A Festive Puzzle

[AndyID]

For several reasons that I won't go into here I like to use highly modified ("hacked" is such an ugly term ) servos as point motors although this could apply to any small DC motor equipped with some type of reduction gearing. Here's the circuit diagram;

Not a lot to it. The servo's electronics have been removed leaving only the motor and gears. The switch in the turnout motor is a micro-switch. The tricky bit is arranging for the micro-switch to flip only when the tiebar reaches full travel in either direction. (Technically this is a form of hysteresis.) That can be achieved with a "lazy" mechanical connection between the tiebar and whatever is used to change the micro-switch position.

 

Now for the puzzle.

 

Can the micro-switch instead be replaced with two limit switches? If it's possible to do that it would make setup simpler and eliminate the need for the "lazy connection".

 

(Nothing to do with the puzzle but if you do use something like this you can control the turnout motor with a two pole changeover relay mounted at the turnout. The control panel only determines the state of the relay. The second set of relay contacts feed the point frog.

[Huw Griffiths]

43 minutes ago, AndyID said:

Now for the puzzle.

 

Can the micro-switch instead be replaced with two limit switches? If it's possible to do that it would make setup simpler and eliminate the need for the "lazy connection".

 

Two microswitches - with "normally closed" contacts - at either end of the travel?

 

The idea doesn't sound unreasonable.

 

After all, it's not as if it's exactly unknown for microswitches to be used as limit switches.

 

 

 

Edited December 13, 2021 by Huw Griffiths

[AndyID]

12 minutes ago, Huw Griffiths said:

 

Two microswitches - with "normally closed" contacts - at either end of the travel?

 

The idea doesn't sound unreasonable.

 

After all, it's not as if it's exactly unknown for microswitches to be used as limit switches.

 

 

 

 

Thanks! That's what I thought too.

 

(I said it was a puzzle )

[Harlequin]

What you are describing sounds a lot like an MTB MP1 or MP5 point motor. They have micro switches at the end of their travel operated by a cam shaft and can thus be operated by a simple SPDT switch.

 

[AndyID]

27 minutes ago, Harlequin said:

What you are describing sounds a lot like an MTB MP1 or MP5 point motor. They have micro switches at the end of their travel operated by a cam shaft and can thus be operated by a simple SPDT switch.

 

That's interesting. Is there a circuit diagram?

[Harlequin]

I can't find a diagram of the actual device after a quick search but I'm sure it will be very simple. (There are many diagrams showing how to wire them up, of course.)

 

To give you a basic idea of the mechanism, there's a photo of the insides of an MP1 here but it's a bit blurry: https://www.rmweb.co.uk/community/index.php?/topic/128007-point-motor-mp1-and-mp5-anyone-tried-them-yet/&do=findComment&comment=2921056

 

 

Edited December 13, 2021 by Harlequin

[AndyID]

11 hours ago, Harlequin said:

I can't find a diagram of the actual device after a quick search but I'm sure it will be very simple. (There are many diagrams showing how to wire them up, of course.)

 

To give you a basic idea of the mechanism, there's a photo of the insides of an MP1 here but it's a bit blurry: https://www.rmweb.co.uk/community/index.php?/topic/128007-point-motor-mp1-and-mp5-anyone-tried-them-yet/&do=findComment&comment=2921056

 

 

 

I have a version with a cam that only needs a single micro-switch like the diagram I posted. It takes advantage of the hysteresis in the micro-switch (cheaper switches are better as they have more hysteresis).

[Nearholmer]

19 hours ago, AndyID said:

Can the micro-switch instead be replaced with two limit switches?

Yes.

[AndyID]

55 minutes ago, Nearholmer said:

Yes.

 

I don't think so . (I did say it's a puzzle.)

 

Can you show us how to connect them?

 

 

 

Edited December 13, 2021 by AndyID
missing word - again!

[AndyID]

I think this will work but I'll be more convinced when I've seen it working

 

The limit micro-switches flip from normally closed to normally open at the limits. Only one can be in the normally open position at the same time. The normally open connection overrides the limit switch when the reversing switch position is changed to allow the motor to move away from the limit positions.

 

The circuit is a bit more complicated but it has the advantage that the limits can be independently adjusted.

[AndyID]

On 12/12/2021 at 18:36, Huw Griffiths said:

 

Two microswitches - with "normally closed" contacts - at either end of the travel?

 

The idea doesn't sound unreasonable.

 

After all, it's not as if it's exactly unknown for microswitches to be used as limit switches.

 

 

 

 

Hi Huw,

 

The snag with that is how do you get the motor to start moving again when you want it to move in the opposite direction?

 

The version above might solve that problem.

 

Andy

[smokebox]

I think this should work

 

The diodes provide a current path bypassing whichever is the open switch at the start of travel.  The open switch closes as movement begins.  When the other end of travel switch opens at the end of travel, it stops the motor.  The circuit is then ready to run in the opposite direction.

[St Enodoc]

4 hours ago, AndyID said:

I think this will work but I'll be more convinced when I've seen it working

 

The limit micro-switches flip from normally closed to normally open at the limits. Only one can be in the normally open position at the same time. The normally open connection overrides the limit switch when the reversing switch position is changed to allow the motor to move away from the limit positions.

 

The circuit is a bit more complicated but it has the advantage that the limits can be independently adjusted.

That looks similar to the old Fulgurex/Lemaco-style point motors.

[AndyID]

15 hours ago, smokebox said:

I think this should work

 

The diodes provide a current path bypassing whichever is the open switch at the start of travel.  The open switch closes as movement begins.  When the other end of travel switch opens at the end of travel, it stops the motor.  The circuit is then ready to run in the opposite direction.

 

Thank you Smokebox,

 

I think you are correct. The Fulgurex motor mentioned by our antipodean colleague (St Enodoc) also uses diodes

(link here  http://www.clag.org.uk/fulgurex.html )

 

It is configured a bit differently in that current to the motor is always supplied through a diode so that the motor voltage does not change as the contacts change position. That could be an advantage for the very low voltage motors used in servos.

 

I'm trying to avoid using a double-pole, double-throw reversing switch and use a single-pole switch instead, mainly because I'd like mount a DPCO relay at the motor controlled from the panel or some automation. One set of the relay's contacts will control the point motor and the other set will feed the point frog. But I'm probably pushing my luck

 

Andy

[AndyID]

I think I've done enough puzzling. Hopefully this will work for me.

 

A simple off/on switch at the control panel controls a two-pole changeover relay at the point motor. Alternatively the relay could also be controlled by another relay or the output from a digital logic output driver. One set of the relay's contacts are used to select the correct polarity for the point frog. The other set determines the position of the points. L1 and L2 are limit switches that cut power to the servo motor when the points are at full-travel. The 2.5 volt supply is created by splitting the output from a five volt "wall wart".

 

Looking again at the Fulgurex motor I realized it is a bit different. The contacts are not simply "limit switches". To me it looks like it's setup more like a hysteresis change-over switch where the contacts all switch at, or close to, the same time.