supplies are needed:

  • Hex key that fits the screws, this would be a 2mm hex key for 3mm button head screws. A 5/64 inch hex key will also work because that's very close to 2mm.
  • Soldering iron, solder (lead free is recommended), rosin flux paste or liquid, wire cutters

Assembly Steps

Solder LEDs and Wires to Printed Circuit Board

The printed circuit board (PCB) has a side with copper and a side without copper. The LEDs should be inserted into the holes from the non-copper side. Each LED has a short lead and a long lead. The long lead of each LED should be inserted such that they come out through the side of the board marked with a + sign. The two holes that are farther apart, near the + and - symbols, are not for LEDs but rather for pieces of wire. Place the end of a 3 inch piece of wire in each hole.

To hold all the LEDs straight while soldering, you can use the circuit carrier 3D printed part. Use the side that doesn't have the deep grooves to hold the LEDs in place.

Once they are set down, double check to make sure all of the long leads are on the + side of the board. Apply a little rosin flux then solder all the connections. It's very important not to bridge the solder between the + and - sides of the board, which are separated by a gap in the copper.

You may now test the LEDs by taking the CR2032 battery and touching the wire on the + side of the circuit to the battery side marked +, then touch the other wire on the other side. All LEDs should light up. If some or all of them do not light up, you may have reversed the short vs. long legs of the LED, or the solder joints may not be good. Carefully check the joints to rule out soldering. To check for reversed wiring, connect the wires to the battery the opposite way and see if the LEDs that fail to light now light up. If they do, you did not orient the LEDs properly and you would need to desolder them and flip them around.

Solder the Centrifugal Force (CF) Switch

The centrifugal force switch will turn on the lights only when the spinner rotates at a certain minimum angular velocity.

One contact is the small wire coming off one side, the other contact is the drum shaped body of the switch. The switch is somewhat more difficult to solder than the LEDs and plain wires. Rosin flux should be applied and you may need to hold the soldering iron on the metal for a longer period to make everything stick. Don't worry, the solder will start to flow. The switch is not a sensitive electronic component so it can take more heat than the LEDs or other circuitry.

Twist the end of the wire coming from the - side of the PCB around the stiff wire coming off the CF switch. Don't get too close to the body of the switch, because if you bridge the wire and the body then the switch will always be "on". Apply rosin flux and solder.

Insert the CF switch in the 3D printed circuit carrier. The wire side should be facing away from the center of the Fidget Spinner as shown. Route the wire from the - side of the PCB to the CF switch in the groove provided for this purpose.

Lay the end of a 3 inch long piece of wire on the body of the CF switch, and apply some rosin flux, and solder. After removing the soldering iron don't move the CF switch for a few seconds to let the solder set.

Solder the Battery Holder

The battery holder has a + one one side and three terminals. Place the battery holder into the circuit carrier as shown in the picture below, with the + side on the same side of the carrier as the PCB.

Route the wire attached to the body of the CF switch to the center terminal of the battery holder and wrap it around the terminal. Needle nose pliers might make it easier to get a tight wrap. Apply some rosin flux and solder. It may be easier to solder from the back side of the circuit carrier (the side where the LEDs are poking through the plastic). Trim off excess wire with angle cutters.

Test the Circuit

You can now test the circuit without even spinning by touching a piece of scrap wire across the CF switch. Join the wire stem to the body using the scrap of wire and the LEDs should all light up.

If some or all of the LEDs do not light up, trace the wiring and check solder connections to make sure everything is solid.

Insert the Skate Bearing And Balance Weights (Nuts)

Screw the Housing Together

Have Fun!