I need help from someone with basic electronic knowlage

[Dalekmun2010]

So I'm trying to make my Pi portable by adding a portable battery pack and a pocket projector. [See Pic]

First problem was turning the thing off without wearing out the micro USB port.
First solution I thought of was to put a switch in line with the USB cable.
Sadly, It appears that I have forgotten nearly everything I used to know about electronics.
So if someone could be so kind as to answer this question for me I would be truly grateful:
I can't remember if I should put the switch along the ground or positive wire.
Does it matter? Do I need to cut of both? If I block the wrong one will the charge damage my Pi, or drain the battery?
I really would appreciate any help I can get.
Thanks in advanced, Yours truly, Ryan Mitchell.

[gordonendersby]

Positive, you want to cut the power.
Ive never come across a device that cuts the negative rather than the positive.


Gordon

[jackokring]

When a device is switched off, as much of the metal should be connected to ground as possible. If you cut the ground, and assumed that resting the Pi unboxed on the ground would be safe, when in fact if the ground of the supply was ground the the Pi would not be grounded and power may flow to ground with sparks if the Pi touched ground.

[Vindicator]

Then you have not dealt with some of the old positive ground cars that used to be around.(yes they exist)
I agree though, you should always switch the positive so the device is grounded at all times.

[jackokring]

Then you have not dealt with some of the old positive ground cars that used to be around.(yes they exist)
I agree though, you should always switch the positive so the device is grounded at all times.

Ground does not have to be positive or negative. The ground state is the electrical balance of the earth's electrical potential. Electrons may be exported from a ground state to achieve positiveness, or imported to achieve negativeness. The effect of a slight weight decrease of a positive charge relative to ground and the gravity potential of the earth is responsible for lightning.

[techpaul]

At a minimum swiitch positive

Best switch both avoids strange signals because a voltage from a signal line is powering up a device through its internal protection diodes.

[Dalekmun2010]

Thanks everyone! I can't tell you how much I appreciate You all taking the time to explain this to me,
so thanks!
Yours truly, Ryan Mitchell.

[mahjongg]

It depends, its tradition for a low voltage DC powered device to put the switch in the positive lead, (but putting the power switch in the negative lead works just as well, and its also what happens with many devices that can be both battery or DC powered, when you plug in the DC adapter the batteries are disconnected using a switch in the DC connector that disconnect the batteries from the negative lead!

For a mains powered device its generally advised to put a double throw switch in which disconnects both leads (for safety, as you do not know which of the two wires will be "hot", that is carries a high voltage relative to earth potential).

In general terms it doesn't matter, all currents flow in loops, if anywhere in the chain you disconnect the loop current stops flowing.

If you put the switch in the lead connected to the "GND" of the board, make sure that the switch cannot be bypassed by something that connects both to the minus lead of the PSU, and the GND of the device. In case of the raspberry PI its GND is also connected to earth through the HDMI cable to a probably grounded TV. But as the PI's power supplies DC voltage is probably not connected to earth it will probably not matter.

[Burngate]

It's interesting that the instructions that come with a pair of automotive jump-leads say always connect the red (positive) first and disconnect red last - in other words switch the ground!

The reason is left as an exercise for the student

[gritz]

It's interesting that the instructions that come with a pair of automotive jump-leads say always connect the red (positive) first and disconnect red last - in other words switch the ground!

The reason is left as an exercise for the student

This is totally a safety thing. Remember that on all vaguely modern cars the negative battery terminal is connected to the car's engine block, body and thence to the metal gubbins under the bonnet. Remember too that car batteries are capable of delivering many hundreds of amps. If the negative lead was to be connected first and the positive lead was accidentally brought into contact with any of the car's metalwork there would be a (possibly explosive) short circuit. Brown trouser stuff! However, the controlled switching of those hundreds of amps into the car's starter motor when needed is done via the +ve feed, because it's done by a big relay (called a solenoid) and not by a chap waving a wire about!

So yeah, mahjongg pretty much has it all covered.

[Burngate]

Two cars, two batteries, both with megawatts available be-it only for a short time.
Scenario 1: connect the negatives - car bodies - together. Then connect one positive. Wave the other end around - lots of negative metal to hit.
Scenario 2: connect the positives together. Then connect one negative. Wave the other end around - you've only got a small target - one of the battery positives - for mayhem.

A psu and a Pi connected maybe to a TV or other stuff with a ground connection.
Scenario 1: switch in positive - when it's off only the negative (ground) of the psu is connected to the Pi. The other stuff may be providing a ground connection, but if you drop the pi in the sink, everything's at ground so no harm done.
Scenario 2: switch in negative - when it's off only the positive of the psu is connected to the Pi. If there's a ground connection elsewhere, then all the pi is at ground potential; drop the pi in the sink and still no harm done.
The only difference is that the switched side of the psu is at either +5v or -5v depending. It's not waving around and likely to touch something.

Something mains powered that you need to work on - a lamp with a blown bulb
Scenario 1: switched neutral. Take the bulb out and stick your fingers in, while standing in the sink. Your fingers find the mains live, and there's a circuit
through you and the bath through earth to mains neutral which is connected to earth either at the substation or your consumer unit.
Scenario 2: switched live. Now you're connected to neutral and earth, which should be the same potential, but could be a few volts out.

[bredman]

I don't know how we got onto the subject of jump cables, but...

Another thing to consider is where the connect/disconnect spark will occur. If you connect a positive lead last, there will be a spark on the positive battery terminal, just next to a battery which could be leaking hydrogen gas.

If you follow the rules, the last connection should be to the chassis of a car. This ensures that the spark will be far away from the battery.

This is of no use to anybody building a circuit for a Raspberry Pi, unless of course you want to use the Pi to switch large currents in the proximity of explosive gases.

[Ravenous]

One bit that is vaguely related to the raspi (and note I am in the UK as I write this):

Many of those little plug in mains power supplies have no earth pin (or a plastic one). If they have, say 9V output it's only 9V difference between the two connections. They are not necessarily at true earth potential.

So getting back on topic (yippee) if your computer or raspi is on and running, then you connect up some sort of homemade gadget to the GPIO pins, the gadget being on a different plug-in supply, there might be a voltage difference between the earths of both circuits. I suppose depending on which pin connects first, there might be a voltage difference high enough to harm a component somewhere. (I'm not talking about explosions or even sparks, just momentary voltage differences.)

I guess this is why, in the instructions we rarely read, they tell you to switch everything off, connect the new peripheral, then switch the power on again. Of course we never do that and I've never seen a problem, but it could happen.

Anyway, for a 5V system my suggestion is switch the power only, and let any earths be connected always (especially if there are other mains powered things connected). Just because sounds neater to me.