Big Mess o' Wires

A few months ago I posted a long essay on my professional future. Should I try to make a career of BMOW blogging? Find an electronics industry job? Return to video game development? Launch a start-up company? Since then I’ve been engaged in many parallel paths, trying to do all those things at once. I’ve drafted business plans, pitched to Sand Hill Road venture capitalists, taken countless coffee meetings with angel investors, rubbed elbows at meet-ups, and generally abused my network of friends, their friends, and their friends’ friends for any worthwhile leads on interesting opportunities. It’s been a crazy, exciting, and confusing time.

I’m happy to say that all those parallel paths have finally converged, and this week I joined Kickstarter wonderkid OUYA as Head of Engineering! It’s software + hardware + games, and I could hardly imagine a better opportunity. OUYA is a $99 video game console, based on the Android OS, where all the games are free downloads. By removing the roadblocks associated with traditional console development, it opens up the TV as a platform to smaller indie developers for the first time. Every OUYA console is also a dev kit, so there’s nothing more to buy. It’s built on Android, so developers already know how it works. Gamers benefit from increased variety and the ability to try any game in the OUYA catalog before purchasing additional in-game content. Hackers are welcome, and rooting won’t void your warranty.

The hardware is based on the Tegra 3 SoC, which combines a quad-core ARM processor with an nVidia GeForce GPU. This is the same chip used in Android tablets like the Google Nexus 7 and Asus Transformer Prime. Those who don’t follow mobile technology closely will be amazed at how powerful this hardware is. Placed in a game console with HDMI output, and freed from concerns about battery life, the Tegra 3 can compete on the level of the Playstation 3 and XBox 360. The bluetooth controller combines analog sticks, buttons, and a touchpad, and boasts a beautiful industrial design from fuseproject’s Yves Behar.

OUYA launched its Kickstarter campaign earlier this summer, and blew past its funding goal within the first few hours, eventually raising more than $8 million in pledges and pre-orders. The response from both gamers and developers has been overwhelming. Clearly the OUYA concept has struck a chord within the community. There’s been some understandable skepticism about OUYA in the press, suggesting it’s a group of well-meaning amateurs with no hope of actually delivering on its promises. But after digging into the details myself and getting to know the leaders of the company, I can tell you nothing could be further from the truth. I’m confident in betting my own future on its success.

This is a hardware blog, so I need to emphasize that I’ll be running the software development part of OUYA’s business. The hardware development is in the capable hands of Muffi Ghadiali, a veteran of past consumer electronics projects such as the Kindle Fire and HP TouchSmart computer. nVidia and its Asian manufacutering partners are also a huge source of assistance. So I won’t be breaking out my soldering iron at the office, but my background in electronics means that I can work effectively with the hardware team. And I get to geek out with pre-release hw prototypes!

My task is to finish development of the OUYA SDK, the console’s browser/store interface, the developer portal web site, the payment system, and all the other technology that’s needed for the March 2013 launch. I’ll be doing all this with a team operating from a satellite office in the San Francisco Bay Area (marketing, sales, and OUYA’s corporate office are in Los Angeles). This is much the same kind of work I’ve done in the past at Trion Worlds and Electronic Arts. At Trion we built the MMO RIFT, which was a great experience in leading a team to build a complex online service from scratch. And at EA I created many console games for the Playstation, XBox, and other systems, and I know what a pain it is to deal with the balky dev kits, tools, and mind-numbing technical requirements. I’m thrilled to be introducing a new kind of video game console that will open up the TV platform like never before.

 

 

 

 

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Frequent BMOW readers have doubtless noticed the lack of project progress  lately. I could claim I’ve been trapped under a piece of heavy furniture, but the truth is I’ve been procrastinating while trying to set a new course for my professional future. That’s a fancy way of saying I’m busy looking for a job. But I can’t post an update here without at least some topical content, so here are the two editor’s choice blue ribbons that BMOW won at the 2009 Bay Area Maker Faire. O’Reilly Media just sent them yesterday, and posted a complete list of winning projects, even though the event was almost three years ago. Their “to do” backlog must be as long as mine!

Despite the contents of the BMOW web site, I’ve never had a professional job in electronics or computer hardware. My career has been in software development, with most of it working in the video game industry in engineering leadership and management roles. Most recently I was involved with the launch of RIFT, a sprawling fantasy MMO game that took five years to develop. I was one of the first employees, and I led the engineering team that developed all the client, server, tools, and other technologies from the ground up. It was an incredible experience, and the game has raked in money since its launch, but I’ll never again run a project whose development lasts half a decade.

