Reed Switch Motor With Transistor

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

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 1A at 6V. 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.
  2. Diagram #1

  3. 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.
  4. Diagram #2

  5. 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.
  6. Diagram #3

This motor can be built from the Kit#5. Complete instructions are provided. However, 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.

The speed control unit, which contains only few additional parts, is included with experimentation kit #1 or #3. Complete assembly instructions describing an easy way to orient the speed control magnet are provided.