A thing about usb cable lengths!

 Well apparently according to many websites I've visted, claim that the longest cable you can use for a usb device is just a little over 15 feet.

That said, I'm one for completely disregarding warnings. According to the text on one website:

Q: How long of a cable can I use to connect my USB device?
A: In practice, the USB specification limits the cable length between Full-speed USB devices to 5 meters (A little under 16 feet, 5 inches). For a Low-Speed device, the limit is 3 meters (9 feet, 10 inches).

So my dilemma is that I have an a$$hole neighbour that is stealing things from me and I want to catch him. Being cheap, I'm going to use my webcam instead of spending $50 on a low resolution color camera that only outputs composite (although that would allow me to make cables easier and I DO have a capture card on my pc) but this is besides the point.

So I found a telephone cable (as it has 4 wires just like USB) and I figure it's got to be AT LEAST 20 feet long, maybe 30, which is long enough for what I need. I thought because It's all over the internet how apparently the limit is 15 feet, that is would just NOT work at all. 10 minutes later I had spliced the telephone wire between the USB connector and the webcam, wiring black to black, red to red, green to green, and yellow to white (usb side).

Upon plugging it into my desktop usb hub, sure enough, my computer recognized it, and the video came through just fine.

So I post this forum-topic to SHOW that it is infact possible to go OVER the so-called usb limit. I got the idea originally from a video I watched last night where a guy had a webcam mounted to the middle of his ceiling for a project he was working on, and it appeared that he had spliced in a longer wire than the webcam came with, and strung it across the ceiling, and then the wall.

I DO have a question and I AGAIN request the help of all you helpful instructables members! My webcam as-is can see IR (infared) light, as I've tested this with a remote control and it can see the white light coming out of it. So the question is, how hard would it be to build an array of IR LED's to light up my yard, not even nessesarily pointing away from the source of the webcam, but say, an array of led's above the door of my shed shining down so if someone does attempt to enter my shed, I will be able to clearly record them.

Long post I know, but this is VERY important to me that this works and I'm hoping I didn't waste my time constructing this, I've had a few things stolen from me recently and it's really pissing me off. For instance, one of my bicycles was stolen, RIGHT FROM MY FRONT DOORSTEP! It has wayyyy more sentimental value to me than actual cash value and I'm literally ready to choke the living sh)t out of someone!

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Festrada0079 months ago

That is because the web cam uses very little power and the video feed is a connectionless protocol, meaning it doesn't care is the other end receive the data. There is less overhead in a connectionless protocol vs a connection oriented protocol. So we know the Camera has low bandwidth requirements and very low power requirements. That's why it works, if you put the camera on a 3 foot cable and again on a 25 foot cable and checked frame rates you would see a huge difference. Now for an experiment, try hooking up something powered by the usb like an external hard drive (not an SSD) and you should get data errors or it will not work. Your USB port puts out 5v @ 500 mA, and the voltage and current drop over 24 gauge wire is significant. at 25' it would be around 13 % or 4.3 volts and 462 mA and that is before a load it put on it. I = V/R is not a suggestion, it's the LAW. nerd humor.

lemonie6 years ago
If you're going to cable this, use something like 15A mains cable - it's voltage drop (I believe) that limits the length.

Punkguyta (author)  lemonie6 years ago
 Ummm..... how does that make sense?

My dad has in principal, taught me that you do not plug say, for example, a thing 22 AWG (Gauge) extension cord into the wall socket, and then plug a thicker (say maybe 18 or 16) gauge cord into the thinner one. This is because of resistance and while the thicker cord may be able to handle the current of say, a power saw or something he's powering outside, but the first, thinner extension cord will start to overheat.

I would also assume that this would work in reverse, as far as voltage drop, me would thinks that hooking a considerably thicker wire up to the thin wires on the usb cable and then the wire that goes to the webcam pcb. Doing this would be a load on the thin power and data wires used on the webcam and would probably cause worse power loss.
.  With DC, you can pretty well bet that bigger wire is better.  Even if part of the run is smaller wire, bigger wire in the other part will help (as long as you don't exceed the ampacity of the smaller wire).
.  The problem is, even though your USB signal is pretty low power, it's not DC. You're dealing with what amounts to an AC signal (IIRC, "pulsed DC" is more accurate, but many of the same principles apply) and things like impedance, capacitance, and inductance become important. It's not so much the difference in size as it is having to make a splice without degrading the signal.
Is it signal or the 5V power that limits first? (Ah divvunt knaa)

.  I'm not sure, but I would think it would be the signal. If for no other reason than the capacitance of the cable.
.  USB power is limited to 500 mA on the DC side, so Ohm's Law and the ohms/foot value for your wire (.00297 for AWG 14, according to http://www.radiolocman.com/shem/shem-cache.html?di=18899) will give you an idea of how far you can pump the 5V without too much loss (depending on how you define "too much"). Splices, done properly, aren't (usually) a problem with DC, ie, they have pretty low resistance.
This isn't a power tool, and you'll have no more than the same power going through the USB socket however you wire it up. If the long distance is dropping 5V to less than will work the device, thicker wire would help.

NachoMahma6 years ago
.  A lot of "night vision" security cameras have an array of ~8-12 IR LEDs around the camera. I see no reason you couldn't put together one or more arrays of your own.
Punkguyta (author)  NachoMahma6 years ago
 I figured it would be as simple as wiring together normal LED's, and yes I know what kind of security cameras you're talking about, but does that really have much "range"?
.  A Q&D Web search says up to 50 feet for 10-14 LEDs.
.  It will probably be easier to make rectangular 4x4 (or 3x3 or 5x5) arrays. Add as many as you need.
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