Comment on Circuit Building

pqe,

I learned circuit building back in the Heathkit days. Everything from TV, test equipment, oscilloscope, to a their train control throttle. In later years I got interested in the digital chips and built circuits with those. Got to the point of where I bought a computer and expensive program to design circuits and do the layout. Routing the traces was the hardest part. It got to be a hobby almost.  Then I decided to stick with model railroading. Did learn a lot about electronics though.

Looking forward to your postings. Your detection circuit will be a great place to start.

Bernd

Reading a schematic

The first circuit I'll cover lights up when a section of track is powered (with DCC). This is handy to show the state of power routing in a yard or following Peco turnouts. This is the schematic of the circuit.

 Schematics are the lingua franca of the electronics business. A schematic is to the technician as a score is to a performer-- It specifies exactly what to do.

A schematic consists of component symbols connected by lines. There are thousands of components available, but they are represented by a relatively small set of generic symbols. Details about components will be written by the symbol (if details are left out, any variant of the component is expected to work, but it may be a good idea to test the one you have before you solder- this is what solderless breadboards are for.) There are several sites that list the standard symbols, such as this one. You will notice that for many there are US (IEEE) and European (IEC) versions.

The lines are the meat of the schematic. Every line indicates a connection between parts (or leads of the same part). They may be literally interpreted as wires, but any sort of connection will do. (Example, the parts may share a pad on a circuit board.) Schematics are usually drawn so the lines do not cross much-- this makes the intent clear. However, if lines do cross, it does not necessarily mean there is a connection between the wires- such connections are indicated by a dot where they cross (old school) or by two different horizontal lines stopping at a common vertical one. Old school schematics may indicate that two crossing lines do not connect by adding a little loop to one. When schematics reach a certain level of complexity, the more convoluted lines may be left out, with connections instead indicated by letters, numbers or names. In most cases, connections to the power supply are indicated by labels such as +12 or -15 as appropriate. There may be an arrow or cross hatch at the label. 
 

Another set of connections are indicated by the ground symbol, which is three horizontal lines at the bottom of a line suggesting a rod shoved into the dirt. This is connected to the neutral or negative side of the power supply. (It's actually a bit more complicated than that. For various reasons, you may run into circuits in which the ground is connected to the positive supply lead, or even with both positive and negative power lines in addition to ground.) The ground wire may be connected to the case of the device- in some designs the case is used for all ground connections. The concept of "ground" is deeply philosophical, but the practical meaning is "put the black lead here to measure voltage."

It is often possible to literally lay out the circuit just as it appears on the paper, but this would probably make for a really spread out board. In practice, the layout is "folded" with parts shown in series actually laid out in parallel but connected with zig-zag wires and other convolutions. Two parts that are next to each other on the schematic may wind up at opposite ends of the board.The important thing to remember is that no matter how tangled up a wire or circuit trace is, as far as the electrons are concerned, it's all one point.

Interestingly, you do not need to understand how a circuit works to build it. If you get the parts connected the way the schematic specifies, it will work. Probably, unless a component got damaged, or the circuit was a dud in the first place. For that reason, I suggest you try all new circuits in a solderless breadboard before turning the iron on. Here's one in action:

The original breadboards were pieces of wood with brass nails in them-- you would try a circuit by wrapping wires around the nails. A solderless breadboard is an array of little sockets sized to accept the leads on most components. The sockets are in groups of five (usually) and anything in one socket of the group is connected to the other sockets in the group. Breadboards also feature "busses" which are long strings of sockets- in the above photo, the red and blue lines indicate busses. (The grouped sockets are vertical in this view.) The resistor and LED are both plugged into group 12 of the upper half of the breadboard, so they are connected together. The clip leads are connected from a lead of each to a rail, and since the track is powered, the LED is lit. Breadboards are handy, but not very reliable. The sockets wear out quickly, so I generally thrown one out after a couple of years of use. Besides, a little wiggling is all that is required to loose a connection. My students would often try to use breadboards in their exhibitions, but they usually came to regret that.

In the next installment, we will make a permanent version of this circuit.

pqe

Edit 8-2-16. Since this is a building tutorial, not a circuit design tutorial, I used the simplest useful circuit I could think of. However, it has come to my attention that some LEDs will not last in this design, because of the reverse voltage that is applied half the time. Here is an improved version.

pqe

Thank you!!!!

