info default logic level labelling

Hi all! this is my first post so hello to everyone ^^

I'm new to sensor interfacing so the first thing I noticed is that some sensors are 5v and some others are 3,3V. I bought some sensors on gearbest and some of them doesn't have a datasheet and I can't find it. for example this sd card reader: http://www.gearbest.com/development-boards/pp_63013.html it has a 3v3 pin, so as far as I can understand all logical inputs must also be 3.3v. but what about this GPS: http://www.gearbest.com/rc-parts/pp_27027.html ? it only say "VCC"; now by experience I know it runs well at 5v but where can I read it? you might say look at the main chip and look for the datasheet. The "problem" is that some of those shields (or "breakouts" as they call them), also have a logic converter and a voltage regulator so they allow 5v and 3.3v. how can I know if I should supply 3.3v or 5v? Is there some sort of standard like VCC means always 5v (logic and power supply) and if instead labeled 3v3 means all logic and power supply must be 3.3v?

thank you!

Welcome to the forum. SD & uSD cards need 3.3V, can't tell from that website/picture how or if it is doing 5V to 3.3V conversion. Maybe you can tell from the manufacturer website, www.lctech-inc.com. Appears to show a 3.3V supply pin, so 3.3V levels are probably needed. Looks like a cap and 3-resistor package on the top?

The GPS - if you can't get a datasheet or schematic for the module or the main pieces on the board, then chase down the manufacturer/supplier for more info. You can find pieces here & there, for example the More Info and Download Tab here http://invention-zone.com/kedai/index.php?id_product=49&controller=product

Haven't seen much info about that module on the actual Ublox website tho. http://www.ublox.com/en/

So it basically is hit or miss - some component suppliers are better about providing info than others.

Thank you for the reply!

before posting here I went to the lctech-inc website but I didn't find a page about my specific board so I thought there should be an un-said rule which I wasn't aware of. I think I have to ask to gearbest guys those infos or send an email to the lctech support. anyway it looks like 4 resistors and 2 leds near the pins and no ICs so it looks "passive" to me.

I read that those little damn things called micro sd could drain even 200 mAs when reading. are they safe to be powered with UNO?

about getting the logic level down to 3v3 I read you shouldn't use resistors because they introduces noise in the circuit tampering the datas, but yesterday I was reading here (sorry: italian site) http://www.mauroalfieri.it/elettronica/tutorial-configurare-xbee-arduino.html that this guy power 3.3v xbee modules with a voltage partitor. who's right?

what if I use a transistor with 5v digital signal on the base and 3.3v on collector? is the transistor's saturation quick enough?

about the hit or miss: 5v -> 3.3v blow things up but 3.3v -> 5v doesn't, so am I right, if I am in doubt, to try always 3.3v first to be shure to don't break anything?