Circuitron TC-3 Remember Last Set on Power Up?

OK - since no one else has stepped up to the batter's box, I'll take a swing.

I don't have a TC-3 and am approaching it as a electrical question based on published documentation:

Presumably you're referring to the "random" direction on startup as per their instruction sheet.

Based on their description I see 2 options looking at the TC-3 standing alone:

(1A). The key is the latching mechanism is a CMOS logic gate.  It has no memory, but will keep its state indefinitely if kept powered.  So the idea is to modify the circuit to install a large capacitor - maybe 50 cents - that kicks in whenever INPUT voltage is lost.  The backup circuit would have an electronic switch (transistor) so as not to drive the output when on backup power.  This would keep the state for days or weeks until you turn on the layout again.  I'd say less than dozen components, less than $5, but a bunch of soldering, cutting traces on the board, etc.

(1B) Or, to retain the state for weeks or months, you could use a battery instead of a capacitor.

(2) For indefinite memory, replace the CMOS logic gate with an electromechanical latch - maybe $2 or so.

You'd want a dual-coil, DPDT type.  One coil sets the relay, the other coil resets the relay.  These would be driven by the two inputs.  You'd need the DPDT type relay to perform the polarity reversing function.  But the key of course is the state is retained mechanically when electrical power is removed.  There would be a few auxiliary components but again, less than $5 but a bunch of soldering, etc.

However, if looking at the TC-3 as typically used (i.e., mated to a Tortoise), here's yet a 3rd option.

(3) Use the SPDT switch position output on the Tortoise as the memory.  Install a circuit which generates a brief pulse (say 1 second) whenever power is applied to the TC-3.  Use the position indicator switch to steer this startup pulse to the TC-3 input corresponding to the last position.  This then sets the TC-3 to whatever it was.  The circuit could be as simple as a resistor, capacitor, and diode - maybe 50 cents in parts - but again some soldering and messing with small components is required.

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Just some half-baked ideas.  I's need dotting, T's need crossing.

Such a simple circuit, almost makes sense to just roll your own with the provision for retaining the position.  If you need one of these, hacking the board might make sense, but if you need a few of them, then the effort of creating a PCB of your own design makes more sense.  The relay Stan mentions makes the most sense to me, no external power and it'll remember forever.

The Kemet EC2-12TNU is a DPDT 2-coil latching relay that required 12ma coil current.  At $2.18, it's a pretty cheap memory device for this application.  I see Stan pointed to the same relay.  Other than a 12V source, not much else needed.  A few parts, a cheap PCB, and you have the TC-3 with permanent memory.

What he said.  From what I can tell, the TC-3 is still available for sale.  Here's the first one with a photo that popped up in a search: 

Yikes!  About $20 (plus shipping??).   I had to chuckle at the photo; the IC chip has a date-code of 9633 which means it was manufactured in 1996.  That's almost 25 years ago!

Anyway, as GRJ says, I am imagining the $2 relay, a few inexpensive odds-and-ends components, and a $1-2 custom circuit board that someone on OGR would surely volunteer to do for the greater good while shelter-in-place is in effect. 

Stan, you draw the schematic, I'll convert it to a PCB and upload the Gerber files here.

Good information guys.  A bit over my head, but I get the gist of it.

Currently I have two TC-3's.  Building my own would probably be a over my skill level and with a the low quantity would probably cost a pretty penny given minimum buys on most components.  I was hoping for a quick, easy fix such as a jumper or such, but I see that is not the case.  If adding the electro-mechanical latch was both cost effective and something a novice could do, I would be in for that.

But I have to say, Stan2004's option 3) sounds like something I might be able to do.  Yes, this is for the Tortoise Switch machine.  Is there anyway you could sketch up a schematic with components and values?  I would really like to evaluate it.  I already have some 1N4001 diodes and an assortment of resistors coming.  Sounds like I might only need the capacitor and maybe a little board of some type to mount them...

