Recreating a Facing Point Lock via electronic locking...

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John Donnelly
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Recreating a Facing Point Lock via electronic locking...

Postby John Donnelly » Tue Jun 15, 2021 10:27 am

I'm hoping I can explain the issue I'm having as it is one of those things that, so far, has proved far easier to show than to tell...

A couple of bits of information to start that are fixed:

1. I'm using servos to move the turnouts and those servos are controlled via MERG SERVO4 boards. For those not familiar with these, they work on the basis that, if there is power, the servo moves in one direction (on) and, when power is removed, they move in the other (off).

2. I'm using DCC Concepts Cobalt S levers connected to the servo boards. Each lever contains 3 switches (an SPDT momentary contact, and two 'break before make' SPDT On-On switches which, depending on how you wire them act as either on-on or on-off).

My current layout has 5 facing point locks and I would like to recreate their use in my lever frame if I can. My initial thoughts have worked on the basis that if I have two levers (one for the FPL and one for the turnout) I can wire them in series so that the turnout lever only gets power if the FPL lever is on which is what I have done so far so the one of the on-off switches controls the servo and another on-off switch on the FPL lever controls power to the turnout lever. So, very simply, the wiring looks like this:

Image

In the photos that follow, it is only the two right hand levers that we are looking at. In the first photo, the FPL is 'locked' and, because the lever is in the off position, there is no power to the turnout lever next to it and the turnout cannot be operated (I know that the lever should be mechanically locked at this point but that is not an option):

Image

I then move the lever to 'unlock' the FPL, so power now goes through to the black lever and moving it, in turn moves the servo and the the turnout:

Image

So far, happy days as the turnout only works if the FPL is unlocked but this is where I now have the problem.

To lock the turnout in it's new position, I now need to lock the FPL again but the problem is that, when I move the FPL lever back to its locked position, it turns the power to the turnout lever off and the servo control reads this and returns the servo to it's 'off' position and the turnout moves when I don't want it to...

So, I need to find a way to return the FPL lever such that it removes power from the turnout lever without it affecting the servo.

As mentioned, I do appreciate that this locking would be done mechanically but that isn't an option with the levers I'm using. My knowledge of electronics is somewhat limited so I may be asking for the impossible but I'd really like to find a way of doing it electronically as a working lever frame has always been one of the things I really want for the layout...

Hopefully the above ramblings make sense...

John

Tony Wilkins
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Re: Recreating a Facing Point Lock via electronic locking...

Postby Tony Wilkins » Tue Jun 15, 2021 11:34 am

Hi John.

I'm sure it's electrically possible. I have had similar thoughts as I have three FPLs on my layout, although I shall ultimately be going down the mechanical locking route. Although I am using Cobalt point motors, they are controlled by relays that have an on / off input, so essentially the same problem. The basic problem is that you need to lock the point one way with the power off and the other way with the power on. I will give it some thought and see what I can come up with unless someone beats me to a solution.

Regards
Tony.
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Re: Recreating a Facing Point Lock via electronic locking...

Postby grovenor-2685 » Tue Jun 15, 2021 11:48 am

We have an already developed circuit within MERG to convert a servo4 to work with pulses for both directions of movement, that would probably be the simplest way.
I'll look up the relevant info later.
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Keith
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Simon_S
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Re: Recreating a Facing Point Lock via electronic locking...

Postby Simon_S » Tue Jun 15, 2021 12:13 pm

I think you need a Gated D Latch. This is a logic circuit where the output (to the servo) equals the input (point lever) when an enable input (FPL) is on, but is latched to the previous input when enable is off.

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Re: Recreating a Facing Point Lock via electronic locking...

Postby grovenor-2685 » Tue Jun 15, 2021 1:00 pm

MERG kit PMP27 does this, each deals with 4 servo controls which can be on different Servo4s so use them for the facing points that need locking both ways and rewire the levers for those points to use the SPDT momentary contacts, still use the FPL lever to supply power to the point lever, then locked there will be no pulses, no pulses point won't change.
Since you are a MERG member you can buy the kits, the kit sales are back open now as Paul is back from holiday.
Regards
Keith
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Re: Recreating a Facing Point Lock via electronic locking...

Postby Tony Wilkins » Tue Jun 15, 2021 1:11 pm

Hi John.
Two suggestions already, here is a third.
FPL circuit.JPG

It relies on using a latching relay to hold the last setting.
Latching relays have many uses and there are different varieties. A double coil version is used here labelled Set and Reset.
The coils are polarity sensitive unlike conventional relays and in applications where reverse polarity is used the suppression diodes cannot be fitted.
The relay can only be changed by the point lever when the FLP lever is unlocked.

Regards
Tony.
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Re: Recreating a Facing Point Lock via electronic locking...

Postby John Donnelly » Tue Jun 15, 2021 1:26 pm

Thank you all for the suggestions, they are much appreciated.

John

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Re: Recreating a Facing Point Lock via electronic locking...

Postby John Donnelly » Tue Jun 15, 2021 4:39 pm

Thanks again for the suggestions. I've decided, as I'm already invested in the infrastructure, to go with the MERG modules recommended by Keith.

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Re: Recreating a Facing Point Lock via electronic locking...

