Neodymium Magnet Motor

Click Here For New Version

Since writing this article, I found the magnet coil length should be short
relative to the diameter for better performance. A shorter coil with about
the same wire length will provide a stronger field to drive the motor and
also provides a greater signal voltage to trigger the circuit. The updated
circuit located in the lower section of this page, uses fewer parts
(3 transistors) and also draws less battery power. The original version
follows here:

**************************** Original Version ******************************** This project is similar to the swinging pendulum idea using a single magnet on a string. This version uses 2 magnets on a rotating armature to illustrate a single pole motor spinning at about 4 RPS, powered with 3 AA batteries. The motor was built on a 6.5" by 3.5" base with 4.5" 3/8 wooden dowel supports for the armature. The armature is a 5.5 inch 1/4 wooden dowel with 12mm neodymium magnets mounted on each end. The armature bearings were made using a brass washer mounted to the side of the supports. The coil was wound with 1200 turns of #34 wire. Actually I used 800 turns (20 ohms) and then discovered I could increase the force and reduce battery drain by adding 400 more turns of a larger #30 wire (5 ohms more or total of 25 ohms). The exact shape and size for best performance is still a mystery without experimenting, (email if you know how to work that out). In operation, as the magnet approaches the coil, a positive going voltage is seen by the transistor Q1 which drives the collector negative and has no effect. As the magnet passes the center of the coil and begins to move away, the input voltage reverses and moves negative which causes the collector to move positive above the reference at the base of transistor Q3. The two transistors Q2,Q3 form a comparator so as one switches on, the other switches off. The rising voltage at the collector of Q1 causes the comparator to change state which turns on Q4 and in turn drives the output transistor Q5 which supplies the full battery voltage to the coil. This results in the coil pushing the magnet away for about 10 milliseconds. The 10 millisecond pulse width is set with the 12nF cap, 120K resistor, and the input resistance of Q1. Note the polarity of the magnets make a difference and may need to be flipped, or the coil connections reversed. The idle voltage at the collector of Q1 is about 2.5 when the battery voltage is 4.5. The signal gain of the first stage is about 3 and the input signal from the coil is about 300 millivolts, so the output of the first stage moves about 1 volt positive from 2.5 to 3.5 or more. The reference voltage for the comparator (base of Q3) is set to 3 volts which is in the center of the range allowing the switching action to take place when Q1 collector voltage moves about 0.5 volts positive or more. The 4 diodes help regulate the voltage difference (about 500 mV) between the collector of Q1 and the comparator reference as the battery voltage falls. The circuit should operate until the supply voltage falls below 2.5 which would be fairly dead batteries. Battery life is about 100 hours using three 2000maH AA batteries.

*********************** New Version using improved coil ***********************

The new version (shown above) only requires 3 transistors, 6 resistors, one cap and the coil and LED. After doing some research, I found the optimum shape for the coil should be similar to a pancake with a wide diameter and very short height (see picture). The coil measures 9/8 inch diameter, 5/16 inch height and was wound with 1700 turns of #31 enameled copper wire. The coil measures 67 ohms of resistance and 20mH of inductance. The inner core of the coil is a 5/16 by 3/16 inch aluminum dowel. Smaller neodymium magnets were used that measure 1/4 diameter by 1/8 thickness and only cost 25 cents. The signal from the coil when the magnet passes is about 300mV at low speed. It might get up to a volt or more when the motor is running faster. The resistor values were chosen by assuming a nominal transistor gain (hFE) of 60 for Q1 and Q2. Since Q2 drives the 67 ohm coil from a 4.5 volt battery, the collector current will be E/R = 4.5/67 = 67 milliamps. If Q2 has a gain of 60 or more, the base current for Q2 will be 67/60 = 1.1 milliamp which is set by the collector resistor of Q1 which would be 4.5 / .0011 = 4.1K rounded off to 4.3K which is a standard value. The bias resistor for Q1 will be 60 times greater, or about 220K. The 10K resistor in series with the cap is used to reduce the gain somewhat and supress oscillations. I found if this resistor is much less than 1K, the circuit will oscillate. So I chose 10K to be on the safe side. The capacitor was chosen to have a reactance about 10 times less than the 220K bias resistor, or 22K which equates to about 0.47uF. The third transistor Q3 and 3 resistors were added to allow adjustment of the output pulse width for optimum battery life. When the output pulse begins, Q3 will switch on and charge the cap through the 100K pot ending the pulse about 15 milliseconds later. The time can be adjusted with the 100K pot. Notes: When the circuit is energized and the motor stopped, the voltage at the base of Q1 measured 0.66 and the collector measured 0.12 using a 2N3904 transistor. Using a 2N2222A transistor, the base voltage was 0.63 and the collector was 0.102 volts. I imagine most any small signal silicon transistors can be used. When the coil is connected directly across the battery, the magnetic push can be felt at a distance of about 1 inch across the coil. The magnets should be orientated so they push away from the coil and are not attracted to it. I found the armature will move about 90 degrees when the coil is manually energized and the magnet is sitting close to the center of the coil.