For the past several months I’ve had no job, by choice. This has been hard on my finances, but it’s provided me an opportunity to consider lots of interesting and unconventional ideas for the future.

Make BMOW a career.

My first instinct was to turn the BMOW projects into a self-financing operation, creating a micro-business from the sales of project hardware and advertising revenue from the web site’s content. You’re probably already familiar with many such operations, like Evil Mad Science, Dangerous Prototypes, and Modern Device. I may still give this a shot, but my guess is that BMOW projects are best off remaining as a hobby. I’d hate to be forced to make all my projects “useful” in a commercial sense. I doubt there’s much market for hand-made 8-bit computers, for example, even if designed as an educational kit for nerds. And I don’t like the idea of filling the web site with a bunch of advertising crap.

Find an electronics job.

If turning BMOW into a business isn’t the answer, then maybe an engineering job at a hardware or electronics company is a better solution. The San Francisco Bay Area is a pretty good place to find such companies, after all. I looked into the options in this space, but came away disappointed. Without an electrical engineering degree or any past professional experience working in electronics, I would have to argue my qualifications based solely on my hobby project experience. Some companies might consider that favorably, but most would not. And even if I could land an EE job, a hands-on hardware engineer role would be something of a step backwards on the career ladder for me. Better would be a technical management job at a company making hardware or software/hardware products, but I don’t think anyone would hire me to do that without past domain-specific professional experience.

Continue on the software or gaming path.

The most obvious route is to continue on my current path, and seek an interesting technical leadership role at a game developer or web-based business. In fact this is what I’ll most likely end up doing. I’m very fortunate that there are many good options for me in this area, and I have plenty of personal contacts at local companies, so it’s more a question of finding the best fit than of finding any job at all. My LinkedIn profile says I’m seeking opportunities that combine technical leadership with wider product development and business responsibilities. Translation: a software technology-oriented role that isn’t exclusively about engineering, but more about the whole product. If you’re in the San Francisco Bay Area and have any leads to share about such positions, let me know.

Bootstrap a software business.

Another option I’ve considered is building a niche software product by myself, and turning it into a small business. With low development costs (primarily just the cost of my own time), the business wouldn’t need a tremendous amount of revenue in order to be successful. I have a few ideas in particular involving casual strategy games for kids, and I may pursue one of these even while I continue to examine my other career options. Bootstrapping isn’t my preference, though. I prefer working with teams to working alone, and the quality level I could expect to hit would certainly be lower for a solo project than for something developed by a team of experienced developers. I’m also acutely aware that having a couple of game concepts doesn’t constitute a meaningful business plan.

Launch an investment-backed startup.

The startup company concept is deeply embedded in Silicon Valley’s culture, yet it was only recently that I began seriously considering it myself. Having now been an early employee (non-founder) at three startups, and having lots of friends and colleagues who’ve done it successfully themselves, I’ve slowly realized that successful founders are just smart, motivated people not very different from myself. I have a few friends at venture capital and investment banking firms to whom I could bring ideas, and many more well-placed friends-of-friends I could meet with an introduction. From a practical standpoint, then, getting my foot in the door of the startup dating game wouldn’t be difficult. Investment backing would enable hiring an experienced team to build a high-quality product, and would also bring referrals to potential cofounders with the operations, financial, and marketing skills that I lack. What’s missing is a compelling product idea, and perhaps another cofounder or two with a complementary background to my own. I’ll be working on both of those needs over the next few weeks.

Been there, done that?

Why am I analyzing my professional future here, as if it were an interesting circuit to debug? My reasons are selfish: I’m hoping for your advice. Have you been in a similar situation with your own career? Ever tried to turn a hobby into a vocation? Ever bootstrapped a product, or launched or startup? How did it go, and what did you learn from the experience?

