morning

hey guys, can anybody recognise or at least help me identify what this device does? i found it in a room i was staying and got super curious as to what it is o_O dropbox.com/s/gcdqf81y59207am/IMG_20140902_000542.jpg?dl=0

Hello...

Ahoy

@rebelliard Do you have a pic of the whole thing?

Maybe you can help me:

my.st.com/public/STe2ecommunities/mcu/Lists/cortex_mx_stm32/…

@Roh Change the code so the sender only sends 1 char per second. Change the receiver so it toggles the LED every time it receives a character. Do you see what you expect?

@ThePhoton i took the picture as close as i could and definitely could not dismount it.

@rebelliard It's mounted in the wall?

Or in a suitcase or the tv set or the ac unit or what?

Is it mounted up by the ceiling or down near the floor?

@ThePhoton it's mounted on the wall, separate and another wall altogether than the TV's, and it's mounted on top right before the ceiling of the room.

@ThePhoton a friend thinks it's a movement detector? But we can't really find any info on the item on the web.

@rebelliard Then my best guess it's a smoke detector. But I have no idea why it has a "mic" port then.

@ThePhoton someone thinks this is the manufacturer , but i don't see the product: www.gaofeiaudiophile.com

@rebelliard Could've been a special project or something.

This is in a hotel? in China?

@ThePhoton a hotel in the caribbean (run by chinamen, though)

@ThePhoton like a motel.

@rebelliard Well it looks over-fancy for a smoke detector, but otherwise why the laser warning? Maybe they hold raves with laser light shows there?

morning

@ThePhoton the room does have "disco" lights?

@ThePhoton it does, is what i meant.

@rebelliard OK, then I'd guess something related to a sound system. Still not sure why you'd have the mic port and a big scary laser (tm) on one panel.

@ThePhoton super weird. thanks for the bounce off, though. :)

But the wording on that laser warning is for something pretty powerful,.

Does anyone here have experience with LED circuits that could lend some advice to a noob?

An LED circuit that lends advice?

haha

that would be remarkable.. but I meant a person

I have noobish questions, and I was told about this chat in a comment on one of my questions

Anyway, if anyone could lend some advice it would be greatly appreciated. Basically, I got some cheap LEDs (forward voltage = 2.2V, forward current = 700mA). I also have an old PC power supply (3.3V, 5V, and 12V). I was playing around with some calculations and actually got an LED to light using a "current limiting resistor".

First I tried with just one LED and a 50 Ohm resistor, in series, attached to 5V rail on a breadboard

then I tried two LEDs in series (4.4V total) leaving 0.6 volts on the resistor.

Ultimately, I was hoping to design a circuit that minimizes the amount of power dissipating in the current limiting resistor, and the best I came up with was 5 LEDs in series (11V) with a single resistor, on 12V.

1V * 0.7A = 0.7W of power dissipated in the resistor.. I think..

vs. 0.6V * 0.7A = 0.42W of power (but for only two LEDS, instead of 5)

am I calculating efficiency properly, or are there other things to worry about in a design like this?

you are correct for your 12V option to be more efficient than the 5V option

however, it is important to look at the value for the resistor you calculated.

R=V/I = 1/0.7 = 1.42 ohm

You can buy 1.2 or 1.5 ohms at 1W for reasonable prices.

1.2 is a bad choice you would stress the LEDs

with 1.5 ohms you will be slightly under max spec, which actually isn't too bad a design decision.

I have been using 50 ohms.. which I think makes the LEDs dimmer (since less current flows)

so.. I take it the "forward current" is the Max current.. then?

Actually when the 1.5 ohm resistor is 5% accurate, a common value, in worst case you'll peak just above 700mA.

yes usually maximum current is listed

the resistor I was using was a tiny little one with color stripes on it (a radioshack beginners kit one)

and the forward voltage is listed at a given current

the forward voltage varies slightly with current.

ah

it said max voltage was 2.4V.. but I was going to shoot for 2.2V..

so they last longer

but I guess I don't really control the voltage across the LED, just the current.. right?

yeah the current is the major factor you want to control

if you have a datasheet for the LED, the voltage curve should be in there too as a graph

ok, I think I'm starting to get the hang of it

unfortunately, I got these on ebay from china.. they didn't provide a proper data sheet..

I was given advice not to buy without a datasheet..

but had already made the purchase

for example if you have a low'ish current, the forward voltage could be 2V or 2.1V and at max current the forward voltage could be 2.2V.

@snapfractalpop With a bit of luck

and a multimeter

ok.. so there is some "play" in the voltate drop across the LED

you should be just fine

nice!

so.. I have some little tiny resistors.. I am starting to think I can't just put a bunch of those in parallel..

hm..

Hey you have a known resistor, so if you know the resistor and you can measure the voltage across it, you know the current in the circuit.

Then you can measure the voltage across your LEDs and you know the forward voltage at the given current

ok cool

Just remember P = V * I and small resistors are usually 250 mW.

maybe it is printed on the card that came with it. The max. power.

so they'd probably break, I assume

it is per resistor

the smallest I could find was 50 anyway..

if you have 50 ohms

so I'd need lots of those in parallel

and eg 2 V across them

I guess I'm better off buying one like you said.. a 1.5 ohm 1 watt..

that'd be P = V^2 / R = 4/50 = 80mW

if you want the LED at full power, yess

awesome!

but you don't want to stare in the light

damage eyes?

I'm guessing that at full power, these will be pretty bright

Don't know about damage, but it'll hurt

because they were really bright at just 40mA or so..

you'll see spots for a couple minutes

just don't stare in them, you only have two eyes

I have 50 actually.. haha

I guess that's overkill

oh.. I thought you meant LEDs

haha

I don't have 50 eyes

good luck with your project. Enjoy the learning and experiments

as for heat dissipation

time for bed for me

oh

ok, thanks for your help