No, they are correct as they are. The transistors in the ULN2803 form a connection to ground when turned on. The free wheeling diode is there only to protect the transistor from the back EMF, aka kickback, when driving an inductive load such as a relay coil. The other side of the relay should be connected to 5V.
Starting to think about these parts for the PCB. Is it best to mount the ULN2803 and the LTV-847 on sockets or SMD direct to the board?
Thanks
Adam
To Adam,
I donāt know you, but it appears to me, at the moment, you have taken on a far larger project then you can fully digest and understand.
I suggest that you scale this project down to one circuit at a time; and bread-board the design to test. At the very least you could Spice your circuits.
Will your part specified opto-coupler function with the actual part numbers you plan to purchase and use? You have provided specifications and calculations, but have not presented any āreal-worldā tested and measured actual values.
You donāt have one specific question in most of your posts. It almost appears as if you are relying on the good people here to finish your final project to graduate EE.
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Hi Jim,
Thanks for the comments.
You are right, these are good people and I do appreciate their kindness, guidance, feedback and their willingness to share their knowledge. I have learnt much already. As a past educator, these are traits I admire and respect.
Iāve been interested in electronics for some time but have not had the opportunity to pursue this interest. Iām long past my first degree and going back to school is not an option so Iām picking up what I can, where I can. I find I learn best by doing challenging projects but am very happy to read a text or 2 if anyone can recommend.
I have made 4 versions of this system on prototype boards and have encountered various problems that I am trying to solve. I donāt have a stock of parts to draw from or a budget so Iām trying to narrow this down as much as possible.
Thanks for the advice, I will look into Spice for simulation.
All the best
Adam
To Adam,
As I said, I have no idea of your intentions. However, my assessment that you appear to be biting off more then you can chew remains.
This can have consequences that end up being negative for you.
At a previous employer of mine, a new board was designed with modern parts to duplicate the individual circuits of an older design where replacement parts were quickly becoming obsolete.
The engineer assigned to the task bread-boarded every individual circuit with āactual part numberā new parts. Even with all the individual testing, there was still some need to tweak some values on the finished board.
Unless you are an extremely talented electronics technician, the likely-hood of you trying to figure out why your finished board doesnāt work is unlikely; to many untested variables in every circuit not physically tested.
3 Likes
You may be correct, but I donāt think so. I have multiple tours on both sides of the classroom podium and I think I can identify that species after this much observation. I agree that Adam is struggling at a task that heās not well prepared for, but he is teachable, asks reasonable questions, and shows initiative. Many comments in this thread have emphasized āwhat to doā more than āwhy we do itā but Adam seems to be picking up some of the background fundamentals.
In the end weāll help him produce a functional product. Optimal? No. The evaluation will happen when he gets into his second project.
Dale
4 Likes
Thanks again Jim,
I appreciate the advice and good intentions. I found the issue with my previous board and built a new board last night (although not completely like the schematic - Iām waiting for some parts). After testing it seems to be working properly.
When I receive the new parts I will test the new design and will begin, as you suggest, by building individual circuits.
A specific question. Like software debugging, where unit test are written and continually run to check code, I was planning on developing a series of tests that can be run āon the boardā with a multi-meter. My plan was to develop a spreadsheet checklist with input and expected output that I can run through. Iāll do this by hand as a good learning exercise, but I imagine there must be plugin or package that does this (I want to verify my checklist) but cannot find one? (Design rule check button in KiCAD EESchemea seems more to test connections and shorts). Perhaps this is part of the simulation in Spice? Any thoughts here would be appreciated.
Thanks
Adam
Here is an example of my first design in KiCAD that I made a PCB from. Its a simple connector board for a group of micro-switches. I wanted something simple that I could learn something about PCB design and fabrication. All comments welcome!
Hi Rob,
FYI as you thought, a wrong connection was the issue here. When soldering I shifted the ground 1 pin over and it was connected to the input.
Thanks! These look interesting and this suggestion is helpful as I doubt I would have learnt about resistor arrays in the near term.
Avoid this 90 degree turn of then traceā¦
See green arrow, straighten out the acid trap ā¦
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Thanks for the advice. Changes made. Will look into tips for PCB routing.
Well this came as a bit of a surprise! Up to now I assumed you were planning a though-hole board otherwise why bother with a breakout board for the DRV8825?
I think if I were you I would plan on a through-hole board, they are easier to assemble and rework.
Havenāt we all at one time or another? And wished there was someone we could ask for help? When I was learning this stuff I didnāt even have the Internet. I spent my time at the local book store or the library until I met an Engineer who was working part-time at the local Radio Shack store. He invited me and my project to his house one Saturday afternoon. I learned a lot that afternoon in this relatively small room surrounded with test equipment I could only dream of owning. I was trying to get my first computer circuit to work and that was when I learned about tri-state buffers.
When Adam mentioned in his first post that he was a beginner I assumed he meant beginner with KiCad. I have since learned what he meant. I agree that Adam inevitably needs to take responsibility for this project and only proceed with parting with his money to have boards made when he is confident he understands the design and layout.
