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Thread: Playing around with DCC powered things.

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    Default Playing around with DCC powered things.

    I've been meaning to electrify a number of things from DCC track power and finally got the first project off my workbench. My plan is to start with simple, lower component count and current things and work up to some more interesting stuff I have in mind.

    With that, my first attempt is a simple FRED. But of course I want a good FRED, so that means using a "supercapacitor". Basically I don't want any flicker, in fact I'd like several seconds of hold-over ability. But I also don't want a battery, they go bad, have to be changed. The only real solution is a super-capacitor.

    I've finished my prototype. It was made with parts from Amazon.com as that's a great place for assortments. For $10 I got 14 sizes of super-capacitors to play with, which is very cool. The finished circuit is below, and it's on my bench and blinking away. My spreadsheet calculations say the flash should be every 1.08 seconds for 0.18 seconds of duration. I think that's about what a real life FRED does, but honestly I'm not very sure.



    Theory of operation is pretty simple:
    • Rectifier D1 turns the track square wave into DC.
    • Voltage regulator U1 gives me 5v regulated output.
    • SuperCapacitor C3 stores the energy like a rechargeable battery. R3 is a 10ohm resistor to limit inrush current to prevent damage to C3.
    • A good old 555 IC, U3, provides the "blink" function. R1, R2, and C2 control the blink rate -- and current usage!
    • R3 is a current limiter for the FRED LED, which the final would be mounted off-board on the coupler via the FRED+ and FRED- terminals. For testing though, it's D, stuck right in the bread board.


    I found https://www.allaboutcircuits.com/tex...d-led-flasher/ a very valuable resource in configuring the 555. Once I understood the theory, I made a spreadsheet to calculate R1, R2, and C2. The values are based on items in my parts bin though, I can probably tweak a bit more by using values I don't have on hand.

    And, it all works. And runs nicely for 20+ seconds when unplugged.

    But, it's way too big. The smallest super-capacitor in the assortment I bought was 0.1F or 100mF. It's the size of a coin-cell battery. I mean yes, it would fit fine in a lot of places, like a boxcar, but I really want to make a small board here I can hide on more interesting cars. So I want to end up with all surface mount components.

    What does the next revision look like? Well, after browsing mouser.com and digikey.com I found the highest capacity literal surface mount (like a square thing that would lay flat) is a 11mF SuperCapacitor, 3.2mm x 2.4mm. Woot! Except that is 1/10th the power of my 0.1F, aka 100mF super capacitor. Boo. So I really need to reduce current by a factor of 10 to get similar performance. Can I do that?

    Yes! I think so. Two simple changes. First, swap out my TLL 555 for a CMOS version of the part. The TTL takes 5ma per the data sheet, the CMOS 0.250ma. Second, my LED today is a nice big T1.5 that's great for prototyping, but takes 20ma. The real LED will have to be much smaller, and can be much more efficient. So I found a square shaped, surface mount RED LED that is 1mm x 0.6mm (wow, soldering that will be "fun") and only draws 1ma. This should take me from 25ma of draw today, down to around 1.25ma. That's actually a 20x reduction in draw!

    The other change is necessitated by the smaller super-capacitor as well. I'm currently running a 5v regulator because my super-cap assortment was 5.5v rated. The SMD supercap is a 3.3v rated. So I'll also be swapping out the voltage regulator for a 3.3v part.

    In short, circuit stays the same, just swapping out for higher efficiency parts all the way around. If it works on my bench, I think I'm going to order a set of custom made boards to make up 20 or so as physically small as I can make them with all surface mount components and FRED up a number of my rolling stock items.

    Now, I said this was the first baby step to understanding how all of this worked. My next goal is to update the lighting in some of my passenger cars using the same trick. Bright white LEDs, a small control board, and a super-capacitor (or two, or three) that will keep them lit for 10-20 seconds with now power ensuring true flicker-free operation on even the worst track. From there, I have more plans!
    Last edited by bicknell; 22nd Feb 2020 at 08:30 AM.
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    Leo Bicknell

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    Nice job!!!!

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    ok... how are you making yer PCB?
    perhaps a group buy for a run?
    a 555 timer can be pretty small... what one did you pick?

    I'd like the car lighting and I can't think that will need a timer

    this is complete ignorance on my part as I do not know how an IC vreg is constructed.
    The only thing I think I'd consider is if the v-reg leaks power once the bridge stops supply.

    perhaps a diode?

    I've been planing a passenger car lamp for a while and have not acted on the thinking at all.

