Hi!
Lately we’ve been doing some research on prices of electrical components.
We have always got our components (sensors, cables, motors, etc.) from AndyMark, But we found them to be much cheaper in places like eBay and Aliexpress.
For example, a proximity sensor costs 35$ on AM (am-2434) and 3$ on eBay (HC-SR04).
Are there any rules against buying components from such sources?
Thanks!
The only rules are in the manual. Nothing specifically against Ebay. It is buyer beware though. Having said that, I have had good luck buyer stuff off Ebay. The pressure transducer we use on the pneumatic system we bought off Ebay.
If you where to buy a custom manufactured part (non COTS) you might run into the COTS definition rule. That would not apply to a commercial product though.
Also pay attention to the sellers reputation. It should always be high.
Thank you for your quick and helpful reply.
We don’t plan on buying any non-COTS components.
Thing is, we haven’t seen any CD threads regarding this question and we were wondering if there is something obvious we’d missed.
Keep in mind you’re also paying for quality. I’ve used the MaxBotix sensor (from AndyMark) before at it works very well.
I looked up the HC-SR04 sensor and read a couple of the reviews on Amazon, I’ve quoted a portion on one below.
These SR-04s are readily available now for under two dollars each. I have a dozen of them from various importers. They work OK with various Arduino libraries. It’s an extremely useful sensor for countless project.
As some people noted, they also return garbage data at times, especially right after the distance to the target changes. Some of the units are better than others that way, some are quite bad. What the issue seems to be is not responding fast enough for the software, or not at all. Many example sketches translate the invalid state to “0 cm”. A filter in the code to discard bad results helps, but that slows average response. If you use the NewPing library’s timer interrupt mode, you get no “0 cm”, the code waits, sometimes a second or more. I’m not 100% clear on this yet. It’s best to buy several and use the better ones. For me, the more expensive 3 pin “Ping” module, that the SR04 apparently copied, performs much better.
We buy from eBay all the time but makes sure to check the delivery estimate. Many of the items ship from Asia and take 3-5 weeks to get to you. We will buy from these sellers during the off-season when we can wait but during build season it’s often better to pay slightly more for a much faster delivery time. Also don’t expect much in terms of customer service, or even manuals or data sheets some of the time.
2014 rules. 2015 rules can change.
DUH. :]
If they purchased the part before season, already have it in stock and it falls within the rules of legally usable parts then how could they not use it? Even if the rules did change for 2015.
Definition of COTS changes.
Or definition of VENDOR.
Or cost limitations on individual components.
One of the downsides of doing business with China (eBay or otherwise) during a build season is that you can spend a great deal of time awaiting delivery. I’ve had packages delayed by as much as 82 days. So I might try to find someone in my country that already has these items such that I can get them faster. I do this with other items like Olimex development boards.
I bought some of the SR-04s while I was buying some cheap logic analyzers from China. I find that the MaxBotix unit from a few years ago may be more expensive but it tends to be more consistent across lots. However even the MaxBotix unit could read wildly wrong during periods of movement especially in the presence of noise sources or electrical noise.
Before anyone demands proof that Maxbotix sensors can have issues let me take care of that for you:
http://forum.arduino.cc/index.php/topic,140936.0.html
I think it greatly matters to me what I am using the sensor for.
If I am just tinkering then the SR-04 is generally fine unless there is danger involved (robotic lawnmower for example).
I might be willing to use it on a FIRST robot but honestly I think these sorts of things are like encoders: time is short and you really do not want to be messing around with inconsistent COTS parts.
Personally I love Ebay for almost everything. I ahve bought several Aduino clones, tons of pin headers for breadboarding (as well as breadboards), tons and tons of ICs, used Mitutoyo calipers, micrometers… the great majority of everything I own is from Ebay actually.
That being said, for FRC I wouldn’t use Ebay. The shipping times can be relatively fast (~3 days max if you I from the USA), however, if a part breaks (which they do) you need to wait at least another 4 days to get another part. I have gotten refunds or replacements for 100% of the items which have broken (in one case I actualy ended up with a refund AND parts which I later fixed…) but because the parts are for hobbies I have no time limit. Support is fantastic, but you still need to recognize that breakages can and will happen on a relatively large percentage of parts (still like 1%) and if it happens at competition, that could be disastrous. For simple stuff like decorative leds it should be fine, but stuff like cables or sensors can go bad pretty easily (particularly cables; of a 40 pack of jumpers for breadboard there’s always one or two that fail).
My $0.02.
Just for safety, I wouldn’t buy sensors on ebay, you might get a good part, but then again you might get junk that somebody is scamming you with
I wouldn’t make a blanket statement like that. 
Ebay is a lot more buyer beware than some other sources, but it is a good resource. Pay attention to the seller reputation. Pay attention to where they are shipping from & what they say about their shipping practices. Read the description carefully so you know what you are buying. Think about how you can test the product you are buying.
A lot of brick & motar companies use Ebay to sell overstock & discontinued products.
We (836, not 365) picked up some pressure transducers for $18 shipped. They work great and have helped a lot in data logging. They were accurate and provided invaluable insight into the transient response and recovery of the high pressure system on the Aerial Assist bot.