Thanks again everyone.

I found an online picture of the front and back of a TC-3.  There are two terminals labeled "BYPASS" which can make the connections somewhat easier.  So here's a starting point.  The 1N4001 can sub for the 1N4003.  If you are getting a resistor assortment, it will undoubtedly have a 100K value.   A suitable capacitor is 10uF, 35V which is 10-cent part...the value is not that critical and I can advise how to make do with anything in that ballpark.   But if your capacitor cupboard is bare, perhaps you know a local hobbyist or tinkerer who would have an assortment of capacitors - they could throw it at you from 6-feet away!  I'd mail you caps but shipping is like $4-5 which is just not right! 

As diagram shows, you might need to swap the 2-wires from the Tortoise's SPDT position sensor.  This hack will either initialize the TC-3 to the last position (what you want) or the opposite position.  Not shown are your existing connections to the TC-3 which remain intact. 

gunrunnerjohn posted:

Stan, you draw the schematic, I'll convert it to a PCB and upload the Gerber files here.

I don't know if it's worth the bother.  From what I can tell, 99% of the guys prefer a pre-assembled module so would just as soon buy a TC-3 or similar.  OTOH, a DIY'er would just wire up the relay to a 12V DC wall-wart and call it done.  I suppose if there was only Accessory AC, it might be prudent to add a 12V regulator in which case a PCB might save some assembly time; the 78L12 circuit is exactly what the TC-3 uses.  I suppose screw terminals would be handy.

Let's see if there's any interest. 

I don't try to get more than about 20-25 milliamps out of the 78Lxx series, those are the ones that overheat and fall out of the Lionel smoke units with a 30-35 milliamp load on them.   I tend to go with the TO-220 package.

At 7 cents a piece, no argument here!  I was thinking it could all fit in a 1 square inch board which, if memory serves, would be about $1 for a bare-board (at $1/sq.in.)?  Anyway, with a 7812 you could probably even power two Tortoises/relays on a single board that's, say, 1-1/2 sq. in.  Pretty amazing how that relay footprint is less than 1/4 sq. in.

I've a bit leery of the TO-92 regulators, I had several fail just running smoke fan motors.  The actual measured current was only 30ma, and they died.  I decided that unless space was truly a premium, I'd just stick with the heavier duty part.

OSHPark charges $5 sq/in, that gets you three boards, so it ends up being about $1.66 sq/in.

stan2004 posted:

I found an online picture of the front and back of a TC-3.  There are two terminals labeled "BYPASS" which can make the connections somewhat easier.  So here's a starting point.  The 1N4001 can sub for the 1N4003.  If you are getting a resistor assortment, it will undoubtedly have a 100K value.   A suitable capacitor is 10uF, 35V which is 10-cent part...the value is not that critical and I can advise how to make do with anything in that ballpark.   But if your capacitor cupboard is bare, perhaps you know a local hobbyist or tinkerer who would have an assortment of capacitors - they could throw it at you from 6-feet away!  I'd mail you caps but shipping is like $4-5 which is just not right! 

As diagram shows, you might need to swap the 2-wires from the Tortoise's SPDT position sensor.  This hack will either initialize the TC-3 to the last position (what you want) or the opposite position.  Not shown are your existing connections to the TC-3 which remain intact. 

This forum is great.  This is something I can do.  Thanks Stan.

stan2004 posted:

gunrunnerjohn posted:

Stan, you draw the schematic, I'll convert it to a PCB and upload the Gerber files here.

I don't know if it's worth the bother.  From what I can tell, 99% of the guys prefer a pre-assembled module so would just as soon buy a TC-3 or similar.  OTOH, a DIY'er would just wire up the relay to a 12V DC wall-wart and call it done.  I suppose if there was only Accessory AC, it might be prudent to add a 12V regulator in which case a PCB might save some assembly time; the 78L12 circuit is exactly what the TC-3 uses.  I suppose screw terminals would be handy.