Thanks for putting this tutorial together.  This really will boost my confidence working on one of these.

-Ed

Great thread

It seem like there was more activity on this around 25 years ago.  It may be DCC, or it may be more of the RTR issue.  Thanks for the links and I look forward to more of your work that your willing to share.

From schematic to board

If you have a known working schematic, you can get a printed circuit board produced for about $70. However, I'm not going to get into what you need to do to design a board, and 70 bucks is 70 bucks. I'm going to build all of these projects on proto boards:

Proto boards (prototyping boards) are a piece of phenolic or fiberglass board with a grid of holes and solder pads.

• The boards are usually 1/16" thick, but this varies.
• The holes are usually on 0.1 inch centers to fit standard IC pin patterns, but other layouts do turn up. Always check this spec.
• Various pad patterns are available, from none to complex ones with power busses and edge connectors. I find two patterns most useful-- the one pad per hole shown above (pads on one side only) and patterns that exactly match a solderless breadboard, like GC Electronics 22-508.
• Pads may be square or round. The round ones build a little neater (less chance of solder bridges) but the square one can be persuaded to accept smd parts. (See plastic diner.)
• They come in all sizes-- I generally keep a bunch of 2" x 3" or similar sizes around and order bigger ones as the need arises. Some are sized to fit specific cases-- handy when building a slick product.
• Good ones have tinned pads-- cheap ones (I'm looking at you, Radio Shack) are varnished plain copper and need to be lightly sanded before use.

The boards can be cut to size by scoring along a line of holes with a glass cutter or razor saw and snapping along the score. This is easy if you clamp it into a vise.

The hardest part of transferring a project from the schematic to proto board is knowing where to start. No, really-- you have to consider the position of any controls on the board, any indicator lights, and how wires that lead off the board will be dressed-- you usually want all wires to come off of the same edge so you can easily turn the board over for service. In this case, there are two LEDs (I'm building two circuits on the same board) that will have to fit holes in the panel. Here are the parts concerned:

• A panel
• a hunk of proto board
• 2 2.2k resistors (Actually I changed my mind after taking this picture and used 2k resistors.)
• 2 LEDs

​Note that I have already drilled holes in the panel. The small holes match the mounting holes in the proto board, and the large ones are a bit larger than the LEDs. I used the proto board to mark the panel for drilling- I lined it up straight and pushed a #61 drill on a pin vise through the holes to make a mark. To get the panel, board and LEDs lined up, I dry assembled them:

Everything is mounted on two 4-40 screws. The proto board needs to be held clear of the panel about 1/4" or so. I use nylon spacers for this, but anything will do, scrap styrene, wood, drinking straws, some folks use extra nuts. The nuts on the top are nylon-- steel works, but will short out a few pads, and Murphy is always watching to see that those particular pads become necessary.

With the work assembled this way and the LEDs pushed firmly through the panel, I soldered the LED leads.

Soldering these is not as difficult as soldering track. For one thing it is quick, and you are not trying to reach behind a delicate water tower.

• If you have never soldered to circuit boards before, practice with odd parts on scrap board.
• Make sure the pad and lead are clean-- they will be if they are new, but it never hurts to give the pad a touch of sandpaper before starting a project.
• Use fairly thin rosin core solder. (No Acid flux here!)
• Use a medium point tip at 650-700° F.
• Apply the iron tip to both the pad and lead and start counting at 60 BPM.
• On the count of 5 touch the solder to the lead and pad-- it should flow.
• On 7 lift the solder.
• On 8 lift the iron.
• Watch for the sudden color change when the solder freezes.

Things that can go wrong

• A huge blob means too much solder-- don't hold it down so long.
• If the pad is barely covered that's not enough solder. The solder should fill the pad and make a cone up the lead-- the rightmost joint above is the best.
• If the solder is rounded at the pad or lead instead of a cone, one or the other was not hot enough. Be sure the iron touches both lead and pad, and wait a bit longer before applying solder. (Many folks put a preliminary drop of solder on the tip of the iron to help the heat transfer. This is fine, but it can be overdone.)
• If the solder is an ugly grainy grey, the lead moved as the solder was freezing- you have to do it again.
• A bit of brown shiny rosin around the joint is natural. A lot of black crispy rosin means your iron is too hot.