    You have my attention.

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    Quote Originally Posted by victor miranda View Post
    ok... how are you making yer PCB?
    perhaps a group buy for a run?
    a 555 timer can be pretty small... what one did you pick?

    I'd like the car lighting and I can't think that will need a timer

    this is complete ignorance on my part as I do not know how an IC vreg is constructed.
    The only thing I think I'd consider is if the v-reg leaks power once the bridge stops supply.

    perhaps a diode?

    I've been planing a passenger car lamp for a while and have not acted on the thinking at all.

    You have my attention.
    As long as the voltage on the output (super-cap) side of the regulator is above the input voltage, current leakage should be negligible or zero. It's basically a comparator/op amp driving (what I assume to be on the CMOS version) a FET, and a FET is about as close to an open switch as you can get when it's off. Besides, once the power source is removed, the LED is going to drain the super cap anyway, so any current leakage through the 7805 won't matter.
    Never mistake a guy who talks a lot for a guy who has something to say...

    CH&FR Site and Blog: http://www.chfrrailroad.net and http://blog.chfrrailroad.net
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    Not sure where I will get the boards. I have several friends who do such things much more than I do and know the best fabs, I will consult with them before I order. I might well be open to doing a "run" of them. The trick here is all the parts are SMD, and very, very, very, very, very, very small. Cuz I want the whole board to be small. I printed out my first draft 1:1 scale, and gulped.

    It will have to be reflow soldered. I've not done that at home but my friends have and I think I can make the process work.

    I haven't finalized the final parts for a board. The group of parts in the mail (some arriving today, some Monday) will allow me to test the 1ma LED and that the 11mF SuperCap has enough hold up to make me happy. That's really the two key details left. But, I can approximate:

    MB2S or similar bridge rectifier in a TO-269AA case.

    LM1117-3.3 or similar linear regulator in an SOT-89-3 case.

    TLC555C in a SOIC-8 case.

    The SuperCapacitor.

    Aside from that there are 4 resistors, I think I can get the specs I need in 0402 SMT parts for all of them. There are also two regular capacitors which I also think I can get in 0402 form factor. The one cap might have to be an 0602, not sure yet.

    I'm using KiCAD, which I never tried before. It has a "3D" viewer, which is very interesting:



    For scale, the solder pads to the left are track power (top two) and LED (bottom two). They are 2mmx5mm. I have no idea if that's a good size for the solder pads or not. It gives you an idea how teeny this is though. And the board is 100% one sided, which means I can even see doing something like gluing it under a flat car!
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    Leo Bicknell

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    Oh, and yes. To drive constant lighting the 555 and it's 3 support chips (1 cap, two resistors) would not be required. It might take multiple super-caps in parallel though to drive enough power for the whole car -- depending on how many LEDs are in it and their current rating. That will be one of the next experiments. While I can get 1ma LEDs, if it takes 10 of them to light a car we're right back to 10ma.

    Anyone know how many LED's Kato uses in their kits?
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    Leo Bicknell

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    Kato uses one LED for the kits I have.
    they have a light pipe along the top of the car with built in scatters to reflect the light down.

    if you are using a v-reg I'd expect a few LEDs can work.

    after being told that a FET doesn't leak when unpowered,
    I am a little nervous to be offering suggestions related to efficiency.
    I am very curious about white SMD LEDs that use 1 miliamp...

    the surface mount stuff is often glued, I am told.
    I once had some stuff that was a solder-paste-glue for prototyping.
    it was rather expensive... and it did work.

    fun stuff tiny SMD...

    victor

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    I went hunting for the latest in white LED...
    found this
    https://www.digikey.com/product-deta...1-1-ND/8535294

    I am wanting to find some sort of light volume claim...
    ah well...
    you are doing a fred and I am thinking car lighting
    the two are not exclusive
    they are different in requirements for the circuit driving the LED
    so please forgive me for jumping in with distractions to your project.

    I have to mention you should consider adding a fuse.

    victor

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    The part I ordered for the FRED is https://www.mouser.com/ProductDetail/755-SML-P11VTT86R

    The light itself is 0.6mm square, the overall size is 1mm x 0.6mm. That makes the LED a scale 3.75" square. Wow! Forward voltage is 1.8 - 1.9, current is 1ma. $0.15 each in quantity 100, and a stocked part. Unfortunately, this particular chip does not come in white.

