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### Topic: Switches rookie problem (Read 1 time)previous topic - next topic

#### Viper98

##### Jan 19, 2011, 03:03 am
I fairly new with everything so this is proably an obvious question. What I'm trying to do is use a single on/off switch to turn on both a breadboard arduino connected to a 9V battery and a laser diode connected to 2x AAA's (3v). If I connected both ground (black) wires to one pole of the switch would flipping the switch cause both devices (breadboard arduino and laser) to turn on? Or is there something I don't know about the power that this would cause an issue?

Thanks for the help!

#### retrolefty

#1
##### Jan 19, 2011, 05:45 amLast Edit: Jan 19, 2011, 05:53 am by retrolefty Reason: 1
Quote
If I connected both ground (black) wires to one pole of the switch would flipping the switch cause both devices (breadboard arduino and laser) to turn on? Or is there something I don't know about the power that this would cause an issue?

Actual you can switch a common ground from the two power sources with their grounds tied together then on to a single pole switch and then on to the arduino's ground and breadboard ground.

It's a little unusual to do it that way, but if you only have a single pole power switch (two terminals) and want both parts to power on and off together, it would work as you wish.

Using a double pole switch for each of the positive voltage sources is the more usual and better method, as if there was ever a DC current path for a device wired between both +3 and +9 sources there could be a discharge path through the two voltage sources. That is not a normal situation with most arduino projects using mixed voltages.

Lefty

#### tkbyd

#2
##### Jan 19, 2011, 09:13 am
Quote
it would work as you wish.

... IF you get the switch in the right place. There must be nothing "below" it, between it and the tied-together grounds of the two power supplies and the Arduino.

But it would still make me nervous.

#### mmcp42

#3
##### Jan 19, 2011, 07:37 pm
single pole in the ground circuit would let current flow from +9 thru both circuits to +3 would it not
a picture of what you have in mind would help a lot
there are only 10 types of people
them that understands binary
and them that doesn't

#### MarkT

#4
##### Jan 20, 2011, 03:03 am
What's the reverse breakdown voltage and leakage current for the laser diode?   It may just work if this diode acts as a diode
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### retrolefty

#5
##### Jan 20, 2011, 03:51 amLast Edit: Jan 20, 2011, 04:56 am by retrolefty Reason: 1
Quote
Current will flow through the circuit through the two power supplies and the Arduino if you simply lift the common ground.

Can you describe the detailed DC current path for say a 5volt battery and a 12volt battery sharing a common  ground but the common ground switched off to the arduino and all external components?

I understand that there is a potential current path if there was a component that was wired directly  to both +5 and +12. Assume a typical +5v battery powred arduino switching a relay with a 12 volt battery. I can see no path for condition between the +5 and +12 voltage sources. But maybe I am missing something.

Use this as a talking reference if you wish: http://www.arduino.cc/playground/uploads/Main/relays.pdf

Thanks;

Lefty

#### retrolefty

#6
##### Jan 20, 2011, 05:01 am
Quote
It is simply NOT good practice even if it MIGHT work in SOME circumstances.

I can live with that.

I just think there are circumstances where it would work without negative consequences. And that the simple Arduino activating a 12 volt relay is such a specific case.

Lefty

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