Examples he gives:

"Current limiting resistors missing (BJTs, diodes, etc.)

Shorting a potentiometer node to a rail

Voltage regulators missing capacitors at input/output

Missing pull-up/pull-down resistors on digital buses

Missing decoupling capacitors"

I mean ... looking at a schematic how should you know that the voltage regulator doesn't have internal caps without seeing the specs? Missing pullups? How do you know those are not internal? It is stupid.

Can you name one voltage regulator that has them? Maybe you've worked with a few, but just off the top of my head, the 78xx, 317, and 1117 don't, and neither do all the other miscellaneous ones that I've seen in consumer electronics.

It's not my field and I had little clue what was happening at the EE level. Maybe you could argue they are not what is normally meant by saying voltage regulators.

In general I agree with the GP look at how many unpopulated areas exist on modern boards.

How should you know if the lack of input filtering is because of poor design vs the fact that the spec of the power supply changed and you no longer need it?

If it’s not universally true and obvious it’s not a good test.

Modern boards are super complex I’m not an EE but I would more likely flag lacking probe points, probe points which are too far or too close apart or too far from common ground as bad design choices than try to figure out if there are enough caps for the voltage regulator without knowing the full specs of the damn thing.

For all i know that voltage regulator isn’t even being used anymore because the IC or PSU specs have changed mid production but because it was wave soldered with a different soldering method than the other SMTs they already had them on a bunch of boards...

If you can’t reverse engineer the board or the relevant within a few min it shouldn’t be in an interview.

Not an EE but I do a lot of HW RE if anyone puts a modern multi layer doubled sided board on my lap and tells me to point out what’s wrong with it in an interview I would ask them what’s wrong with them.

That's exactly what everyone says when candidates fail FizzBuzz.

The best explanation I know is that good candidates get hired quickly, so at any given moment, the candidate pool is dominated by people who keep failing questions like this.

- Stress-levels of the candidate - Tone / demeanor of interviewer - Communication prior to interview

I mean, sure, if you fail fizzbuzz interviews 10 times in a row, there seems to be some pattern going. But you could might as well fail it the first time, and get blackballed as incompetent.

It would be great if they'd stumble over conundrums like "what if there are non-numeric entries in the array." Most often, they just stare and don't even start on the problem.

Are you expecting some map-reduce-like parallel sum, or something else?

If the role's primary function is to do validation prior to manufacture this seems like reasonable. it assumes that new circuits will have some number of fairly common mistakes. in my experience this is very true, despite the skill of the engineer who designed the circuit. missing termination, pullup/downs missing, etc. if the role is applications engineering...well...its far less likely you would be troubleshooting common circuit design issues so much as you would be troubleshooting common system design issues. im sure you can extend this to other functional roles an EE may take on. if the job is to design a MCC and size breakers to minimize arc flash hazards i doubt the applicant will be up on a correctly build half-bridge rectifier.

in that list quoted i've encountered all of it in new designs, including missing stuff plainly stated in the part's application notes (like those regulator caps).

This is different than for bulk capacitors, but even those should still be on the same PCB assembly because connectors have higher resistance and this will limit their ability to handle transient surges.

Not all LDOs are stable without external output capacitance in a particular range / ESR.

Understanding that the capacitor and the trace form an RC circuit and that distance matters is what separates a professional from a beginner.

I would still argue that logic IC have alot of internal pull ups. Or that eg. "Reset sequence of digital component is wrong" is a bad whiteboard question.

Otherwise the person may just ask: "where are caps, or is this thing a module or what?"

Definitely not a switcher or module, could be an LDO.

They're useful if the voltage drop isn't too large and/or if you need a particularly stable output voltage; contrast them with switching regulators, which are more efficient but have some amount of output ripple.

One design that I worked on used a switching regulator to generate 5V, then linear regulators to generate something like 3.7V and 2.9V for various sensitive analog circuits.

I think my best one was shorting a (slow) clock to power, which made the whole thing blink on and off.

Digital systems classes can cover it too, when going over power distribution, but that’s usually a graduate level class.

The digital VLSI or signal processing tracks of EE might not require this stuff anymore.

Interviewer: "Can you draw me the schematic symbol for an LED?"

Me: Draws LED schematic symbol.

Interviewer: "What would you say the forward voltage is on that LED?"

Me: "That depends - maybe between 1 and 4 volts depending on the color?"

Interviewer: "Good, let's call it two volts. What would you say would be a reasonable current if we're just using it as a power indicator?"

Me: "I usually set it to around one milliamp on my boards."

Interviewer: "That sounds good. Can you show me how you would connect that to a 5 V supply?"

Me: Calculates ballast resistor size for 1 mA current from a 5 V supply with a 2 V LED drop, then draws a circuit with an LED, resistor, and 5 V source.

---

And on it went. We ramped up through complexity: next we did simple BJT and FET switches, then basic op-amp circuits including single-pole filters. I stumbled on a differential BJT amplifier question, but the interviewer helped me through it. And so on. I didn't nail everything, but I got most things right. I ended up getting the job.

When I asked the interviewer later about his experiences using test for hiring, he told me a sizeable chunk of new graduates flunked out on the first question, which I found pretty shocking. A lot of students with excellent resumes didn't even know how to hook up an LED!