 

This too is awesome.  A bit over my head though.  Thanks for all the info.

I have a couple of these that I will try.  A good friend from the club tells me 18 vac to the "C" on the output, a couple of 1N4001 diodes facing opposite directions, one on the "NC" and one on the "NO" outputs, to the tortoise switch motor will do the trick.  I should know in a week or so (diodes are in the mail)...  Price not too bad considering the minimal quantity I need.  We'll see.

That looks to be the same functionality that we were discussing with a custom board, so it certainly should work.

What he said.  I attached the datasheet of the CZH-LABS D-1022A SPDT latching relay module.

So as your friend advises, just add a couple diode and 12V DC power supply

I am impressed that the datasheet explains how it works, nice diagrams, complete schematics, etc.   I found it interesting how they use non-volatile "flash" memory in a microcontroller chip to save the whopping 1-bit of data with the last position of the relay.  So when the module turns on, the microcontroller looks up the last position and drives the relay accordingly.  There will be an ever-so-brief interval, measured in milliseconds, where the relay might be in the incorrect position but surely not long enough to move the Tortoise out of position.

Well, I use the non-vol memory in my Chuff-Generator to save one byte of data, I need somewhere to store the cal value.  Actually, now that I think of it, it's two bytes.

And while on the topic of LATCHING DPDT relays for driving Tortoises, I just looked up and see that MTH still shows the NTE 50 relay for driving a Tortoise using DCS command-control using the TIU-AIU.  

From what I can tell, the NTE 50 relay is no longer in production though I see it can still be purchased if you hunt around:

A little spendy to say the least! 

In addition to the ~$2 Kemet EC2-12TNU discussed above, I found the HongFa HFD2-012-M-L2 12V DPDT dual coil latching relay on eBay, datasheets of both relays attached.  It's physically a tad larger than the Kemet but might save a few bucks.

@gunrunnerjohn posted:

Well, I use the non-vol memory in my Chuff-Generator to save one byte of data, I need somewhere to store the cal value.  Actually, now that I think of it, it's two bytes.

Well, considering the cheapest latching DPDT relay seems to be $2, I suppose an interesting thought exercise would combine the relay-less TC-3 design with the 1-bit memory of a microcontroller chip.

A 8-pin uC with at least 1-bit of EEROM is, what, 50 cents in small quantity?  The uC would have 2 inputs for set and reset, and 2 outputs for motor+ and motor-.  If powered by 12V DC, that would require a 7-cent 5V regulator, a few 5-cent power supply caps, maybe 10-cents of resistors and caps to protect the inputs, and a pair of 6-cent 555 timer chips used for their bi-polar output drive capability to level shift the uC outputs to something higher.  But apples-to-apples it would be less than $1 in parts vs. $2 for a relay.  Still probably want 3 x 2-position screw-terminals (DC+, DC-, set, reset, motor+, motor-).

 No job is so easy as the one you imagine someone else doing...

@stan2004 posted:

What he said.  I attached the datasheet of the CZH-LABS D-1022A SPDT latching relay module.

So as your friend advises, just add a couple diode and 12V DC power supply

I am impressed that the datasheet explains how it works, nice diagrams, complete schematics, etc.   I found it interesting how they use non-volatile "flash" memory in a microcontroller chip to save the whopping 1-bit of data with the last position of the relay.  So when the module turns on, the microcontroller looks up the last position and drives the relay accordingly.  There will be an ever-so-brief interval, measured in milliseconds, where the relay might be in the incorrect position but surely not long enough to move the Tortoise out of position.

Stan,

Last Friday I received the diodes and this weekend I was able to try this out.  It works like a charm.  IMO a cost effective way to control tortoise switch motors with momentary switches and the DCS AIU.

Thanks all for your contributions to this thread.  Much appreciated...

Best regards, Frank