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Author Topic: Micro Switchery  (Read 916 times)

GG

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Micro Switchery
« on: April 01, 2020, 05:49:57 pm »

I have to confess to a mild aversion to "electronic solutions" when a simple electro-mechanical method is available.  If only because in the event of failure a piece of electronics tend to just sit there giving you no clue as to why it isn't behaving itself, whereas electro-mechanical problems can often be solved with little more than a voltmeter and thinking about it.  The latter is however something that doesn't come naturally to a significant fraction of the population.


For many simple electrical circuits that need switching either ON/OFF or diverting, the humble "Micro-switch" has a lot going for it.  What follows is not intended to be technically correct in all aspects but good enough for the average person to understand how these switches work and how they can be used.  At this point the pedants could perhaps find something else to use their talents upon.


A micro-switch is a box with three external contacts and a small operating rod on one side.  Push the rod into the switch body and you ought to hear a small click, release it and a second click should be heard.  What is happening inside the switch is shown in simplified form in Fig 1.


A metal conductor which is connected to Contact 1 is normally pressed by a spring firmly against Contact 2.  Push the rod in and it overcomes the spring and breaks the connection with Contact 2 and makes it with the third contact, Fig 2.


Thus, in the original state, an electrical current can flow from Contact 1 to 2 but not 3.  Press the rod in and the connection with Contact 2 is broken an current flows from 1 to 3.


The actual construction is more complex than that and an "over-center" action is employed rather than a simple spring.  Push the rod in slowly and nothing happens until the connection to Contact 2 is broken and immediately made with Contact 3.  Interestingly if the rod is allowed to move slowly out, nothing happens until the connection is broken with Contact 3 and immediately remade with 2. This is an example of "hysteresis"  in that once contact is made, any slight movement of the rod will not break this positive connection so current will flow from Contact 1 to 2 or 3 with no hesitation.  Unlike simple mechanical switches which can make poor or intermittent contact (like our kitchen radio does sometimes!).


In practice the "proper" terms for these features of a micro-switch are shown in Fig 3.  The Common connection is so called because it will go to either of the other two connections.  The connection that is normally in contact with the Common is called Normally Closed since a switch that allows an electrical current to flow is "closed".  The other connection is, not unreasonably, called Normally Open.

Next - how to use them (short break 'cos I'm hungry)

Glynn Guest



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GG

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Re: Micro Switchery
« Reply #1 on: April 01, 2020, 08:20:23 pm »

How to use the damn things?


The simplest way would be to just switch something ON or OFF at will, Fig 4. Connecting the device you plan to operate, lights, sound or whatever. Via the micro-switches Common (C) and Normally Open (NO) connections to the battery would do this.  Should you want to switch something off on command then just use the Normally Closed (NC) connection rather than the NO.  Pushing the button will now break the circuit and stop the lights, sound or whatever.


A variant on this can be whenever you need to operate one device or another but not at the same time, Fig 5).  Do note that either device 1 or 2 will be on at any given time.


Using two micro-switches can offer interesting possibilities.  A electric motor can be controlled to give forward-stop-reverse on command using the circuit shown in Fig 6. Both NO connections being connected to the battery positive whilst both NC connections go to the battery negative.  The motor, which has to be brushed, is connected to the micro-switches C connections.


Without operating either micro-switch, both wires from the motor are connected to the battery negative so the motor just sits there motionless.  Now if the button on the left-hand micro-switch is pressed, the C connection makes contact with the NO contact and hence the battery positive.  So now the current can flow from the battery positive through the left hand micro-switch NO to its C connections, from left to right through the motor and then the C to NC connections in right-hand micro-switch and finally to the battery negative.  (Just for the "picky".... I'm using the concept of "Conventional Current" which is defined as flowing from positive to negative when we all know in our models, the electric current is actually negatively charged electrons flowing the other way!)


The clever thing is that were it the other micro-switch button that was operated then the current would flow from right to left through the motor i.e. the motor would run in the opposite direction.  So, by operating one micro-switch we can make the motor run clockwise, operate the other micro-switch and it runs anticlockwise, operate neither and the motor doesn't run.


True, with no intermediate speeds between full ahead and full astern, it is not perhaps the best way to operate the drive motor of a model boat.  But, adequate for many auxiliary functions, it's always been used to operate bow-thrusters in my models.  Only requiring a gentle moment from the bow-thrusters means that the lack of fine motor control was never a problem.


Switching on one or another device can be achieved with a couple of micro-switches as shown in Fig 7. More micro-switches and devices could be added but things would be getting complex but where reliable operation without trying to remember how many times you have to waggle a transmitter stick, it has some merit.


