Control with Meccano
The Snap Action Switch is actually a push button switch in which the button is activated by lever action. The term “Micro Switch” is the name of the company which first manufactured this switch and is frequently used in describing this particular type of switch. The value of this switch in a model is the small amount of pressure which is required for the switch action
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.
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
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.
Mounted Snap Action Switches
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
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 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.
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 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.
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
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.
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
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.
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
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
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
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).
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.
review of the literature will suggest many uses of these Electrikit components
to the modeler.
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.
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