As to the cause - it's just a guess, but I think many "good students" these days are very good at regurgitating information and memorizing procedures to complete formulaic exam problems, but aren't very good at actually developing the physical understanding of what's really going on. When I was in my senior year, I designed a buck converter as a small part of my capstone project. One of my team members remarked that he didn't realize a buck converter was a power supply, and that he had seen it in his power electronics class but had no idea what it was used for. He could do the duty cycle calculations for one all day long, but he didn't even realize it was a DC/DC power supply!

Universities these days do not seem to successfully evaluate deep understanding. Rather, they seem to optimize for the superficial appearance of understanding. Even "lab courses" when I was a student were formulaic: plug this wire in here, put your probe here, read this value here, do this calculation here for your report. There were very few courses that were actually designed to encourage real experimentation and practical learning. A good EE needs to develop a mental model of the underlying physics and be able to relate it to practice, and that is a tricky thing to teach in the classroom.

Anyway, all that said - I really like the idea of the "bad schematics" approach for an interview question. If I ever find myself in a hiring role, I am 100% using it.

At our university we had basically only TWO courses that required you to "understand" to pass them and not just mindlessly puke out formulas and insert values. These were theoretical electrodynamics and integrated analog circuits. These were exactly the exams that kicked people out of university, since you had only 3 attempts to pass them.

So, no, uni's will never try to force understanding, since so few are actually capable of that. Unis teach you to know the basic set of symbols, be a mindless symbol manipulating monkey and hope that industry would pick you up and enlighten you afterwards.

Indeed: as a teacher, I have had the experience that there's pressure to pass underachieving students in the elementary courses (we don't want to scare everyone off by flunking them out of their first courses!) and then there's pressure to pass underachieving students in the advanced courses (it's not fair to pass them to this point, and then suddenly hold them to higher expectations!—or even, directly, "if we don't pass them then they won't be able to graduate").

In the software industry we have a similar problem; alot of people can follow instructions on how to glue together lots of libraries and frameworks, but have no idea how to go beyond that --- or understanding of how computers work in general --- much less debug it when something goes wrong. There's a classic article about this:

https://blog.codinghorror.com/why-cant-programmers-program/

Most small LEDs will tolerate per datasheet 20mA; 4mA is "well-lit". A little less than 1mA is going to be pretty anemic looking on lots of LEDs.

So I'd be more inclined to put down 470 to 1k5.

For use as indicators for firmware debugging under typical office lighting, I find 1 mA to be the "sweet spot" - bright enough that I can tell it's lit with a quick glance, but not so bright that if I'm staring at the board it becomes uncomfortable to look at, or distracting out of the corner of my vision.

However, if it's for indication for an operator of a device, and it's inside a case going through some lossy elements (eg: aforementioned light pipe or diffuser), I'll definitely use more current to make it a bit brighter.

(Maybe I'm biased because I often have my office window open and it's bright... and having couple orders of magnitude of current above what a microcontroller pullup/pulldown will source/sink has been nice sometimes-- that or I'm just stubbornly trying to justify following the very-old 4mA rule of thumb).

But I'm modestly good at math and the EE classes were essentially an elaborate sequence of math riddles (complex functions? Fourier? I'm great at that), superficially excused as representing some "circuit".

1. Extremely theoretical, minimal practical elements (labs, demos, etc.). These would have classes in Electromagnetism, Signal and systems, ICs, RF, Optics, etc.

2. Very practical, but only skin deep on theory. Power systems, electrical circuits, electrical machines, etc.

The very few classes where we had a nice 50/50 ratio of theory / work were perfect, but I always felt like missing out on the other classes. Unfortunately the coursework was quite heavy, so you didn't get much time to do self-study either. Labs always crammed, always some projects / HW / home exams / labs that needed be handed in.

However, you’re often working in groups, so it’s not uncommon for one person to really “get it” while the other members of the group nod along but don’t fully understand the choices being made. In addition, TAs in the lab probably don’t have industry experience, so there are basic practical circuit design practices that aren’t taught or enforced.

Labs are a good way to get initial experience, but I think there’s a lot that you inevitably have to learn on the job. There’s a reason you can’t earn a license as a Professional Engineer until you have 4 years of work experience.

For one thing, many of my professors (in fact, probably all of them) were very theory focused, and had very little experience implementing electronics. I think they figured that practical topics like this were too specific to lecture about, and we'd pick it up on the job anyway. I'd have to assume that mindset is not wholly exclusive to my undergraduate degree.

For another thing, I find that commonly, large employers are frequently not doing much to train their new grad EEs, or even let them near much design work. As a result, they miss out on practical experience that would teach them about concepts like decoupling.

Same! I didn't realize you needed them until I entered industry. I think that's a little disappointing - it's a very simple explanation, just one that most professors don't choose to spend time on.

I'm trying to put together a good demonstration of why you need decoupling capacitors in a lab setting. Best one I can think of is paralleling a bunch of inverters with no decoupling caps, and then measuring the current draw on the rail using a shunt resistor.

Explain please?

Yes, there are some considerations when you're dealing with RF, but a modern 0402 0.1uF cap works pretty much everywhere because it has such a great frequency response due to its small size.

Now, explaining to people that their 10uF 6VDC 0402 across the power rails probably doesn't have as much capacitance as a 2.2uF 15VDC 0402 is a little subtle.

It is also an issue that decoupling caps can hold too much energy and keep a power domain operating when you wanted it shut down. That makes for some interesting debugging. Power domains now require discharge circuits because IC's use so little power.