Tortoise Control

[LKOrailroad]

The same is posted on MRH so to those on both forums, I apologize for the redundancy. I seek as much input as I can get.

I am very nearly finished cutting sub-roadbed and will be laying track soon. The plan is to build each track "section" on the workbench for transfer to the benchwork once complete. Think of it as building fully functional track work modules off the layout.

To avoid confusion of terms let's call a railroad switch a turnout and an electrical switch a switch.

Starting with the very first section I will need to mount and make functional my Tortoise turnout machines. I want them controlled with the pt# momentary contact pushbuttons I used for my lighting setup. They will be fascia panel mounted with multiple panel/switches for the same turnout in some instances. The fascia panels will have a track diagram with green/yellow LED position indicators. Nothing revolutionary there, huh? BTW there is no desire for DCC turnout control.



I am going to breadboard the schematic below I found on the Internet to make sure it performs as expected. Although I don't totally understand them, I have had great luck building circuits taken from my old copy of CMOS Cookbook (now very tattered and torn from use). I know there is likely many circuit ways to go about this but I tend to favor CMOS because the circuits always seem to work even with my very limited understanding.



In addition to being CMOS, the feature that I really like about this circuit is the defined relay state during power up. This will allow the entire railroad to have a default power up turnout position. Parts from Digikey and circuit boards from Pad2Pad are within budget for this part of the railroad construction so cost is OK.

I invite your comments good and bad about my approach. I don't have to make a firm decision for a couple months or so. Thank you in advance.

[DKS]

If it was me... not sayin' what you're doing is good or bad... but I look at the parts count and the investment of time required to build this sort of controller. I've accomplished the same with a part count of one per Tortoise, at a cost of $2-3 each (surplus pricing). I just use a simple DPDT latching relay. It can be actuated with pushbuttons (or any other momentary contact switch), and it can never lose track of its last set position during power cycles.

A SPDT latching relay would work, but I've found the DPDTs are actually more readily available. If you use a center-tap +/- power supply, you only need one of the two circuits, so you can use the other for panel indicators.

[LKOrailroad]

Is a latching relay what it sounds like it is - every time the coil is energized the contacts alternate position? That sounds to simple to be true. Relay like this? http://www.digikey.com/product-detail/en/D3046/PB1117-ND/1634003

[DKS]

Is a latching relay what it sounds like it is - every time the coil is energized the contacts alternate position? That sounds to simple to be true. Relay like this? http://www.digikey.com/product-detail/en/D3046/PB1117-ND/1634003

Yes, except that's a single-coil latching relay, which alternates position with each press of a single pushbutton. If you want to use two pushbuttons, you need a dual-coil latching relay, like this: http://www.digikey.com/product-detail/en/V23079B1203B301/PB1085-ND/1633971

Here is a schematic, which includes optional LED panel indicators.



Myself, I like to use illuminated pushbuttons. They are more money, but they simplify panel construction. http://www.digikey.com/product-detail/en/RP3508BBLKREDREDNS/EG2543-ND/379007

[LKOrailroad]

OK, I am getting a grip. Thanks to your drawing (much appreciated) and doing some Internet reading I think I have an understanding of latching relays. I see some issues:

1. Two push buttons are not desired. I want one SPST momentary push button to toggle the Tortoise.
2. Single coil latching relays require the coil polarity to reverse, something not practical with a single push button. DPDT on/off-off/on switches are expensive and don't appear to be available in my switch style.
3. The only single coil, latch/unlatch on same coil polarity relays I can find are the mechanical type. They are too big and too expensive.

Am I missing something? 

[DKS]

I don't think you're missing anything; I missed the point that you wanted a single pushbutton to toggle the turnout position. (Just out of curiosity, how come? Seems kind of counter-intuitive, but maybe that's just me.)

As an alternative, this would do the trick, with no need for any electronics; might be able to find them from a surplus supply for less: http://www.digikey.com/product-detail/en/MB2085SS1W01-CA/360-2177-ND/1014873

[LKOrailroad]

Thanks Dave for hanging in there with me.

To your first question... brace yourself, this is one of my quirks showing through. I want all the switches on the layout, regardless of their function, to be the same style and size as the lighting push buttons. It's a Monk thing 

I used a similar kind of switch in another project a long time ago. http://www.radioshack.com/product/index.jsp?productId=2062542. While it fits the bill electrically, it was clunky in its operation. I know I sound picky, and probably am,  but the pressure to operate the Radio Shack switch was considerably greater than I want. The momentary push buttons on my lighting control require only a very light touch to actuate. I like that. The data sheet of the switch you directed me to indicates the alternate action version requires less actuating force than the standard momentary version. That is very interesting. This evening I am going to get the brand & part number of my momentary switches and see if their data sheet specifies actuation pressure so I have something to compare to. I have no sense of what 7.06 newtons of force feels like.

Thanks much for finding the switch.

[LKOrailroad]

Dave, I am going to be real eccentric here. Remembering I have the PDF sales receipt from ordering the push buttons I found the part number and their data sheet. The momentary push buttons require 150 grams of force to actuate. The on/off-off/on push button requires 7.06 newtons which is equal to 713 grams of force. As I suspected the push on/push off switch requires much more pressure. Yeah, I am anal about this I know but I really like the action of the momentary switches.

[DKS]

Well, there you go, can't argue with the boss. I'm sure you realize that you'll be paying for that über-consistency--by the cost of the materials and/or the time required to achieve the end result.

Out of curiosity, that photo of your lighting control panel has two rows of pushbuttons. Just wondering what the lower row does...

[railnerd]

FWIW, I'm quite partial to this board here— works quite well on a friend's Free-moN module.

http://home.cogeco.ca/~rpaisley4/556Stall08.html

I've also found latching pushbutton toggle switches in the local electronics supply places out here in Silly-Con valley for about $3.50.

-Dave

[LKOrailroad]

I earlier had looked at this circuit. The missing part is the automatic default turnout positioning upon railroad power up. Otherwise, it is perfect and much simpler.

[bdennis]

Wouldnt the circuit have a default power up position? I cant imgaine that it would be left floating until the button is pressed?

[LKOrailroad]

I certainly no very little about electronics but I would think the power up state would be determined by the charging rate of the two 0.47uF capacitors. I suppose one could increase the value of one of the caps to ensure the other charged faster but then again I couldn't say without testing the circuit.

[railnerd]

Wouldnt the circuit have a default power up position? I cant imgaine that it would be left floating until the button is pressed?

Yep.  In practice, circuit will force a turnout "normal" on power-up— or loss of power.  That second bit is actually the downside of using the circuit on a modular layout: power loss on the aux bus causes all turnouts to "normalize".

-Dave

[railnerd]

I certainly no very little about electronics but I would think the power up state would be determined by the charging rate of the two 0.47uF capacitors. I suppose one could increase the value of one of the caps to ensure the other charged faster but then again I couldn't say without testing the circuit.

Yep, on power-up, a Cap will look like a dead short.  Having built two of these boards, I can say that they do stabilize just fine.