 

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Have you always wondered what would happen to a Backwoods Logger Mini if it were crushed under your own body? No, neither have I, but today I found out anyway. I took out one of the newly-assembled Mini prototypes for a trail run, stored securely in a plastic case in my hip pocket. I wish I could say I was chased down a cliff by a mountain lion or something equally exciting, but the truth is that I tripped on a sidewalk crack before I even made it to the trail. I was running downhill and moving pretty fast, so I went skidding and bumping down the sidewalk with pieces of my hands, knees, elbow, and hip left behind on the concrete. As I hobbled back home, I heard some ominous rattling noises in my pocket. Not good…

Further examination releaved the sad truth: the Mini took a direct hit when I fell, with all my body weight coming down on it, crushing it between my hip bone and the concrete. The plastic case was completely destroyed and smashed to pieces. The OLED glass was crushed, and part of the ribbon connector ripped off. The NEXT button was flattened and the spring mechanism killed. On the back of the Mini, the header pins were bent nearly 90 degrees over, the negative battery terminal was ripped straight off the board, and a bit of wood got stuck in the RTC crystal.

No, it does not still work.

I’m upset at having lost a prototype, since they take considerable time to assemble and the parts aren’t cheap. At least this makes a more interesting story than losing a prototype to a soldering error!

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I don’t like to solder. Nor am I very good at it, despite having soldered loads of boards. The temperamental solder never seems to flow where I want it. Surface-mount soldering in particular used to scare me enough that I didn’t even attempt it. A few years back, I gave Wired my opinion that surface mount chips are out of reach for non-professionals, or at least for anyone without advanced skills and special equipment.

Since then I’ve changed my outlook, and I now believe that successful soldering is primarily a matter of self-confidence. The only soldering project you’re 100% guaranteed not to complete is the one you don’t attempt. Sure, you might mess things up horribly, but you probably won’t. And even if you do make a mess so awful that it can’t be salvaged, what’s the worst that can happen? You’ll lose a few dollars worth of parts, but gain valuable practical experience. Consider it the cost of your soldering education.

I arrived at this conclusion after some reflection about my own soldering track record. I promise you, I am not a very skilled solder technician, and yet in all of the many boards I’ve assembled with countless hours of sweat, tears, and swearing:

• I have never destroyed a part
• I have never failed to get the board working

Some day I’ll screw up and ruin a part or a board, but the fact remains that the biggest thing holding me back from more complex soldering projects was not any particular skill or tool, but just the confidence to attempt the project in the first place.

 

BMOW Soldering Q&A

There are tons of great tutorials on the web about the mechanics of soldering, like how to bring the iron and the solder to the joint, and how to drag-solder SMD parts, so I won’t discuss that. Instead, I’d like to share some ideas about my overall approach to soldering, in the hopes that it may help others.

 

Soldering: Now With Undo!

Q: How can I prevent making mistakes when I’m soldering?

You can’t. I used to believe that soldering had to be done perfectly on the first attempt, and mistakes were fatal. Maybe I was vaguely aware of some techniques to fix mistakes, but assumed they were “advanced” techniques that didn’t apply to me. Nothing could be further from the truth! You wouldn’t expect to type a long document on a keyboard without a backspace key, and making and fixing dozens of typos is a normal part of the process. Soldering is just the same. Even on a small board, I normally make five or so soldering mistakes like bridges or misaligned parts, but I’m able to fix them quickly and move on.

Q: So how can I fix mistakes?

Solder wick (desoldering braid) is your friend. Got solder where it shouldn’t be, or too much solder on a joint? Lay the braid on top of the solder you want to remove, then press down on the braid with your iron. The excess solder will be sucked right into the braid. If you’re not soldering with solder wick, then your keyboard doesn’t have a backspace key. Get some.

Flux is even more useful than solder wick. Your solder may have some flux in the core already, but trust me, you need more. I use a flux pen that works just like a magic marker, enabling me to paint flux easily on to any joint or pad. Got a solder bridge between two chip pins? Apply some flux, then touch the pins briefly with the iron, and the bridge will often disappear. Got clumpy solder that doesn’t seem to want to flow where it should? Add more flux and reheat. Still not working? Add more flux. Then add still more. Flux fixes practically anything.

Q: I want a nice, professional-looking board. Won’t all that fixing look messy?

Yes, your board will probably look like crap, but it’s not a beauty contest. Let go of the idea that the board should look pretty. The only important question is whether it works.

Q: What if I make a mistake that I can’t fix?

Whenever I’m assembling a board, I always buy enough parts to build two of them. That way I’m less nervous about screwing up the board, because I can always trash it and try again with the second board if I make a huge mistake.

