After playing the SD card shell game for the last week, I think I have finally found a way to make sure that I have authentic Sandisk cards for my data loggers. Muve music microSD cards were custom built by Sandisk for Crickets music service, and they put some kind of blocking routine into them that prevents you from seeing 3/4 of the space on the card. As you might imagine, extra custom circuitry which makes a product less functional affects its market value, so they are unlikely to be seen as a profitable target by counterfeiters. Of course I put these used cards through a full test routine, formatting them with SDformater, and then making them sweat through a good bout of H2testw’s verify. And even though what I am after is stable low sleep current, I keep an eye on the meter during the testing. Some of the fake cards I ran into earlier draw unusually large amounts of power during write operations, even if they sleep OK.
More adventures in eBay land:
Going through all that motivated me to make a post on the approximate nature of buying cheap electronic components on the internet. Of course there are warnings about this all over the place. But when I was just starting out, very few people took the time to provide details so I had to learn this from scratch. The easiest way to convey this is with a table showing the photo from the listings vs what actually shows up in the mail somewhere between 2 & 8 weeks later. I would say the current hit rate for actually getting what I thought I was ordering from vendors outside the U.S. is about 60%. The rest of the time you get something that resembles the item in the listing…more or less:
I could go on, but hopefully that gives you a general idea. And this is in no way limited to eBay. But these kind of replacements are usually not hard to work around, and since the parts only cost a buck or two, I’ll make parallel orders from other vendors, just to be sure I get something I can work with. Order different quantities from each vendor when you do simultaneous orders, because the parts could arrive weeks later, and there will be nothing written on the package in English to help you figure out which vendor sent which shipment. Always do a “test” order of a small quantity, before ordering multiple units from an eBay vendor, and don’t be surprised if the product changes like this from one batch to the next anyway.
More concerning are part substitutions directly on the boards. The more components on a breakout, the higher the chance that at least one of them will not be what they claim it is. Complex combinations like you see on IMU’s are a red flag:
|GY-81-3205 10DOF IMU modules are usually described as having the a 1g BMA180 accelerometer (which I use with the HMC5883L in the flow meters) but from places like Miniinthebox, Gearbest, Amazon, etc. they actually ship with crappy 2g BMA020 on board. Of 5 vendors I tried, only NYplatform actually sent this board with a BMA180.|
If I’m rooting around on eBay for a sensor board to try out, I get one with as few components as possible. For example, I use the ADXL345 in my drip sensors. These things are common as dirt, and almost as cheap, but the boards are often laden with dubious quality voltage regulators, power LEDs, etc. In general, the less cruft there is, the more likely it is that it will work, and I always look for boards that have a 3.3v power input.
|Avoid:||Ahh, that’s better…|
Of course this is still no guarantee, especially when a conditioning procedure is needed for the sensor to meet spec. But when the parts are only a couple of bucks, I don’t have worry that much about frying them when I am just noodling around. (I killed several of these things before I realized it that JB 5-minute plastic epoxy dissolves IC housings before it cures…) And it’s just the magic of eBay economics that you usually pay more, sometimes much more, for the breakouts with fewer parts on them. The other thing to watch out for is chip variant substitution on the same board. I use a fairly common DS3231 Real-time Clock module on my loggers, but about half the time the chip that arrives on that board is the DS3231-SN and the other half the time it’s the DS3231-M. While similar in overall function, those chips have dramatically different drift characteristics over time. The vendors just play dumb when you complain about showing photos of the better chip, and then shipping modules with the poorer quality one.
If you paying peanuts for a sensor that normally sells for $20-$30, you should expect a 20-25% failure rate, with about half of those failures being subtle, like for example, one axis on an accelerometer not reading at the same bit depth as the other two, or only reading in the positive, but not negative direction, etc. Always test your cheap sensors against a a “known good” board and one thing I have found to be particularly diagnostic is how much current an IC draws. If the milliamps match the data sheet, there is a good chance that the sensor is Ok. This is especially true of low current modes, as I have run into several cheap sensors that give decent readings, but never go into low current sleep modes. This makes them useless for data logging applications.
It’s usually a much safer bet to get simple passive components on the web, and the difference between low end parts, and brand name stuff is often an order of magnitude or more. When you are just starting out, finding cheap component kits can get you rolling for without breaking the bank (although the thin wires & non standard labeling will eventually get on your nerves) Sadly, I did not find out about those until I had already paid extortionate amounts to radio$hack, etc.
And speaking of places where staff react like I a speaking another language if I use the words like “capacitor” in conversation, I have to add that it really does make a difference if you order stuff from a vendor who actually uses the products they are selling. For example, I recently ordered a large amount of wire from HobbyKing. Not only did they take quite a while to ship it to me from their US warehouse (for items listed as “in stock” when ordered) but they looped the many different types of wires into a single large intertwined bundle. It took ages to untangle that mess, leaving me with the impression that they are just a warehouse operation, charging a markup simply to repackage stuff that they too are ordering from China.
I guess that if I was to sum it all up, I’d say I still get lots of stuff from dodgy online vendors if I am risking $5 or less. The more expensive an item is, the more likely I am to go upmarket to a vendor like Sparkfun or Pololu. For example, when I need a tool like a soldering iron, or a multimeter, I check what they reccomended at EEVblog, then I see what they think about it on Adafruit. Those guys really know their stuff, and I never have to worry about dropping a couple of hundred bucks on their recommendation. (something I would never do with an eBay vendor from Hong Kong)
Anyway, I hope that helps someone out there who is just getting started (like I was not so long ago). Even with the best research beforehand, I still end up wasting about 30% of the money I spend on components, because there are alot of elements to any item that one just has see & use to understand. And there is always seems to be some “physical” factor that the experts thought was too obvious to mention, that dramatically affects your particular project… like… for instance…never putting your coffee down on the work bench beside your newly built data logger 😉