After the long thread on weak DCS output signals from the TIUs there was some interest in a simpler tester for those that don't want to finance or support a full oscilloscope. Here is a really simple $10 answer (I priced parts at $9.44 on digikey... battery not included!).

DCS_test1

Concept:

It's super simple. The filter [A] extracts the DCS packet. The offset diode [B] makes the voltage go between 0 and 12V (instead of -6V to +6V). The trimpot [C] is a voltage divider and makes it so the DCS amplitude needs to be about 12V to successfully trigger the one-shot. The one-shot pules a blue LED [E] for 1 second set by time base [D]. It runs off a 9V battery that's regulated. [F]

How to use it:

Connect the red and black wires to the TIU port. If the TIU port is good (Vdcs > 12V), the LED will blink. If the TIU port is weak/broken/bad (Vdcs < 12V) the LED will not blink.

I made a breadboard version and it works well. I'm a designer but maybe someone who's both a designer and good at making things (so basically GRJ) might do a pcb with a box if there is interest.

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Original Post

Thanks for posting this!

So to set the trim pot, do we set it up on a TIU output we have reason to believe is good and just adjust the pot until the LED starts blinking?  Or is there a better method during construction (maybe applying a DC  signal directly to the B/C node of the circuit before installing the filters on the front end?)?

Thanks for all the good discussions you have been posting on the topic.

-Dave

Dave45681 posted:

Thanks for posting this!

So to set the trim pot, do we set it up on a TIU output we have reason to believe is good and just adjust the pot until the LED starts blinking?  Or is there a better method during construction (maybe applying a DC  signal directly to the B/C node of the circuit before installing the filters on the front end?)?

Thanks for all the good discussions you have been posting on the topic.

-Dave

Hey!

Yup, the trimpot just sets how many volts of dcs signal you need before the light flashes. If you turn it all the way up it’ll flash at 2.5V. In the middle it would flash at 5V. Near the top maybe 50V or more. I think it’s easiest to just tune it against a good channel like you suggest. Tune it to the point right where it barely flashes on a working channel, then leave it set there as your threshold of good and bad for future tests.

I would like to try this out. Not sure I can make a PCB? If so that would take me some time, but I might try it anyway? Breadboard is probably what I will start with. I know GRJ can probably put one out in a few minutes. He may even have one posted here by the time I post this? 

My only question (I think) would be what type of caps did you use and were the resistors precision or just regular old 5% resistors? I am going to place a Digikey order for all that I don't have. Or maybe just order it all along with some extras.

Thank you again Adrian, for doing all this stuff and posting here for the rest of us. I find it all very interesting, make that fascinating! As I have said before most is over my head, but still very interesting and some may even rub off on me one of these days. 

rtr12 posted:

I would like to try this out. Not sure I can make a PCB? If so that would take me some time, but I might try it anyway? Breadboard is probably what I will start with. I know GRJ can probably put one out in a few minutes. He may even have one posted here by the time I post this? 

My only question (I think) would be what type of caps did you use and were the resistors precision or just regular old 5% resistors? I am going to place a Digikey order for all that I don't have. Or maybe just order it all along with some extras.

Thank you again Adrian, for doing all this stuff and posting here for the rest of us. I find it all very interesting, make that fascinating! As I have said before most is over my head, but still very interesting and some may even rub off on me one of these days. 

Hey!

The RC network at the front blocks 60Hz and keeps 1 MHz. Even if the values were off a factor of 10 times it should work fine. The filter as drawn cutsoff signals below a few KHz, but 10Khz or 100Khz is also fine. The r and c for the timebase sets the light flash at about a second. Depending on which version of 74h123 you buy the equation for the duration is different so you may want to try different combinations until you’re happy with it.

RJR posted:

Adrian, going to Digikey, I see many variations of the CD74HC123.  Why the differences and which is best?  Note Adrian asked a question about resistors & caps.

