There appears to be much interest in the control of electric motors for Meccano Models. Information has been posted to Spanner and articles have appeared in magazines and other literature pertaining to Motor Control with Meccano. This series of articles will be an attempt to bring all such information together, adding some new material which has not been posted in order to present a full discussion of the many ways in which motor control can be accomplished.
The obvious starting point in the total discussion would be the subject of using switches for the Off - ON control of the motor. The discussion will thus treat this subject first, followed by the control of the direction of rotation, and then the control of the speed of the motor. Practical circuits will be included using easily obtained electronic components to control a Meccano M5 motor (Sometimes called the 'Richards" Motor). This is the 6-12 volt motor with the six ratio gear box.
The series will include among other subjects, the sequential control of a motor with mechanical switching circuits and the control of an entire model using a Microcontroller (the Stamp 2).
If for any reason, you wish to contact the author, feel free to do so either through Spanner or by direct e-mail: firstname.lastname@example.org.
Motor Control with Meccano - Part 1, Switches
The OFF - ON control of a motor is a rather elementary operation. It can be achieved by the simple process of connecting and disconnecting the power source. Such a power source can be a battery or even in the case of a power supply, pulling the plug or using the switch on the power supply. But, suppose you desire to have an Off - On control mounted on the model itself. Then one must make a choice of the type of switch to use. There are many such switches available as illustrated by the many types and sizes of switches available in electronic parts catalogues. The types applicable to Meccano models include:
1. The Knife Switch - Old technology, but effective.
2. The Toggle Switch.
3. The Slide Switch.
4. The Rotary Switch
5. The Momentary Contact Switch (Push Button Switch)
6. The Snap Action Switch or Micro Switch.
7. The Magnetic Reed Switch.
8. The Mercury Switch.
9. Relay Contacts.
10. Electrikit Components.
The Knife Switch:
The Knife switch itself was the first effective Off - On device used to control the current in an electric circuit. While no longer widely available in the supply catalogues, there are doubtless many still around and if it suits the model can readily be used for this purpose.
The Knife switch has the disadvantage of having exposed contacts thus making it dangerous at high voltage levels. However, when requiring only 6 to 12 volts as the power source, this danger is minimized. They are, however rather bulky.
Fig 1. Knife Switch
The switch shown in Fig. 1 is a Double Pole, Double Throw switch and is capable of reversing the direction of rotation for a DC motor. This circuit will be discussed later in this series of articles.
The Toggle Switch:
The Toggle switch is a small convenient switch to use for the control of models. The operation of the switch is achieved by flipping the handle of the switch in the desired direction. The name, "Toggle" is derived from the handle remaining in the position to which it is moved. These switches can be obtained in a variety of switch contact configurations. The most elementary is a single "make" contact. That is, when the handle is in one position, the switch is open, while in the other position, the switch is closed. When open, no current can flow through the switch. When closed, current will flow though the switch. Such switches are identified by having only two terminals. They have the electrical designation SPST (single pole, single throw).
Fig. 2 Toggle Switch
Toggle switches are available with many contact configurations. Those with three terminals are called SPDT (single pole, double throw). On occasion, this switch action is termed a "1C" contact. This simply means that it is a transfer switch, transferring the current path from first one contact to the other. With a switch (or other switching device) designated as 2C, it means that two circuits transfer the current path for two circuits simultaneously. If the switch is designated as "N.O.", then it is a normally open contact; "N.C." indicates that it is a normally closed contact. The N.O. can also be called the "A" contact, while the N.C. can also be called the "B" contact. If needed for some specific purpose in a model, toggle switches are available with up to 4C contacts.
There are also toggle switches available which have three positions. These are found in catalogues listed as "on-none-on". When the handle is in the center position, no contact is made to either side, while if the handle is pressed to either end, then contact is made to the terminals on that end of the switch.
