Figure 10  The Snap Action Switch – plain end

The end of the lever can be plain or contain a small wheel (roller end) depending upon the intended use of the switch.  The plain end is used where a rubbing or physical contact is required.  The roller end is used when the end of the switch must roll on a surface.  

Figure 11  Snap Action Switch – roller end. 

Several uses suggest themselves for this switch.  The first might be the limit switch in which the switch is used to detect the position of some moving part of the model.  For example, if in a crane model, the motor must be disconnected less it carry the boom beyond a desired position.  The position of the switch can effectively limit the motion of the boom being driven by the motor.  In this application, the switch element would be a break contact. 

In some applications where a continuous back and forth motion is desired, two such switches might be employed with the switches activating a relay which itself will reverse the direction of rotation of the motor.  In this application, the motor will drive the moving part of the model to one extreme until that part activates the limit switch.  This then causes a relay (or other control device) to reverse the polarity of the voltage applied to the motor causing the motor to reverse direction.  In the reverse direction, the same action will occur when the moving part of the model reaches the other end of its motion. 

The limit switch is thus a sensor, a class of devices which will be covered in a later part of this series.  A sensor is a device which senses a physical quality such as position, light, sound, etc. and converts that change into a useful input signal for an electronic circuit. 

The snap action switch finds another usage in sequential control in a complex model.  As will be shown in a later discussion, these types of switches lend themselves to following the profile of a cam or to sensing the presence of raised portions of a drum or barrel.  Such a construction in Meccano will be shown in a later part of this series when the topic is sequential control.  

The mounting of the snap action switch in the model offers little difficulty to the modeler.  Most of these switches have two holes in the body of the switch which can house a machine screw with which the switch can be attached to a Meccano strip or bracket.  Two such mounted switch units are shown in Fig.  12, the smaller one mounted on a #55a and the larger on a perforated strip.   

Figure 12  Mounted Snap Action Switches 

The Magnetic Reed Switch: 

The Magnetic Reed Switch consists of two or more reeds of magnetic material contained in  glass envelope.  The switch is activated when a magnetic field of sufficient force causes the reeds of the switch to become magnetized.  When this magnetic effect is present, the reeds are attracted to each other, thus closing the circuit. 

Figure 13  Magnet Reed Switches. 

Reed Switches are available in a vast variety of shapes and sizes and configurations.  While most have N.O. contacts, some are available as SPDT.  When the reed switch is enclosed in a solenoid (coil or wire), the result is a reed relay since the solenoid is intended to provide the magnetic field.  

Figure 14  Magnetic Reed Switch mounted in coupling. 

Reed Switches are available with their glass envelopes smaller than 0.16” diameter and can thus be placed within a Meccano coupling or collar.  If the set screw is carefully set to just hold the switch and not crush the glass envelope, then the mounting problem is solved.  

The reed switch is somewhat difficult to adjust for proper operation.  Since the metal components of Meccano are themselves magnetic, some shielding of the magnetic force field is to be expected.  Therefore the placement of the switch at first try may not be the optimum position and so some “trial and error” placement is suggested.  

Figure 15  Magnetic Reed Switch action. 

Then too, the orientation of the switch affects its operation.  When placed in position, the glass envelope should be rotated within its holder in order to obtain optimum sensitivity.  Up to a quarter turn is generally all the adjustment that is necessary while testing the switch during the entire turning process.   Fig 15 shows the limit of sensitivity for the reed switch that is being used.  The pot magnet has sufficient strength to close the contacts of the switch at that distance. 

The Mercury Switch: 

The Mercury Switch is a glass enclosure with two contacts at one end.  Within the enclosure is a globule of Mercury which is free to move from one end of the enclosure to the other by gravity.  Thus this switch is sensitive to its position.  As such it has many applications which are of value to the model maker. 

Figure 16  Mercury Switches. 

Some mercury switches have a glass envelope small enough to fit within a Meccano coupling in the same manner as was used with the reed switch.  In such usage, the switch is placed within a coupling and the coupling mounted on a component having a boss.  When so mounted and placed on a shaft, the switch can serve as a limit switch for the rotation of the shaft. 

Figure 17 Mercury Switch in Short Coupling.

The Mercury Switch is generally a simple make-break switch, however there are some which have a SPDT action.  One of these mercury switches is shown in Fig. 16 at the far left. 

In practice, the switch action occurs when the switch is tilted.  If the switch is mounted on a component with hub, it may then be used to detect the rotation of the axle.  

Relay Contacts: 

The relay used in electrical / electronic circuits is simply a set of switch contacts controlled by an electromagnet.  There are two types of relays in general usage, the common relay and the reed relay.  

Figure 18  The Common Relay. 

The common relay is composed of the magnet and an armature which is attracted to the magnet core piece, which causes the contact springs to move either making or breaking contacts.  The reed relay is composed of a magnetic coil in the center of which are one or more magnetic reed switches. 

Figure 19  The Reed Relay. 

The magnetic structure of the relay determines the sensitivity of the unit.  Each relay is specified as the level of current or voltage at which the relay activates.  The relay supplied by Meccano in its Electronics Kit of the 70’s is a common relay having SPDT contacts.  The necessary voltage requirement for the operation of the relay as suggested by the Electronics Manual is 12 volts, however a measurement of the relay coil reveals it has a resistance of 680 ohms and will operate at a voltage as low as 6.5  volts.  

The relay has an interesting characteristic which is termed “hysteresis.”  This means that while the relay requires a level of voltage to activate, once activated the voltage can be decreased and the relay will remain activated down to a certain critical voltage.  These two levels of voltage are called the “Pull-in Voltage” and the “Drop-out Voltage”.  With the Meccano Relay #606, the pull-in voltage is 6.1 volts and the drop-out voltage is 1.4 volts.  When using simple switching to control the relay, this difference in voltages presents no difficulty.  However, when the relay is driven by an electronic circuit, these voltage differences become critical. 

Figure 20  The Meccano Relay #606.

Common relays can have several switch units incorporated in its construction.  The most often encountered are the DPDT and the 4PDT.  The switch contacts are rated in terms of maximum voltage and current just as the manual switches are rated.  When switching inductive loads ( such as a motor), arcing occurs at the switch contacts.  This will eventually burn the contacts causing the contact to become pitted.  This pitting over time will cause the switch contacts to stick or become inoperable.  A diode connected across the motor winding will help eliminate this problem.  This diode is connected in opposite polarity to the voltage operating the motor. 

Electrikit Components: 

The Meccano Electrikit (#4EL) contained many components useful in the making of switching devices.  The manual includes suggested models of selector switches (E1), push button switch (E2), and the DPDT knife switch (E3).  There are also suggested relay models (E14 and E20). 

Many models using the Electrikit components have been described by their originators in Meccano Magazine and particularly in the Model Plans series.  Perhaps, one of the best illustrations of the use of Electrikit Components would be the Automatic Elevator (Model Plans No.58) by Keith Cameron.  This model was produced in 1975 when most models were made with only available  Meccano components.   

A review of the literature will suggest many uses of these Electrikit components to the modeler.

Comments: 

In the next article, more sophisticated switching circuits will be discussed and the way in which they may be used in models.  Included in the discussion will be limit switches, reciprocal action, and sequence action initiated with cams and cylinders.