Difficulty level: 2 (simple, but requires the use of a soldering iron)
Kits covered: Kits #5, 8-10

Motor, assembled from the kit #5

The reed switch motor is one of the simplest motors and works very well under low voltages. However, on high voltages a spark may appear between the reed switch contacts. This significantly reduces the lifetime of the motor. A spark is created because the reed switch is connected directly to the electromagnet, which is an inductive load. Size of the spark depends on the diameter and amount of wire in the coil.

There are several ways to address this problem. Usually the best results are achieved by separating the reed switch from the inductive load. The motor described on this page uses a transistor for this purpose. You may take a look at how easy it is to assemble this motor from the kit.

A transistor is a small electronic device invented in 1947 by Shockley, Bardeen, and Brattain. Almost all of the transistors today are made from silicon. Pure silicon does not conduct electricity. Therefore boron atoms are added to silicon to create a P-type semiconductor (positive), and phosphorus atoms are added to silicon to create an N-type semiconductor (negative). A transistor consists of a stack of three layers. The arrangement, PNP or NPN, determines which way the current flows.

NPN and PNP transistors

Transistors have three leads, known as base (B), emitter (E), and collector (C). A very small emitter-base current allows a much larger collector-emitter current to flow. Therefore transistors may amplify the signal, or act as a switch. The switching ability of PNP power transistors is used in these motors, but NPN transistors may be used as well.

This is how power transistor looks like:

PNP power Darlington transistor

It is important to select the transistor with maximum electrical ratings significantly higher than the electrical consumption of the motor. For example, the motors described on this site may experience peak current over 1 A at 6 V. Usually this happens when the rotor is stalled and the transistor is open. You should not leave the motor in this state as it may destroy the transistor. When the rotor spins with a proper speed the transistor turns on and off, and the average current flowing through the transistor is about 10 times smaller.

If the collector current through the transistor is high, the transistor may get extremely hot and burn the fingers if you touch it. A heat sink (a special metal piece attached to the transistor) could be used to dissipate heat. Normally, the motors described on this site do not need it, but you may still add a heat sink if necessary.

Some motors require the transistors that are more sensitive. For that purpose certain kits use so called Darlington transistors. Darlington transistors are actually two transistors in one case where the second and more powerful transistor amplifies the signal from the first transistor. Darlington transistors look the same as regular transistors.

From the engineering point of view (according to different electrical diagrams found in books on electronics) some of the transistor circuits shown on this site may require additional elements, such as a resistor, connected to the base of the transistor (which limits the base current); a protecting diode, connected to the inductive load; etc. The designs on this site represent the most simple circuits. However, all of them were tested extensively and proved to be very reliable and worked very well.

During the experiments Stan found that one additional element helped to prolong the life of the transistor motor. It is a capacitor connected to the ends of the electromagnet. The capacitor helped to smooth the voltage spikes that occur at the moment when the contacts of the reed switch connect or disconnect. It is not required to attach this capacitor for the motor to work.

You can find additional information about transistors and their usage in books about electronics and on the Internet (see Links).

This is how the motor works:

  1. When magnet #1 gets close to the reed switch, the two contacts inside the glass tube get magnetized and touch each other. A small current flows through the base of the transistor. The transistor opens, and allows a bigger collector-emitter current to flow through the electromagnet. The electromagnet pushes magnet #3 away.

    Reed switch motor with transistor - diagram 1

  2. When the rotor spins away, the reed switch demagnetizes and the contacts move back to their original position. Since there is no more current flowing through the base, the transistor turns off. This disables the electromagnet.

    Reed switch motor with transistor - diagram 2

  3. The rotor continues to spin due to inertia until magnet #2 gets in working range of the reed switch. It becomes magnetized again and its contacts connect together. The transistor opens and allows a current to flow between the collector and emitter. The electromagnet turns on, and pushes magnet #4 away. This process continues until the power is disconnected.

    Reed switch motor with transistor - diagram 3

New! Watch How Reed Switch Motor With Transistor Works video on our YouTube channel.

This motor can be built from the Kits #5 and #8-10. If you decide to design this motor yourself, you may order only the parts you need (transistor, reed switch, magnet wire, magnets, heat sink).

You may add a speed control unit to this motor. It will allow you to control and change the speed of the motor from a complete stop to maximum speed, which may be 10-25% greater than the normal speed of the motor. The speed control unit is described at the How It Works: Reed Switch Motor section.

10 Comments

  1. Quite interesting and beneficial ideas. Saved to fav .

    Thank you for tthe great read.

  2. i saw youtube video,you can use capacitor instead powersupply

    • Theoretically you can use a capacitor. However this motor (as any other motor) draws some significant current. In order to let it run for a certain time the capacitor, unless it is really big, cannot store enough energy.

  3. I would like to know what voltage and amps are transfered through the magnet. switch to the transister and what volts and amps finally power the coil via the transister. I have made a few brushed motors without transistors, and have an interest in building brushless motors using transistors I will.probably purchase some components fro
    You..Thanks : Al

    • Voltage depends on the power supply (the battery in most cases) you use. We experimented with voltages as high as 30 Volts, but recommend 6-12 Volts. The current through the coil normally does not exceed 200 mA though peak starting current may go up to more than 1 A. That is why we use power Darlington transistors with max parameters exceeding these numbers.

  4. What is the item attached to the reed switch on your motor, it looks like diode?? Also is the curreny through the magnet switch less.then at the coil to prevent damage to the switch?? Thanks for your advise!! Al

    • This is not a diode, it is ZNR. All that explained in “How it works” section. You are correct, the main purpose of using a transistor is to reduce the current through the reed switch

  5. I was wondering why you recomend using the PNP trans. Over the NPN unit..Also does the NPN require a small power source in addition to the main power source, as I am new to transistor usage and have seen videos indicating that a seperate small power source was required to switch unit on?? I know you need a switch of course to turn on power, but am confused as why the NPN needs this small power to turn unit on, I don’t see that requirement with the PNP unit. I am going to order some items from you when I understand these transformers a little better..Thanks again; Al

    I

    • NPN transistor could be used but it requires changes in the schematics and we do not have that circuit diagram any more. The transistors do not need a separate small power source.


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