All switches have a voltage and current rating. When using them, always be certain that the switch is operated at a lower voltage and current than its rating. Overloading a switch can cause it to fail. There is one caution - motors are magnetic devices and possess a certain amount of inductance. This can cause arcing of the switch contacts when the current is interrupted. The arcing can be minimized by placing a diode in the circuit to prevent the "back emf" from causing this effect. Diagrams (particularly those using electronic devices) used in this series will have such diode protection.
Toggle switches are normally mounted through a hole using the washer and nut supplied with the switch to secure it to the device used. A plate can be made for this purpose using any type of plastic material following the Meccano pattern.
Variations on the Toggle Switch:
There are several variations of the toggle switch which should be included in this discussion. One such switch is the "Rocker Switch" in which a rocker mechanism is used in place of the handle of the toggle switch. The action is similar in that when one end of the rocker is pressed, it stays in that position until the opposite end is pressed. This type of switch requires a square hole for mounting.
Fig 3. Rocker switch
Another variation of the toggle switch is the "Key Switch" or "Lock Switch." A removable key is placed in the lock of the switch and is used to complete the circuit when it is turned. In some models, the key is removable in either position. The key switch is mounted in a round hole. This might be a good solution to keeping an unauthorized person from activating your model.
Fig. 4 Key switch
Yet another variation is the "Dip Switch." This is a switch mechanism mounted in a Dip plug-in. The designation "Dip", refers to the integrated circuit socket called a "dual in-line" and is thus a small unit containing 8 type "A" switch units mounted on the Dip plug-in. This provides for a control of 8 circuits with a small compact unit. Specialized uses of this type switch will be covered in a later part of this series.
Fig 5. DIP switch
The Slide Switch:
The slide switch is similar to the toggle switch in its operation. The major difference is that the slide switch is activated by sliding a small bar in the desired direction. The same switch contacts are available in the slide switch. Mounting the switch is different, however in that a rectangular hole needs to be made in the mounting plate to effectively mount the switch.
Fig 6. Slide Switch
This switch can have two or three positions similar to a toggle switch and can have a variety of switch contacts.
The Momentary Contact Switch (Push Button):
The Momentary Contact Switch, or the Push Button Switch again is similar to the toggle switch in its operation. A similar range of switch contacts are available as in the case of the toggle switch. The major difference is that the switch is activated only a long as the button is depressed. For a short run of the motor, this is possibly desirable. The switch is mounted in the same fashion as the toggle switch with a hole placed in a plate made for the purpose.
Fig 7. Push Button Switch
Momentary contact switches do not necessarily have to be a push button. There are several variations on this switch action that should be mentioned. One of these is the "Foot Switch." As the name would indicate, this switch is activated by pressing the pad with one's foot. This could be an eloquent way of showing the action of a model when describing the model to an
audience. Most of these switches are furnished with a special line plug to control some device working at the household voltage. This plug can be easily removed and the two wires used as the switch leads to control the model.
A larger foot switch is the "Floor Mat." Standing on the mat will activate the switch. Again, this lends itself to a display model which you do not want to run continuously. The viewer when standing on the floor mat will activate the model.
Fig 8 Foot Switch
The Rotary Switch:
The Rotary Switch is a broad classification of switches which can have a variety of switching action. It can even contain momentary contacts. The switch elements are mounted on a plastic wafer with contacts fastened to the shaft so that the turning of the shaft brings contacts on the shaft into contact to those on the plastic wafer on the body of the switch.
Fig. 9 Rotary Switch
Rotary switches found in electronic catalogues have a variety of contacts. At one extreme is a single pole switch with up to 12 contacts. This would be called a SP12T arrangement. The other extreme would be a six pole switch, each pole being a double throw or a 6PDT switch. Many applications suggest themselves and will be included later in this series. This switch is mounted in a hole in a similar manner as the toggle switch.
This ends the first article of the series and will continue in the next
part with a continuation of the discussion of different types of switches.
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