    I will note for car lighting there is no need for an LED that physically small. There appear to be no 1ma white units at all. I see 2ma white units on mouser, none stocked. I get 37 in stock choices at 5ma. Digikey has 1ma white units, all non-stocked. They also have 2ma units, and appear to stock two units:

    https://www.digikey.com/product-deta...4-1-ND/8535297

    https://www.digikey.com/product-deta...1-1-ND/7430418

    The first is 1mm x 0.8mm, the second is 0.65mm x 0.38mm! That is beyond tiny. The second shows 2ma @ 2.8v. Wow.

    Fascinating. The car lighting project will be very interesting when I get there. Today though, time to finish the FRED. I got my capacitor assortment so I'm working on dialing down the current used even more. Monday the CMOS 555 will show up along with the LEDs and the super capacitor.
    --
    Leo Bicknell

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    Hi Leo,
    I like what you are doing.

    I am so used to thinking my own thoughts I sometimes forget there are other
    kinds of ... thinking... or in this case other kinds of LEDs from white.

    about once a year I survey the tiny white LEDs to make sure creating Headlamps have not
    gotten brighter or such. I think I've been hoping that square LEDs get smaller...

    None of that is helpful with your rather good idea for lighting a FRED.

    having lost a car and nearly having a small fire
    I do recommend you consider adding something like this
    https://www.digikey.com/product-deta...2-1-ND/2776946

    you got me thinking and for that I am grateful

    victor

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    I'm no circuit expert, so wondering how the capacitor doesn't feed current back to the DCC bus during half the square wave cycle. Does the rectifier prevent that?

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    yep. the magic of diodes. one way.

    :-D up to some voltage and then they become zeners
    and then they smoke
    and then they sometime fail open
    and sometimes fail closed

    I have personal experience with all that...

    :-D ooops

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    This morning I woke up before everyone else and looked at my little circuit again. I re-read https://www.allaboutcircuits.com/tex...d-led-flasher/ twice.

    I can't change the power used by the 555 (other than to go to a low draw CMOS part). I can change the LED to the 1ma unit in the mail to reduce the output draw. There's really only one other bit of "waste" energy, and that's C1. C1 is charged and discharged each flash, so the smaller C1 is, the less energy is used to charge it and to discharge it. I already knew that, and the circuit on my bench is working with a 0.1uF capacitor which requires a 1Mohm R1 and a 200Ohm R2 to get the blink rate right.

    I've made a super-simple spreadsheet with the timing formula in it, and I played around with that. More or less, cutting the capacitor to 1/10th the capacity means going to 10x the resistance on both resistors. So I went to Mouser, selected 0402 resistors (I could probably fit a 0602 if I had to), in stock, and looked at what sizes are available. The largest is pretty much a 22Mohm. Ok, so let's make that our top resistor. That's 22 times my 1Mohm now. Alright, so 22*200K is 4.4Mohm. Woot, a 4.42Mohm is available in 0402 for $0.10. Now, what about the capacitor? That's 0.1uF/22, or 0.045uF, or 45nF. The closest standard size is 47nF, which should be close enough. Ok, let's plug that into the formula.

    22Mohm R1

    4.42Mohm R2

    47nF C

    time High = 0.86 seconds (this is "off" on the LED)
    time Low = 0.14 seconds (this is "on" on the LED)

    That's almost exactly 1 pulse per second, and 0.14 should be plenty long to see. And I just reduced the capacitor loss by a factor of 22!

    If the other parts work out as expected on Monday I may order the necessary resistors to try this ultra-reduced power version. The highest resistor I have in my assortment is 1Mohm, and I don't have 22 of them to string together! I have the 47nF capacitor. I kinda wonder if there is a limit to how small C1 can be with the circuit still being reliable -- but I guess the only way to find out is to try.

    I also played around with the board design a little bit trying to make it even smaller. Here's what I came up with:



    The yellow lines represent the edge of the board. It's 18.67mm wide, by 10.92mm high. The width means it should fit under just about any car. I'm thinking my first place to try and hide it might be under a fish belly type gondola, empty. That's the sort of thing today that can only be done with the firefly FRED.

    Speaking of which, I wondered how that worked. He can't be fitting all of these IC's in a truck. So I found the patent application and watched a couple of videos. I now get it. The trick is a self-blinking LED. Something like https://lighthouseleds.com/led-compo...ace-mount.html. Basically LightHouse (and maybe others) have integrated the 555 timer into the LED itself. This is also why his is polarity sensitive on DC, I think it's just the pickup strips, a small resistor, and one of these self-blinking LEDs. A pretty cool solution! But of course it is polarity sensitive (doesn't work when a DC train is backing up), and does not feature the SuperCapacitor here for hold over. In fact, being a DCC guy I hadn't even thought about that aspect -- mine would work much better on DC. An operator could stop a train and back up and the FRED would blink the whole time due to the capacitor hold over.