Glynn Guest
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GG

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Re: Micro Switchery
« Reply #2 on: April 01, 2020, 09:22:58 pm »

Agh....!
Just tried to post last part only to find "file too large".
Hit the back button and found, well couldn't find what I had spent an hour typing, guess its lost forever.
Ah well, try to morrow maybe?
Glynn Guest
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Colin Bishop

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Re: Micro Switchery
« Reply #3 on: April 01, 2020, 09:27:06 pm »

Glynn,

If you are making a long post then draft it out on your word processor first then cut and paste it into Mayhem. That way you won't lose it from a Forum glitch. Quite a few of us have learned that particular lesson the hard way.

Colin
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Capt Podge

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Re: Micro Switchery
« Reply #4 on: April 01, 2020, 09:42:54 pm »

Thank you for taking the time to explain this Glynn, now I can understand where I've been going wrong!
I'll be using this as my guide for future projects  :-))


Regards,
Ray.
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GG

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Re: Micro Switchery
« Reply #5 on: April 02, 2020, 10:42:11 am »

Colin,
      Yes, I could "cut and paste" but this would excuse the designer(s) of this system (not Martyn I quickly add for those who cannot tell the difference) for incorporating a serious flaw, presumably not testing it and, as I cannot believe it hasn't been reported before, falling to correct it.  Perhaps "Simple Machines" should be renamed "Stupid Machines".
Enough on this sore topic, back to Micro-switches.


Glynn Guest
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GG

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Re: Micro Switchery
« Reply #6 on: April 02, 2020, 11:36:31 am »

How to install the damn things,


Firstly, micro-switches can come in many shapes and sizes.  I like to use the "V3" type which is a convenient size and had contacts that can accept spade connectors, Photo1. You will notice that one switch has a simple button to operate it, the other two have levers (one with a roller) to act on the button.  Levers just give you a little more to press against.


The C, NC and NO connections are usually marked on the switch body which is handy.  They may also be described as suitable for 15 Amps and 250 Volts AC (Alternating Current).  Now AC is easier for mechanical contacts to cope with as it cycles through zero Volts and reduces the chance of arcing as the contacts open and close.  But DC (Direct Currents) as favored in our models will support arcing across small air gaps. But, with the modest currents and voltages used in our models this has never been a problem for me.  Manufacturers data suggests that these switches can have an operating life of millions of cycles, oh that we should live that long to test it..! 



You could operate the micro-switches remotely but I prefer to place them directly on a servo.  Servos are now relatively cheap so you might be tempted to fit switches permanently to them.  But, for a little extra effort I usually make a detachable servo mount.  It makes any trouble shooting much easier and, as we all know, any inaccessible installation just attracts the attention of any passing malevolent Gods!


The micro-switches are bolted onto a rigid plate which has a hole made to match the servos case around the output shaft, Photo 2.  The bolts allow small adjustments to be made to ensure positive operation of the switch buttons/levers. An aluminium saddle is glued to the underside of this plate so that it will sit securely on top of the servo, Photo 3.  In this example the solid plastic disc that accompanied the servo was used to secure the switch plate on the servo, Photo 4.  Part of the disc was cut away to clear the rollers on the switches levers when the servo was at its central position.  Rotating the disc either way would operate one switch alone.


Using the V3 micro-switches  and spade connectors allows you to "program" them to operate single or multiple devices or a motor which I find to be convenient. Photo 5 shows one switch unit installed in a model to operate a discrete bow-thruster in a model.  I say "discrete" 'cos the vessel the model was based on never featured a bow-thruster but was notorious for its poor steering.


Hopefully this has encouraged a few to consider the simple micro-switch.  Cheap, reliable, easy to "trouble shoot" and, unlike some electronic equivalents, not upset when connected the wrong way around to battery terminals.


They also have potential use as "limit switches" to halt the movement of working functions on scale models.  Things like gun turrets, cranes, opening bow doors and such like.  But, that is for another thread maybe?


Glynn Guest
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Terry

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Re: Micro Switchery
« Reply #7 on: April 02, 2020, 11:48:25 am »

Many thanks Glynn. Very informative and superb timing. I have been scratching my head for the last few days over how to control a bow thruster motor using the K.I.S.S. principle, you solved it for me, brilliant.
Cheers, Terry.
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GG

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Re: Micro Switchery
« Reply #8 on: April 02, 2020, 02:43:16 pm »

Terry,
       You really should thank Martin since without his efforts such a good Internet site for we model boaters would not exist.
As for me, I'm just going through the stacks of stuff that's in my files and posting what might have some value.
Regards,
        Glynn
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