Postby andrewnummelin » Wed Jun 16, 2021 11:06 am

Having had problems with a somewhat different, but related, set-up, I wonder if you may run into trouble when powering up or down the whole system. Do check the logic of all circuits to prevent things going wrong if you, for example, shut down the whole system with some levers not in their normal position and then you return the levers to normal and power up the system.
In my case the problem arose with a crossover where the motors are both on the same side of the track - so one is always in "pulled" position while the other is "pushed" - powering down resulted in no motor movement but both electronic switches for supply to the track returned to their normal positions. It took me a long time to discover the cause of a short circuit when running a train with power to the point motors off... Powering up the point motor system also does not return the one switch to its correct setting so it proved necessary to reverse and return to normal every point - a nuisance on a small layout but it would be a real pain on a large one. I'm convinced I also had a case where it was one point motor that moved on powering down and which did not reset on powering up - no short circuit, just a derailment! (Not fun with a scissors crossover/double slip combination in a difficult to access location - I resorted to changing to stall point motors incorporating mechanical switches.)
Regards,

Andrew Nummelin

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Re: Recreating a Facing Point Lock via electronic locking...

Postby Tony Wilkins » Wed Jun 16, 2021 11:10 am

John Donnelly wrote:Thanks again for the suggestions. I've decided, as I'm already invested in the infrastructure, to go with the MERG modules recommended by Keith.

Hi John.
Given your situation, I would have chosen that option too.
Regards
Tony.
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John Donnelly
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Re: Recreating a Facing Point Lock via electronic locking...

Postby John Donnelly » Wed Jun 16, 2021 3:09 pm

andrewnummelin wrote:Having had problems with a somewhat different, but related, set-up, I wonder if you may run into trouble when powering up or down the whole system. Do check the logic of all circuits to prevent things going wrong if you, for example, shut down the whole system with some levers not in their normal position and then you return the levers to normal and power up the system.
In my case the problem arose with a crossover where the motors are both on the same side of the track - so one is always in "pulled" position while the other is "pushed" - powering down resulted in no motor movement but both electronic switches for supply to the track returned to their normal positions. It took me a long time to discover the cause of a short circuit when running a train with power to the point motors off... Powering up the point motor system also does not return the one switch to its correct setting so it proved necessary to reverse and return to normal every point - a nuisance on a small layout but it would be a real pain on a large one. I'm convinced I also had a case where it was one point motor that moved on powering down and which did not reset on powering up - no short circuit, just a derailment! (Not fun with a scissors crossover/double slip combination in a difficult to access location - I resorted to changing to stall point motors incorporating mechanical switches.)


You raise a very valid point Andrew. So far, with about 8 servos wired up, I've had no issues when powering up or down although another layout I'm involved in that uses servos with MERG components does have an issue with servos twitching when the power is switched on although it uses CBUS to send commands which I'm not doing.
Last edited by John Donnelly on Thu Jun 17, 2021 8:41 am, edited 1 time in total.

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Tim V
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Re: Recreating a Facing Point Lock via electronic locking...

Postby Tim V » Wed Jun 16, 2021 4:40 pm

I'm glad I used mechanical locking on my frame ...

By the way, I spot Scotchlocks (the blue connectors), I hope these are temporary?
Tim V
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John Donnelly
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Re: Recreating a Facing Point Lock via electronic locking...

Postby John Donnelly » Wed Jun 16, 2021 6:51 pm

Tim V wrote:By the way, I spot Scotchlocks (the blue connectors), I hope these are temporary?


Very much so, the whole frame is temporary just to get things working for now.

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Re: Recreating a Facing Point Lock via electronic locking...

Postby John Donnelly » Tue Mar 28, 2023 9:33 am

grovenor-2685 wrote:MERG kit PMP27 does this, each deals with 4 servo controls which can be on different Servo4s so use them for the facing points that need locking both ways and rewire the levers for those points to use the SPDT momentary contacts, still use the FPL lever to supply power to the point lever, then locked there will be no pulses, no pulses point won't change.
Since you are a MERG member you can buy the kits, the kit sales are back open now as Paul is back from holiday.


Just to revisit this, I finally got the PMP27 installed on Saturday and I'm pleased to say that it does exactly what I wanted so thanks again to Keith.

Whilst mechanical locking would, of course, have been more prototypical it wasn't an option given my choice of levers but at least I can now replicate it electronically.

John

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Re: Recreating a Facing Point Lock via electronic locking...

Postby Dave K » Thu Mar 30, 2023 3:57 pm

grovenor-2685 wrote:MERG kit PMP27 does this, each deals with 4 servo controls which can be on different Servo4s so use them for the facing points that need locking both ways and rewire the levers for those points to use the SPDT momentary contacts, still use the FPL lever to supply power to the point lever, then locked there will be no pulses, no pulses point won't change.
Since you are a MERG member you can buy the kits, the kit sales are back open now as Paul is back from holiday.

Keith,
I have used Tortoise point motors and a Society’s Mk 1 lever frame on my layout. Would the MERG kit work with this set up or does it only work if you are using servo to switch the points, if so is there are another MERG kit that would work with Tortoises?

Dave

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Re: Recreating a Facing Point Lock via electronic locking...

Postby grovenor-2685 » Thu Mar 30, 2023 6:31 pm

Dave, the PMP27 essentially converts the pulses intended for solenoids to an on/off signal for a servo driver circuit.
If you want to convert such pulses to work a tortoise I would think the simplest way would be to use a latching relay.
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Keith
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Re: Recreating a Facing Point Lock via electronic locking...

Postby Tony Wilkins » Fri Mar 31, 2023 2:44 pm

Hi Dave. I posted a suitable circuit earlier in this topic.
viewtopic.php?f=4&t=7552&p=96888#p84852
Regards
Tony.
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