Category: Troubleshooting

Building a Reed Switch Brushless DC (BLDC) Motor

Today was one great fall day. The morning temperature was in the 40s and gradually it warmed up to the high 60s. After reading the Bible, cutting the grass, raking the leaves and doing some other honey-do work around the home, it was time to build a Reed Contact Motor from Kelvin Educational products. This kit can be ordered from Kelvin also has many other interesting science and robotic kits that you or your parents might be interested in ordering for a birthday, Christmas, or to provide something for you to do during a spring or fall break.

Although this instruction manual said it came from Farmingdale, New York, the parts list was written in millimeters (mm) and the dimensions were provided as if the kit was designed in the United Kingdom using 52,0 mm vs. 52.0 mm as seen in the United States. The reason I mention this is the author of the instruction manual used terms I was not familiar with like Mignon cell batteries, PMMA, Pillar drill, etc. A clever youngster can read the instructions and figure out how to measure and diagram the parts for drilling and sawing, but some of the above terms might through you off – that is why you might need an adult nearby.

Here are the main pieces that come in the plastic pouch that was shipped to me:

1 kelvin reed contact dc motor contents

Kelvin Electronics Reed Contact Motor

This above photo shows the instruction manual, the plywood base, the copper wire, the acrylic side brackets, the reed contact, the pine coil holder/magnet rotor, battery holder, and various sundry items like the magnets, brass nails, and the set screw that you use to make the electromagnet.

The first few pages of the instruction manual give you the dimensions that are required to build the motor. Here is an photo of the components I put on the three main components.

2 reed switch motor pieces marked

The three pieces that need to be dimensioned prior to cutting

In order to build this kit you have to provide two different saws (one to cut the acrylic and one to cut the wooden base, rotor, and coil holder), two needle-nosed pliers, glue, masking tape, soldering iron and solder, and a AA battery. It took me about 2-1/2 hours to cut out the side supports (acrylic), the base, the rotor, the coil holder, glue the magnets to the rotor, and wrap the 30 feet of wire to make the electromagnet. Here are a couple of photos of the finished product before testing.


3 closeup of reed switch assembly

Close-up of the reed switch (note how the reed switch was soldered to the brass nails)

5 top view of reed switch motor

Top view of the completed motor (note how the reed switch is lined up with the magnet)


4 side view reed swith complete start testing

Side view of the completed motor

Once the motor was complete and pictures taken, the next step was to insert the battery to see if the motor would start. I carefully put the battery into the battery holder and … nothing happened. What went wrong? I had checked to be sure the polarity of the magnets were correct before I glued them onto the rotor. Once again I checked to be sure I had wired it all correctly. As I started looking at the rotor and moved the motor slightly it started to run! Immediately, I knew what had happened. In order to make this motor run you have to have the magnets be a little off center in order to pull onc part of the reed switch away from the other part of the reed switch. Since I had a little “give” in how the rotor lined up, it was easy for it to automatically move to the right spot on its own. That is what is nice about most DC motor kits – most of the time you don’t have to make them perfect to have them run.

Here is a photo of the rotor spinning merrily along.


6 reed switch motor complete and workingReed Switch Brushless DC motor working!

Here is the video of the motor as well:

I get a lot of enjoyment making these DC motor kits and hope you do as well. This BLDC kit cost around $7.95 plus I had ordered additional motor kits to save on postage. Now that it is getting colder it would be a good time to start making a few DC motors for Boy Scouts, Girl Scouts, science clubs or even enter the next Science Fair at your school. Again, it was a great fall day!

Quote for the Day: “Do what you love and what you love will reward you.” — Dave Martin



Last time we were left wondering why the first motor I had built was not working. The instructions were reviewed twice before going to bed. The next day after school I was back at it again and started to troubleshoot or figure out why the motor wasn’t running. First, I checked the power source, we called them dry cells in the day, today we call them batteries. At that time we didn’t have a sophisticated battery checker from Radio Shack like we have today. In order to check the dry cells, I made a simple electromagnet and wired it to the posts on the dry cell. We wanted to be sure the cell was not worn out. Sure enough after wiring the electromagnet, it worked and it picked up a nail and held it tightly to the end of the electromagnet. The dry cell was working.

Next, I had to check the armature to be sure it was working. The armature is a lot like an electromagnet but it uses what is called a commutator to help switch the circuit. It is a little hard to figure out if it works or not. The best idea I came up with was to get two other wires and wire each to a different post on the dry cell. Then I would take the opposite ends of the wires and touch the two tin plates on the commutator. These two tin plates had the wires from the armature coils held in place underneath each plate. The thought was that when I touched the wires from the dry cell to the tin plates on the commutator, the armature would become like an electromagnet and nails or paper clips would be attracted to either coil. So, after making two wires about a foot long each and gently removing the plastic insulation, I attached them to the dry cell posts. Then, I pressed them on each of the tin plates. Sadly nothing happened – no reaction from a loose nail or paper clips. The good news is that I knew I was getting close to figuring out the problem.

Since this was my first motor I didn’t understand about “tight” connections to create a circuit. Also, I had not learned to solder, however, I knew that this was my problem. The excitement of figuring out the problem was energizing. After looking at the diagram of the tin strips, I thought it might be good if I would take the wires from the armature and bend them such that I would have a larger surface for the brushes to touch and not use the tin plates at all. I replaced the two tin plates by taking off the adhesive tape and then bent the two wires from the armature windings and left a little space between both wires. Next, it was time to put adhesive tape back on the commutator wires.

Once again, I tested the “new” commutator to see if it would pick up a nail or paper clips. If my suspicions were correct, this new method would make a circuit and the armature would act as an electromagnet. The moment of truth had arrived – will this work or not? I took the two wires attached to the dry cell posts and touched the two wires on the commutator. Yes, the circuit worked and the nail moved toward the armature and the paper clip moved toward the coil on the other side of the armature. The troubleshooting had worked!

All that remained was to assemble “The Tin-Can Wonder.” I was moving faster now because I could see the motor turning in my mind’s eye. Once again, the crucial moment arrived. When I wired the last wire to the dry cell post I saw the armature begin to move and then it stopped. Faster than swatting a fly, I gave that armature a spin and the motor took off humming! What a beautiful sight, what a relief – my first motor actually worked. Since that first motor I’ve never lost that same feeling of “eureka” when the motor turns for the very first time.

What did I learn through this? It is important to check your batteries with a battery tester or a multimeter (a device that can measure direct current and help you to decide if you have a created a circuit). The multimeter can help ensure you have at least 1.2 volts DC available. Secondly, be sure you follow directions and wind your wire correctly. In a few of my motors you have to remember to wrap the wire the same way when you move from one armature coil to the next. Third, it is important to have tight connections to complete the circuit. Fourth, the brushes should gently touch the commutator plates. Granted, the solution I came up with would not last very long in a production environment because the wires would get worn down and break off. But for this first motor build, it worked to great satisfaction for the builder and for my brothers, sisters and parents.