Before you trash a failed board, though, give it some extra effort in the attempt to fix it, and don’t be shy about it. Solder on some jumper wires. Burn things. Use a hammer. If you break it, you’re no worse off, but you’ll have gained more experience. Then try again with board number two.

 

Small Doesn’t Mean Difficult

Q: Some of those SMD parts are awfully small– those can’t really be hand-soldered with an iron, can they?

Yes, they can, easily. The surface tension of molten solder naturally makes it flow where it should, and soldermask deters it from flowing where it shouldn’t, even for very small parts. Mistakes are easy to fix with soldering wick or flux.

Q: OK, but what about those really tiny parts, with like 0.5 mm pin spacing?

Yes, those too. Easily.

Q: What about those plastic ribbon connectors on LCD screens, with the tiny metal contacts?

Those are even easier.

Q: But what about QFN parts that don’t even have exposed pins?

You can hand-solder those too, using an iron, by tinning all the pads on the board first. A hot-air tool makes it easier.

 

It’s Not About The Tools

Q: What do I need in order to do surface-mount soldering?

Not much beyond what you already have, probably. A regular iron with a regular tip is fine. I use the chisel tip that came with my iron, which is several times larger than the pins on the chips I’m soldering. It’s fine if the tip is larger than the pins, because you’re not going to do pin-by-pin soldering of SMD chips. In fact, for all but the largest SMD packages, the iron tip will always be larger than the pins.

Other essential ingredients:

• Flux. Use a flux pen, flux in a jar, or whatever other form works best for you. Flux everything liberally before soldering. Rule #1: If you’re having difficulty soldering a part, you need to add more flux. If that fails, refer to rule #1.
• Magnifier. Most SMD joints will be too small to see clearly with the naked eye. To inspect finished joints for tiny solder bridges or other defects, you’ll need something to magnify them at least 3x and preferably 10x. I use a cheap 10x loupe, which is like a monocle with a magnifying lens. USB microscopes also work well if you want something fancier.
• Tweezers. You need something to maneuver those little parts into place, and hold them there while you solder them. You’re hands aren’t going to cut it.
• Drug-Free Bloodstream. Your experience may be different, but I can’t do fine pitch SMD work within a few hours of drinking coffee. Even tiny amounts of hand twitching make it nearly impossible to align parts with sub-millimeter accuracy.

 

Circuit Debugging Is Fun

Q: How can I make troubleshooting easier when assembling a board?

Don’t assemble the entire board in one pass. I always assemble boards one functional section at a time, and test that section as much as possible before beginning the next one. This greatly reduces the number of possible causes that must be investigated if a problem is discovered.

The first section on nearly every board is the power section. Solder in the battery or power jack, voltage regulator, and other related parts. Next, measure the resistance between power and ground. Don’t just perform a continuity check, but actually measure the resistance. For most small electronics projects, it should be unmeasurable, or else in the megaohm range. If it’s below about 10K ohms, then you may have a very small solder bridge somewhere. If it’s zero ohms, then you have a dead short between power and ground, or a chip that’s soldered in backwards or something.

If the resistance checks OK, then turn on the power and measure the voltage on all the VCC pins. If it’s not what you expected then stop, and don’t proceed with further assembly until you’ve first fixed the power problem.

For microcontroller projects, I normally add the microcontroller next, as well as the programming header and any other components required to get the MCU running. Connecting with the external programmer and reading the device ID or fuses is a good way to verify that the MCU is working properly.

The remaining functional sections vary greatly depending on the project, but often there’s an LCD or other info display that can be tested first, and then buttons or other input hardware. After each section, recheck the resistance between power and ground before turning the device on, since each new part added brings a risk of an accidental short-circuit.

Q: I assembled the board, and the soldering all looks good, but it doesn’t work. What can I do?

In my experience, failures are almost always caused by short circuits (usually due to solder bridges) or open circuits (cold solder joints, or too little solder).

Short circuits are easier to diagnose, since they can usually be seen under magnification if you look in the right spot. Use your multimeter in continuity check mode, and test for continuity between every signal you suspect has a problem and its neighbors. If you’re fairly certain there’s a short, but can’t find it, then test again by measuring the actual signal-to-signal resistance instead of continuity check mode. For some types of signals, even a very small solder bridge with a resistance of 10K+ ohms can be enough to break the circuit, and those typically won’t produce a beep when in continuity check mode.

