mouseusbusb devicewiring
I know that USB cables usually have 4 wires – power, ground, data+ and data-.
In this optical mouse cable I have there is a 5th. It is black, but also thicker than the others, almost suggesting that there is more than one wire in it.
What is it? My best guess is that it is another kind of ground wire.
I would have a bit of a reservation about powering a baby swing from a random wall-wart supply. Normally, of course, such supplies work fine, and the baby isn't supposed to touch other grounded surfaces, but the swing wasn't really designed for it.
There are two areas of concern:
Electrical Safety
If your supply's isolation fails, you might have a dangerous situation as the baby is normally strapped into the swing. A grounded adult touching such a swing might then electrocute themselves and/or the baby.
Physical Safety
The supply cord becomes a nice strangulation hazard. Make sure that it's properly zip-tied to the supporting structure and routed out of reach of the baby!
When it comes to devices that are in forced, constant contact with the body, such as in a baby swing, I'd not use a random supply, but buy a medical grade power supply, such as this one. It's only $15 at this time + shipping.
Before you spend on such a supply, it'd be worthwhile to calculate how much money you're going to save in batteries over the life of the device. I'd also not randomly switch over to rechargeable batteries: they can be a real fire hazard in devices not designed (read: at least fused) to accommodate them.
My first reflex was to say that you should disconnect D+ and D- lines. Leaving VCC and GND connected is obviously safe for both the mouse and the host: VCC and GND leads in USB connectors are even made longer to insure this safe state is reached before D+ and D- are connected.
However, disconnecting VCC alone may be enough, and even if not, it should at least be safe. Take a look at this figure from USB specification:
The host will only communicate with the device when it sees D- line pulled up. However, once VCC is disconnected, Rpu is effectively eliminated from the circuit, leaving both D+ and D- at ground potential. As a result, no further communication will take place.
Once you reconnect the VCC line, the host will again see D- line pulled high, and will react as if a new USB device was just plugged in. In the best case, your mouse will just work. In the unlikely worst case scenario, the mouse won't have enough time to initialize (since normally VCC is connected before D- and the device has more time to start) and will fail to reply to the host within the allotted time. In that case, you'll have to keep VCC and GND connected and put a DPST switch on D+ and D-.
Best Answer
That would be the cable shield, which is connected to USB connector metal chassis.