Hi again... I went with exactly this chip. There's no design reason, it was just in stock the week I needed it. All the people who make 74xx chips (TI, ADI, ON, ...) have different underlying fabrication processes for the CMOS technology so you get different timing numbers depending on which brand you buy. Again on the resistors and caps 5% should be more than enough. Everything is order of magnitude-ish insensitive so if you buy 1K,5K,10K,100K resistors and 0.01uF, 0.1uF, and 1uF caps, you will definitely have a few combinations that work. The only one that's critical is the one for the LED so you don't blow it with too much current.Note the LED doesn't have to be blue.... I just think they look fancy.

just to be clear here, you're just testing for +12volts out?

Most users will know if the TIU doesn't put out voltage. Is there a condition you're looking at that puts out partial voltages?

A meter or MTH PS2 or 3 engine could tell the user what the track voltage is.... right? or even a light bulb?

" on Sour mash and cheap wine " ??

Why go back to DCC when I have DCS!

Engineer-Joe posted:

just to be clear here, you're just testing for +12volts out?

Most users will know if the TIU doesn't put out voltage. Is there a condition you're looking at that puts out partial voltages?

A meter or MTH PS2 or 3 engine could tell the user what the track voltage is.... right? or even a light bulb?

Adrian talks about DCS signal in volts not to be confused with volts used to run an engine. 

He should be able to explain it better

Engineer-Joe posted:

just to be clear here, you're just testing for +12volts out?

Most users will know if the TIU doesn't put out voltage. Is there a condition you're looking at that puts out partial voltages?

A meter or MTH PS2 or 3 engine could tell the user what the track voltage is.... right? or even a light bulb?

Here's a little doodle I made just for you!

voltage

The track has a 60 Hz voltage to power the trains (usually like 10-25V depending on what power source and such). It also has super-imposed on top of that the digital signal for DCS. We are talking about measuring this digital voltage, not track voltage. The digital packets are infrequent and at a high frequency which is why you need to filter them with the above circuit to measure them independently of track power.  It's this DCS digital voltage (formal name is excursion voltage) that limits your DCS link quality. A normal working part should have it about 12-14V, but when parts fail it can be a lot less. This is what the tester is validating.

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Digikey parts ordered! Luckily, I selected the exact CD74HC123E chip as Adrian linked above too! I didn't get the assortment of resistors and caps, but I have those in quite a few different values here in ceramic caps and 5% 1/4 & 1/2 watt resistors. Oh dear, now for the PCB design...eek! I think GRJ uses Diptrace, so I'll try using that. Maybe he will have mercy on me with this project if it's in his native language? 

What would be cool here is several LED's so you could see lower values.  If this were assembled by someone with a 'scope to calibrate it, you could have maybe four LED's to know exactly what you have.  What would probably be easy would be to create a circuit board for such a design with thru-hole parts so many people could build it.  With a 1% divider, you could probably do this kind of thing that requires no calibration.

gunrunnerjohn posted:

What would be cool here is several LED's so you could see lower values.  If this were assembled by someone with a 'scope to calibrate it, you could have maybe four LED's to know exactly what you have.  What would probably be easy would be to create a circuit board for such a design with thru-hole parts so many people could build it.  With a 1% divider, you could probably do this kind of thing that requires no calibration.

I like it ... but then you need 4 one-shots, 4 dividers and 4 leds to do it using this approach. If you only want one led at a time you need a thermometer decoder too.

If you're going to all this trouble you might just give up and and go to a microcontroller with an 8-bit ADC inside and a $10 LCD with numbers on it (like my telemetry train one). That's closer to a $50 solution though. At some point writing code becomes simpler than so much analog!

Everything I ordered was Thru-Hole so if I make it out alive after getting into Diptrace it will be something most anyone can make. I don't have a scope, nor would I know how to use it if I did so the calibration is a bit above me right now. I would be happy to add more LEDs, if I get something made up without getting spanked by the PCB part. 

I also ordered extras of each part so I could fiddle a bit with the boards if I get that far that is.

Here's how mine looks.

After more playing around side-by-side with the scope I suggest a shorter time base so you can see long gaps between packets like the one in the video (10K and 100nF). I don't have a pot handy so I used a 3K and 1K resistor for the divider.

 

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