    Still, finding these self-blinking LEDs has me thinking. I could drop the 555 timer and it's support IC's and just have the rectifier, regulator, and charge the super capacitor. Then the super-capacitor could drive the self-blinking LED directly. That would cut out one more IC, and let me re-arrange the board.....
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    Leo Bicknell

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    Quote Originally Posted by NtheBasement View Post
    I'm no circuit expert, so wondering how the capacitor doesn't feed current back to the DCC bus during half the square wave cycle. Does the rectifier prevent that?
    The M2BS http://www.vishay.com/docs/88661/mb2s.pdf rectifier is rated to block up to 200v in the reverse direction. Since the capacitor is 3.3v max (they "explode" at like 5v) I'm not worried about it feeding back through the rectifier.

    The linear regulator is rated for 20v input. If anyone is running their track above 20v they deserve everything blowing up! Ha! I could probably find a 35v part if I wanted to be paranoid.
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    Leo Bicknell

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    Quote Originally Posted by NtheBasement View Post
    I'm no circuit expert, so wondering how the capacitor doesn't feed current back to the DCC bus during half the square wave cycle. Does the rectifier prevent that?
    Let me be a little more educational. First, find Capacitor C1 in the drawing. It's an input filter for the linear regulator, but pretend everything to the right of it doesn't exist. Just C1 and the bridge rectifier. If you follow the positive output of C1 back to the bridge rectifier, you'll see there is a diode in both directions blocking the reverse current. So starting at this point there's no way to feed back current onto the rails. That's why 99.9999% of track powered things (like every decoder you've ever bought) start with a bridge rectifier.

    But C1 is a super-small capacitor and has no real energy to damage anything. It's C3, the Super-Capacitor, that has the giant amount of energy we need to worry about. Now look a little further in the diagram. After C1 is a linear regulator. It's job is to take 0-20v of input and deliver 3.3v of output, rock solid. So pin 1 is the 0-20v in (filtered a little by C1), and pin 3 is the 3.3v out. That's where we connect it to the capacitor to charge it up.

    The linear regulator _also_ blocks reverse current though. If the input is 0v and the output is 3.3v (from the super-capacitor), no current will flow across the regulator in the reverse direction.

    So there is actually double protection from the super-cap feeding back into the track.

    On a board I put on a model, I would do one of two things for further shorting safety. Either wrap the whole board in heat-shrink so metal bits can't be accidentally touched. Or, place epoxy over the entire board to cover any exposed metal bits. The heat shrink is easier, but adds a few fractions of a mm of thickness, the epoxy can be done so no thickness is added, but is messy.
    --
    Leo Bicknell

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    Nice project!

    A few thoughts I have, ignore them at will

    - Check the 555 datasheet (the specific variant you are using) and the C2 capacitor data sheet for leakage currents. At 22MOhm for the resistor, there is a risk of capacitor leakage current and 555 leakage current running larger than the charging current through the 22MOhm resistor, which means the capacitor will never charge, the 555 will not oscillate. Or at very different frequencies than you expected.
    - Get an extra thin PCB - regular is 1.5mm / 1.6mm, but at that size, you can get away with 0.5mm.
    - Connection pads can probably be much smaller - I know of a project at http://opencarsystem.de/decoder/decoder.html that uses 1mm square pads IIRC.
    - Checkout https://pcbshopper.com/ for PCB suppliers, if you are open to working with Chinese suppliers. Great overview and comparison (they do include US based companies as well)
    - If you want to stay open for hand soldering, use larger pads for the components - makes life easier...

    Have fun with the project, I'm looking forward to seeing the results,
    Heiko

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    One of my friends recommended MacroFab and PCB:NG. I think PCB:NG is the easiest to understand the pricing, and if I understand it right I can get the boards made for approximately $10 each, assembled. But that assumes quantity 100. To sell them and recoup my costs I think I would have to be in the $15-$18 (shipping included) range, minimum. There's some postage, but they can mail first class as they are so small, tax, etc. If I could get like 15 people to buy 5 boards each, it might be worth doing. Some feedback on if folks would buy at that price would be useful.

    None of them do 0.5mm boards for the small runs, as far as I can tell. They seem to all be 1.6mm.

