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Author Topic: Arduino switch without access to ground line.  (Read 453 times)
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Ok, so first off I'd like to apologize if this has been answered before.  I did my best to search the forum, but all I could find were the solutions that I couldn't use.

So my problem is one that has cropped up every other day, and that is that I need a 12V digital array.  So I want the arduino 5V to turn on an external 12V logic.  The problem is that every transistor switch I've seen really just switches in the return to the load.

I do not have access to the load.  I am controlling an array of 8 logic pins which are in reference to case ground inside of the case.  This means I need to have the switch drop in the +12, rather than having it be hot all the time.  Simply put, I need the switch to be on the other side of the load - is this possible with a simple transistor switch?

Thank you for any assistance offered!

-John
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Its not clear from your description what the circuit is - a picture is worth a thousand words for situations like this (even if hand-sketched and photographed!)
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Ok, so first off I'd like to apologize if this has been answered before.  I did my best to search the forum, but all I could find were the solutions that I couldn't use.

So my problem is one that has cropped up every other day, and that is that I need a 12V digital array.  So I want the arduino 5V to turn on an external 12V logic.  The problem is that every transistor switch I've seen really just switches in the return to the load.

I do not have access to the load.  I am controlling an array of 8 logic pins which are in reference to case ground inside of the case.  This means I need to have the switch drop in the +12, rather than having it be hot all the time.  Simply put, I need the switch to be on the other side of the load - is this possible with a simple transistor switch?

Thank you for any assistance offered!

-John

Yes, do a search on the term 'high-side switch'. This usually involves using P channel mosfet or PNP transistor to switch on and off a positive voltage wired to a ground based load.

Here is an article on using a PNP transistor circuit to implement high side switching from a uP digital output pin like an arduino output pin.

http://www.w9xt.com/page_microdesign_pt8_pnp_switching.html
Of course a good old fashion relay can accomplish the same task.

Lefty
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How can you provide 12V relative to a ground you can't connect to?  How is the negative side of the 12V power supply connected?!?
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I do not have access to the load.  I am controlling an array of 8 logic pins which are in reference to case ground inside of the case.  This means I need to have the switch drop in the +12, rather than having it be hot all the time.  Simply put, I need the switch to be on the other side of the load - is this possible with a simple transistor switch?

A 'P' transistor will work on the other side of the load, no problem there.

OTOH you need some sort of reference between the load voltages and the Arduino voltages, it can't work otherwise. If you don't have a common ground then what do you have?
« Last Edit: November 08, 2012, 04:30:22 pm by fungus » Logged

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Alrighty!  I believe retrolefty answered my question, but I'll go ahead and post my little picture just to make clear what I meant.



And, in case I messed up the image above: http://imgur.com/L7pYI&v1fjs

Logic pins in gray, internal loads in red, case/ground in black.

All I meant is that I do not have access to the ground on each individual pin.  They are all case grounded together.  This means that if I used a typical transistor switch to complete the circuit on the load's return, I'd complete the circuit for all of the loads inside the case.

Thanks a bunch!
-John
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Yes, you need high-side switching. If the switched current is low (say 100mA or less) and you can tolerate a volt or two of voltage drop (so that the load sees 10 to 10.5V instead of 12V), then there are some 8-channel ICs that will do the job. Otherwise, use P-channel mosfets driven either by mosfet driver chips (if the load current and switching speed are both high) or NPN transistors.
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