If that fails, visually reinspect every chip pin for possible hairline solder bridges to its neighbors, as well as bridges between pins and vias. If you see anything that looks like it could possibly be a short, then apply some flux and reheat the joint to clean it up.

Open circuits are a bit trickier, since they can’t be conclusively diagnosed just by looking at them. If the solder in a joint looks cloudy, or there’s very little solder, then check the continuity with your multimeter to make sure everything’s connected as it should be. When checking continuity to a chip pin, it’s important to touch the probe to the actual pin, and not to the pad underneath the pin, since the pin-to-pad joint itself is often a point of failure.

As with testing for shorts, it may be useful to test open circuits by measuring the actual resistance instead of merely performing a continuity check. If two points that should be connected measure 1K ohm of resistance between them, it may be enough to elicit a continuity check beep, but something is clearly wrong.

If all the continuity checking and resistance measuring fails to identify the problem, then you’ll need to perform live circuit debugging. Turn on the power, and start measuring voltages at various places in the circuit, verifying that the voltages are as expected. For DC voltages, this is easily done with a multimeter. For something like a clock signal with a 50% duty cycle, you can still use a multimeter, and expect the measured voltage to be 50% of the high-t0-low voltage swing of the clock.

For other time-varying signals and data signals, you will probably need to use an oscilloscope to observe what’s happening. A few common symptoms and their causes are:

• stuck at VCC or ground: short-circuit to power, ground, or another DC signal, or an unconnected signal.
• stuck at some middle voltage: contention (two different sources attempting to drive the signal), or an unconnected signal (open circuit).
• sort-of correct-looking signal, but reduced in amplitude and shifted far towards VCC or ground: hairline short-circuit to power, ground, or another DC signal.
• random wavy noise pattern: unconnected signal (open circuit).

If all else fails, then you may need to cut traces or remove parts from the board in order to isolate the problem. Remember, if it’s already broken then you’re unlikely to make matters worse, so don’t be shy about getting in there and banging away to debug the board.

 

8 comments

What do small scale open hardware projects do about RF interference compliance testing? I’ve been looking into selling assembled versions of a few of my projects like the Backwoods Logger, to people without the time or skill to build their own. If I’m lucky, I might sell a few hundred such units, through a dedicated store web site, or just a page attached to the BMOW blog here. I would want to do this right, which means complying with any applicable certification requirements for consumer electronic devices. In the United Sates, that means FCC Part 15.

After searching for information about FCC requirements, it appears that anything operating at frequencies above 9 kHz requires FCC verification testing, which costs several thousand dollars. This is true for both intentional radiators (WiFi modules, Bluetooth, remote controls, etc)  and unintentional radiators whose emissions are accidental. By that rule, everything from an Arduino clone to a data logger to a robot control board requires FCC testing. You don’t actually need an FCC ID, but you do need to perform the testing and keep your certificate of compliance on file, should the FCC ever ask for it. And your product must include a phrase like “This product complies with FCC requirements for a Class B device.”

I looked for information about how the “major” hobby electronics vendors handled FCC testing, and found that this is a topic no one wants to talk about. It’s like a dark family secret. Discussion threads get responses like “we can’t legally comment on this” and are then locked. Reading between the lines, it seems that while FCC testing is required for virtually every electronic board and module, almost no one actually does it. But because the penalties for non-compliance are worse if you knowingly sell an untested electronic product, nobody is willing to admit that they didn’t perform the tests, or even discuss the subject at all. I’m not going to name any specific vendors, but if you have any circuit boards on your desk that contain a microcontroller or USB chip or other interesting gizmos, check to see if it was FCC tested.

Have you ever sold an electronic product that you designed yourself? Have you ever taken a product through FCC compliance testing? What was your experience? Leave your feedback in the comments.

9 comments

Sometimes the simplest things give me the most trouble. I’ve been working on a downloader cable adapter for the Backwoods Logger, with the goal of supporting both 5V and 3.3V FTDI cables. Because the Backwoods Logger is a 3.3V design, the incoming TXD (transmit data) signal from a 5V cable needs to be lowered to 3.3V for safe operation. However, the incoming TXD signal from a 3.3V cable should be passed through unmodified. Outgoing signals from the Logger require no conversion, because a 3.3V output is still a valid logic “high” for a 5V system. I need a level converter for a single input, that operates correctly with both 5V and 3.3V inputs with no configuration or jumper settings.