There are more parts to add, so at this point the panel was removed.

Next I added the resistors. Note the colored stripes. These reveal the value of the resistor. They read from left to right (in this picture anyway- the gold or silver stripe comes after the value)

• First digit- in this case 2
• Second digit-- 0
• Number of zeros to follow (or multiply by ten to this power) again 2

So the value is 2000. AKA 2k ohms.

The color code is published many places- it is basically the colors of the spectrum:

• 0 black
• 1 brown
• 2 red
• 3 orange
• 4 yellow
• 5 green
• 6 blue
• 7 violet
• 8 grey
• 9 white

Well, sort of a spectrum. Like I said, it's easy to look up. Download a chart, color print it and hang it on your shop wall. These colors are used to mark a lot of things. The gold or silver stripe is tolerance- gold is 5%, silver is 10%, no fourth stripe is 20%. What, did you think electronics was a precision science? 1% tolerance is available, but expensive. Instead, we design so 5% is close enough.

Note that I placed one lead of each resistor close to a lead of the LED. I then turned the board over and bent the resistor lead around the LED lead:

Then I soldered the leads together and clipped off the excess. 

Next was to bend the remaining leads in a  curve over a nearby pad. Wires will be soldered to these that will connect to the track:

The wire is pushed through the pad and bent over the associated lead. In this case I used 26ga solid wire, but if the wire runs very far I recommend stranded. Tin stranded wire before soldering it, and enlarge the hole with a #58 or #61 drill if necessary.

It's a trick to visualize both sides of the board at the same time, so here's a close up of one of the circuits with the symbols drawn in:

This is a situation where dyslexia is helpful! That extra resistor lead ws soon clipped.

Putting it back together:

I envisioned attaching the wires to some sort of connector to complete the run to the track, but which kind will depend on your preference. A barrier strip attached to the panel would be a good choice. 

And the user view:

More holes are needed to mount the panel to a fascia, but again that should match what is already in use.

Next: a bit more complexity, but not much.

pqe

Great thread!!!!! Thanks!

Great thread!!!!! Thanks!

Good Stuff...

....I don't need the tutorial myself being an Electronics Technician by trade (I could never spell Teknition now I are one) but it is always good to see how others do things. I like your precise explanations and the pictures of in progress are excellent. Well Done.

... Good old "Roy G. Biv"

One of my favorite Christmas presents was a "150 in 1 experimenters kit" from Radio Shack.  That and all the Forrest Mims experimenter handbooks.

WIll try it soon...

I should have the parts and the time available in a couple weeks.  I can't wait to try it. I especially appreciate the specifics on the soldering and how to mount each item and the board to the panel.

Thanks,

-Ed

Excellent!

Love the thread and hope you continue with more. 

Circuits

Great thread.

Been doing that since 1954 with first Heathkit.

Etched my own PC boards. Have bought some circuits through

http://home.cogeco.ca/~rpaisley4/CircuitIndex.html

Doing SMD circuits is a real challenge.

Rich

Vero board

I have used Very board for circuit building. There is an app for designing a circuit PC board. Google Vero board.

Rich

Veroboard

Yes, Veroboard is a proto board with all pads in a row connected as a single strip. You then cut the strips where you don't want connections. You don't get as many wire jumpers, but it is more difficult to change your mind.

pqe

Good stuff!

Good information! Thanks!

Tim

Vero board

Just Google Stripboard Veroboard design software.

I got my supply from the UK vie ebay about six years ago. Price and supply was very resoanable.

Rich

Think inside the box

The previous project was built on a panel, suitable for mounting on a fascia. Sometimes we want to put our projects in a full enclosure. Enclosures can be had in any size and shape from a pill box to a refrigerator, and made of plastics, aluminum, steel or more exotic stuff. You don't have to use purpose built enclosures, many household items of a box-like nature will do. (I once taught a class in building audio mixers in lunchboxes.) You should pick a size that can hold your circuit board without crowding. The convenience of a small footprint can cost you in building difficulty and reliability.