    The 22Mohm resistor is also expensive ($1.40) compared to most of the others which are like literally less than a penny. It may not be worth chasing as a result. I suspect the power savings really isn't that great. I'm going to see what sort of runtime I get with the 1M/200K/1uf combo I'm currently using on my board first. If it's acceptable, we'll just go with that as the parts are dirt cheap.

    I swapped out the regulator for a version in a different case, which let me make the board a bit smaller. I'm down to 11mm x 17mm overall. And that's with the large-is solder pads for the wires. The tallest component the regulator at 1.45mm max height. With a 1.6mm board the whole thing would be right at 3mm thick.

    Which gives us N scale dimensions of:

    Width: 70" aka 5' 10"
    Length: 108" aka 9'
    Height: 19" aka 1' 7"

    Which gives you an idea of where this could be hidden. It would trivially fit inside any enclosed car like box car, hopper with load, auto rack, etc. It would likely be easy to hide in a gondola under a load of some sort, being only 19" tall. I'm wondering if it could be stashed under a flat car in a way that isn't visible, not sure about that yet.

    Of course, power pickup is also required. I have purchased some Kato caboose trucks to experiment with, wanting needle point pickups. They are unfortunately arch-bar friction bearing trucks, but most people can't tell from normal viewing distances. My plan is to run #29 magnet wire from the trucks to the board, and from the board to the FRED.

    Now, if this all works mounting for the FRED is also required. My next trick would be to try my first 3D printing exercise. It would be the FRED box, which is well, a box. A simple rectangle with a piece cut out of for the LED to recess into. My thinking is to glue the LED into the recess, use a small amount of clear epoxy to then fill in the rest of the hole, and finally paint. I suspect the result would be fairly realistic.
    --
    Leo Bicknell

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    Quote Originally Posted by bicknell View Post
    Or, place epoxy over the entire board to cover any exposed metal bits. The heat shrink is easier, but adds a few fractions of a mm of thickness, the epoxy can be done so no thickness is added, but is messy.
    Do you think a conformal coating would work?
    Daniel Dawson

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    Got some more time to play with this today, and man am I impressed with the state of the world.

    I switched to 4x0402 diodes rather than the bridge rectifier part to save some space. A bit more work on my part, but a bit of space savings. I also found an error where I had set some the component sizes wrong. With all the re-arranging the board has shrunk to 7.4mm wide, but is a bit over 17.2mm long now. Here's a current picture:



    I've been recommended to try jlcpcb, macrofab, and pcb:ng to make the boards. I wanted to try jlcpcb as they can do the thinner 0.4mm boards. But jlcpcb says their SMT assembly is not available right now. So I went on to PCBNG and gave that try.

    I have to say the interface is very impressive. Guided me right through the process, clearly mentioned my mistakes, and had good links to KB articles to fix. My problems were very minor, I had the edge too close in two spots and the auto-routing settings I was using was off by a fraction giving me traces too close (3.94mm v 4 required). A couple of quick tweaks and I could at least get an assembled price.

    6 boards is $80.

    24 is the first price break, $262.

    120 is the next price break, $1032.

    That last price is $8.60/board, fully assembled including making the boards, the component cost, SMT assembly, soldering, and shipping to my house. That really is very impressive. A few clicks and a completely modern board with all pick and placed SMT components can show up at a fairly affordable price. I really expected it to be more expensive, as my friend said "this is a golden age for making things yourself".

    If my bread board testing goes well I will absolutely be ordering a run of 6 to see how they do and start testing on the layout. And I guarantee I will be doing other projects now that I know how this works!

    - - - Updated - - -

    Quote Originally Posted by Mobile One View Post
    Do you think a conformal coating would work?
    I have no idea. I've never tried one, and I had to look it up to know what it was. Very interesting.
    --
    Leo Bicknell

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    @bicknell So I guess this board is single purpose in the sense that it just flashes a single led. But I would like to point out that beside FRED's, it could be used for airplane warning lights on tall buildings, a feature I have have wanted to add on at least one building. So now it is a dual purpose board. Unless there is a cheaper way for doing this on buildings? So this thing will run off DCC track power, but I guess a 5-20 volt wall wart would run it just as well with the regulator built in? What about multiple LEDs? Does an additional LED alter the timing or exceed the current load of the board? I'm thinking of structures that would have multiple lights or lights on multiple structures, though the synchronized blink might be undesirable in that case.
    Daniel Dawson

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