Level Converter Chip

One solution is to use a 3.3V chip with 5V tolerant inputs, like a 74LVC244. That would work, but I’d prefer something simpler and smaller if possible, since I only have a single input to convert.

Clamp Diode

A second solution is to use a series resistor and a clamp diode, like this (image from daycounter.com):

That prevents the voltage at the 3.3V Backwoods Logger input from going more than a diode drop above the 3.3V supply. With a standard silicon diode’s drop of 0.6V, that clamps the voltage to 3.9V. For the ATmega328, that’s not safe: its maximum rated voltage on any input is just 0.5V about VCC. A germanium diode has a drop of 0.2 to 0.3V, so that would work, but it’s not a part that many people typically have handy in their parts bin.

This solution also has the drawbacks of consuming current from the 5V output, and dumping current into the 3.3V supply, raising the supply voltage. The FTDI outputs have a maximum output current of 24 mA. Assuming a germanium diode with a 0.2V drop, that means R1 needs to be at least 62.5 Ohms. Frankly I’m not sure how to quantify the risk of dumping current into the power supply. In the case of the Logger Classic with its tiny CR2032 battery, dumping 24 mA into the battery in the wrong direction definitely doesn’t sound good.

Zener Diode

The approach that appealed to me most was to use a series resistor and a Zener diode connected to ground, like this (image from daycounter.com):

The Zener has a known reverse-bias breakdown voltage called the Zener voltage. Raise the voltage above the Zener voltage, and the diode conducts. The series resistor produces a voltage drop, so that the voltage at the Backwoods Logger input never rises above the Zener voltage. You can get 3.0V or 3.3V Zeners (or lots of other values too).

So I ran out and bought some Zeners, and built this circuit, and it didn’t work at all how I’d expected it to. I used a 3.0V Zener, and a 100 Ohm series resistor, to limit the current drawn from the FTDI cable to 20 mA. When I connected a 5V dummy output, I got 2.91V at the Logger input. That seemed odd, it was off by 0.09V, but it was still close enough. Then I connected a 2.95V dummy input (the actual voltage from my crummy “3.3V” breadboard regulator), and I got 2.4V at the Logger input. Huh? That’s not going to work. I had expected that for any voltage below 3.0V the Zener would do nothing, and for anything above 3.0V it would clamp it to 3.0V, but that’s clearly not what happened.

What went wrong? Truthfully, I’m not exactly sure. The datasheets talk about a minimum current necessary to get the Zener effect, but I’m not sure that applies here. I can’t safely increase the current further anyway without damaging the FTDI cable. But would more current even solve this problem? It makes sense that the Zener wouldn’t really turn on instantaneously at 3.0V, but rather would begin to conduct more and more as the voltage approaches 3.0V. With a voltage of 2.95V, the Zener would already be partly conducting, pulling the voltage seen at the Logger input below 2.95V. But how much below? How can this be quantified?

One thing in particular bugs me about all the Zener diode datasheets: every datasheet lists values for standard measurements called Izt, Rzt, Izk, Rzk, Rz, and a few others. These are standard measures from some hypothetical standard Zener graph, but the datasheets never actually show this graph, and I’ve never been able to find one anywhere. I know “k” is for “knee” and I believe “t” is for “test”, but what I really need is an actual IV curve for a Zener with these values labeled. Then I think I’d understand this better.

Further Complications

Just to make things more interesting, there’s one more constraint to consider. The Logger Classic uses an unregulated battery as its supply. It can work just fine at battery voltages of 2.8V, and probably down to 2.5V or even lower. In order to stay within the VCC + 0.5V margin of the ATmega328P, the input voltage must not go more than half a volt above the potentially fading battery voltage. A standard 3.3V input when the battery is 2.7V would actually damage the ATmega. That’s why I chose to use a 3.0V Zener rather than a 3.3V one. That should be safe down to a battery voltage of 2.5V, below which I could configure the ATmega’s brownout detector to engage.

The Way Forward

I’m going to sleep on this, and see if anything brilliant comes to me. If anyone else has a suggestion, please reply in the comments. Assuming I can’t find a way to make the Zener work while still meeting the other constraints, then I’ll probably cave in and use a level converter chip. Without really understanding the implications of current flowing into the supply battery under the clamp diode method, I wouldn’t feel comfortable relying on that approach.

18 comments