Not knowing what his previous electronics experience is I have been trying to mention terms like āflybackā, ākick backā, āfree-wheeling diodeā, āclamping diodesā, āPWMā, etc. in the hope that he would research any that he wasnāt already familiar with. Iāve tried to direct him to use some of the details found on data sheets. With a bit of guidance and not much hand-holding I think he has done well at incorporating the many suggestions weāve all had and his schematic has certainly improved. I hope he is understanding the reasons for the suggestions, if not it would be in his best interest to make the effort to do so.
Seriously? His project is actually not that complex and being predominantly digital would not really benefit much from the time it would take to simulate in Spice.
Looking forward to the next revision of the schematic. 
2 Likes
Youāre worried about the impedance of a trace that connects to a microswitch?
What acid?
Hi Rob,
I am happy to go either way on the mounting. I have good dexterity and am confident in my soldering experience. We also have a class coming up here at my local maker-space on SMD that I was thinking of taking. If it would make assembly easier, then Iāll go through hole.
I was asking about the sockets for the LTV-847 and UNL2803 in case I burn these out it would be nice to be able to pop a replacement in rather than work with a soldered part. The socket seems cheap, but perhaps there is a good reason not to use them.
The DRV8825 from Polulu comes as a breakout board. I like the modularity because I have burnt out a few drivers and its nice to simply pop a new module in. Iām open to any advice here.
As for the terminology, I look them up and spend time reviewing and have found this very helpful.
The last time I did any real electronics design was a class in high school. We performed basic math with logic gates, made our own masks and etched some PCB boards and learnt about basic componentsā¦that was 30 years ago.
Iām a biomedical engineer by training but specialized in implanted systems and not electronics. I taught for some time and found that most students did well with a bit of a nudge in the right direction as it saves time, focuses effort and keeps morale up. I find the same here, and hope it is not too much effort on anyoneās behalf.
I make a sincere effort to follow up on all advice.The forumās nudges and suggestions are very valuable, and often it really helps just knowing where to start. Iāve learnt a lot in the past week from this forum at a pace that is much faster than I would have learnt alone and am very grateful for the patience, insight and guidance.
Adam
Here is a modified version of the PCB. I also widened the trace widths to 0.5mm from 0.35mm
I have 3 questions:
- I see that many boards have a large fill area of Cu that is the ground. Is this a better approach then using a trace?
2.On this board for the MS cable connector the ground is a trace (pin 1) Positive is pin 2 - would separating them as far as possible be a better idea (positive to pin 6)? Same for the microswitches pins 1 and 2 are also - and +.
- The layout was fairly simple and Iāve managed to use one side of the board. Wondering if I should use both sides as a way to learn about using vias.
Thanks
Adam
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One of the processes for etching away the copper on a circuit board used a chemical solution containing hydrochloric acid. (Perhaps a mixture of hydrochloric acid and hydrogen peroxide, but I am so far removed from college chemistry that I donāt know if that combination makes sense or not.)
āAcid trapā must be close to an archaic term. It refers to the tendency for various chemicals used in the printed circuit fabrication process to either become concentrated, or diluted, where pieces of copper join at acute angles. The end results are defects such as over-etching, under-etching, undercutting the copper foil, etc. To avoid acid traps you used mitered corners on traces and kept all angles at 90 degrees or greater where copper sections joined.
Twenty years ago, acid traps were a significant concern. Some layout programs had DRC algorithms that identified potential acid traps and urged the designer to correct them. I donāt know if the original concern was exaggerated, or perhaps fabrication machinery has changed, or the chemical processes themselves have changed. At any rate, it has been more than 5 years since I heard a board fabricator use the term āacid trapā. Perhaps there is equipment or processes still in use where acid traps may be a problem, but it doesnāt seem to be a concern among contemporary board fabricators.
There is still a justification (in addition to aesthetics) for avoiding 90 degree corners on tracks: a mitered or radiused corner usually lets you squeeze more traces into a tight space.
Dale
If you have trace enter pad at 45 degree , it traps acid easily leading to it not able to fully wash off easily, potentially causing over-etched condition. In thinner traces, at times leading to open.
CL
Well, it all depends upon perspective I presume.
Iāve done both the bread-board and Spice for small circuits. And, I have learned from both methods.
For example, Iāve seen enough changes in LEDs over this last few years that I would definitely test the optocoupler circuits by themselves and read the actual current draw on the power supply at the expected voltage range.
That is much easier to do on a bread board instead of trying to swap resistors on a finished board.
My posts have been slanted with a slightly negative tone then posts by others. Only trying to be realistic in that having knowledge of why things are done greatly increases the chance of the finished product performing as expected the first time around.
Yes, I have heard about the potential for problems with this type of angle was an issue, but I never knew why.
A couple of days ago I extensively studied an HP printer main board. The traces broke every old rule I could think of.
If a major manufacture that makes millions of things can get away with tossing out the āoldā rules then I figure that I probably can also. Time will tell as my latest design does not follow these āoldā rules.