Here's the circuit of the day:

 This is a version of the  DCC polarity tester I have already published. If you want to confirm you have wired your tracks with correct polarity, or if the polarity is being switched appropriately, this is the tool to use. This will be built with the point labeled DCC A buss attached to a green wire and an alligator clip, the B buss to a red wire, and the frog connection to a blue wire. Why these labels? The circuit is also handy for detecting the position of turnouts with powered frogs. Build it on a panel with the connections as shown, and the red LED will light when the turnout is one way and the green one otherwise. (To change the light colors, just swap the buss connections.)

The circuit works because the two DCC rails are mirror images of each other- when one is positive, the other is negative. If both sides of an LED are connected to the same rail, it won't light up, but if connected to the A and B rail, it will light half of the time. The D in LED stands for diode, and current will only flow when the anode (back of the arrow) is more positive than the cathode (the crossbar).

I've picked a pretty small box for this build, because there's not much in it. Here it is, along with the board and some of the parts:

I have already cut the board and drilled the box. I used the same procedure as before- cut the proto board to fit inside the box, clamp the board on the back of the box (foil side down) and use holes in the board to mark out the holes in the box. The mounting holes are 1/8" and countersunk for a 4-40 screw and the LED holes are just a bit bigger than 3/16". (Drill a 3/16" hole and touch it with a tapered reamer.)

As before, I dry fitted the board into the enclosure to align the LEDs for soldering. It is important to get the LEDs in right way around. The marking on an LED is pretty subtle, just a slightly flattened spot next to the cathode (crossbar) lead.

I used 1/4" standoffs, which can be tricky to persuade into place. (Remember those kid's games where you had to roll a marble into a hole by tilting the box?) For bigger projects I prefer to use standoffs that are threaded for 4-40 screws. Then they can be permanently screwed to the panel, with the board held in by screws in the back. I'll demonstrate that on the next build.

Next step, remove the board and add the resistor. One end of the resistor needs jumpers to each LED:

Here's a trick I use when soldering several short wires into a circuit. The wire is just bent around the lead it will connect to, and if the other end is loose it tends to flop around and may even fall off. If I use a long wire wire I can connect the other end for the next jumper. I solder both ends, then cut the wire to the appropriate lengths to finish both connections:

At this point I discovered a mistake. I had one of the LEDs in backwards. I had to remove the connections I had just soldered to turn the LED around. Solder is removed with solder-wik. This is a braid of fine copper wires saturated in rosin. If you lay it on a solder joint and heat everything, the solder will get wicked up into the braid and the part will be free.

Once that was corrected, I could get on with the build.

Next parts needed are the 1N415 diodes. They are there because while LEDs are diodes, they are not particularly good diodes. Some will pass a fair amount of current backwards. When I was testing early versions of this project, I discovered that if the frog wire was not connected one LED might glow a bit as the other leaked current from buss to buss. Adding two real diodes fixed that. The cathode on a normal diode is marked by a black or silver band. The part number is stamped on the side,and you have to look it up to discover details like power rating. The currents involved here are so small that practically any diodes will do.

Here is the board with everything wired in, including the wires to the outside world.

It's not shown, but those wires had to be threaded through the box before connecting, because the alligator clips were already on them.

And here is the finished box in action:

Next, something with an IC in it.

pqe

(Portions of this build were edited for narrative purposes.)

Diode choice?

I was thinking of using this circuit, but the closest diode in the variety packs of electronic components I bought is 1N4148. Is that close enough for the circuit or do I need to buy the one listed?

Thanks,

-Ed

Diode choice

1N4148 is fine. That's probably the most common diode out there.

pqe

Doing the math for the resister...

Sorry for so many questions - but I like to understand the details of anything I do.

So my question is about the selection of the 2K resistor.  I am trying to understand why that value.

Here is how I would have done the math, and I am hoping you can explain why I might end up selecting the wrong resistor if I went by my math. 

So the red and green LEDs seem to both be listed as between 1.8V and 2.2V forward voltage with suggested current of 16mA-18mA.  So playing it at the lower current end I started out figuring to shoot for 16mA.    Forward voltage for a 1N4148 is listed as1V.

Based on Ohms law with DCC being a 12V source, to get 16mA current I get:

R = V / I

R = (12 / .016) = 750 Ohms

But if I understand correctly I need also to account for the voltage drop across the LED and the diode. Using 1.8V for the LED and 1V for the diode, then the voltage drop across the limiting resistor I calculate as 12V - (1.8V + 1V) = 9.2V.  If so, then I would have.

R = (9.2 / .016) = 575 Ohms.  

In either case, of the resistors I have the closest that is greater than or equal to these would be a 1K resistor.

So doing the math the other way with a 1K resistor, I get:

I = (9.2 / 1000) = 9.2mA

My understanding is that the forward voltage drop may be a bit lower across the LED and diode at lower than their rated current, but just to be safe if I assume only the limiting resistor then I get:

I = (12 / 1000) = 12mA

which is still well below the rated range of the LED and diode.

So this leads me to ask, what drives the selection you made of the 2K resistor?  By my calculation, use of a 2K resistor would give me somewhere between 4.6mA and 6mA of current.  Is it simply a matter of LEDs performing well even at that low a current and wanting a higher resistor to cause less stress on the components?  Or is there something in my math or reasoning that I am misunderstanding that when applying these principles in other circuits later could cause me problems?

One last question - I have 1/4W and 1/2W resistors.  By my calculations, even at the rated max current (20mA) of the LED and attributing the full 12V voltage drop to the resistor, I get I*V = .24W, so given the lower current and the nominally lower voltage drop across the resistor I am guessing I can safely use a 1/4W resistor, but I wasn't sure how much leeway I should have on the power rating of the resistor and whether I should therefore go for the larger 1/2W resistor.

Thanks,

-Ed

P.S.:  As an experiment after initially posting this, since I was fairly confident the 1K resistor would not cause damage, I tried it out on the breadboard both ways: with a 1K resistor versus with a 2.2K resistor (I don't have any 2K).  In could not tell the difference by eye in terms of the light output of the LED.  So my guess is that the higher resister value is just to reduce power consumption and therefore might be better for long term life of the components - but I still would appreciate any lessons or advice you might have on my reasoning/math above as well as this conclusion I came to on why the 2K resistor.  

Your math is impeccable

However, we have to look at the peak voltage in the worst case. When the A rail is in its positive phase, the B rail is in its negative phase. (I'd put a diagram here, but I'm 2500 miles away from my computer.) The NMRA spec allows a voltage swing of +/- 22 volts, so the peak voltage can be as high as 44 volts. (When the A rail is +22, the B rail is -22.) With a 30 ma LED, 2k makes sense.  Typical for HO and O seems to be around +/-14 volts. You can adjust the resistor for your own situation, of course.

In any case, a 1/4 watt resistor will be fine.

pqe

edit-- After all of this was written, I discovered the the DCC spec was written in a confusing manner-- the actual voltage swing is 0-22 volts, with the +/- symbol indicating the two rails are out of phase. So Ed's math is exactly correct.

Got it - thanks

Aha - I see.  In addition to the potential swing allowed by the NMRA spec, I see I was starting wrong at 12V rather than peak to peak.  I checked my LED specs and while they recommend 16-18ma they can handle up to 35ma.so I will stick to the 2K (or closest I have: 2.2K) resister.

Thanks for the explanation.

-Ed

One last question - the diodes...

I have one more question.  The original simple circuit to test polarity at the beginning of the thread used an LED without an additional diode.  But the turnout direction detector uses the 1N415 diode in series with each LED.  I am wondering what the purpose/advantage of the additional diode for each of the LEDs is.  Is this another peak handling situation of some sort?

Thanks,

-Ed

Think I figured it out...

By reading some other articles on the web plus reading the specs for the LED versus the specs for the 1N4148, I think I figured out why the extra diode for each LED.  It looks to me like the reverse breakdown voltage is far higher for the diode than for the LED.  As a result, the diode should limit current more effectively in the reverse direction ensuring we can't draw more reverse current through the LED than it can handle.  I have seen other circuits that put a diode in the reverse direction in parallel with the LED instead.  But I am guessing this series arrangement accomplishes the purpose.  

Am I understanding it correctly?

Thanks,

-Ed

I added the extra diodes after testing.

You will note the circuit connects the two rails through back to back LEDS. In theory, a backward biased LED should block current through the other LED, but in practice, an LED can leak enough current to allow the other one to light. When I tested a version without the extra diodes, both LEDs would glow a bit when the A and B clips were attached. The extra diodes prevent that.

pqe