So what exactly is the voltage output of the Arduino Output pin ?

Is it 5 Volts ? Or it depends of the Power I feed it ?

If I feed it a 9v battery, is the output 9v ?

5V at the high side, with low current load, drifting down as the load increases, and with 5-5.5 volts on the ATMega Vcc pins; higher than that and you fry the chip.

9V into the barrel jack, or plugged into USB port, will yield 5V on the ATMega Vcc pins.

On the low side, around 0.3V with low current load, drifting up as the load increases.

You really should browse the data sheet to learn what the chip can do outside of the 5V regulator and the USB interface chip; your circuits will be connecting directly to the chips I/O pins.

http://www.atmel.com/dyn/resources/prod_documents/doc8271.pdf

CrossRoads, you are very very helpful, but I simply cannot comprehend it in the same detail way you do.

this...this is not working for me, all I am hearing is complicated...complicated...complicated.

Let me try again....

Is the output pin when pinMode is OUTPUT and DigitalWrite is HIGH alway 5V ? Like die die 5V, it's always going to be 5v ?

No. It won't "always" be 5v.

It will be around 5v as long as you don't connect anything unsuitable to it. If you connect something unsuitable, it may never work properly again.

If you sit in the cockpit of a fighter jet, you can't just say "how do I make it go", and expect anyone to be able to tell you in two sentences.

I know, I'm stupid.

So what you are saying, is under normal circumstances, e.g no "unsuitable load", the output will ALWAYS be 5v yeah ?

Regardless of whether the main power is 5v or the 9v right ?

Regardless of whether the main power is 5v or the 9v right ?

Yes.

If the main power is 9V, you must connect it into the barrel jack or the Vin pin. If you have a regulated 5V supply, it goes into the Vcc pin. The third alternative is to use USB, which is also regulated 5V.

“The Arduino” works at 5v. Only a tiny corner of the PCB carries anything else.

It is perfectly acceptable, but not always convenient, to supply the 5v running around the Arduino board from 5v arising somewhere off the board. (It has to be a “regulated” 5v… i.e. more consistently 5v than the “9v” which comes from a wall wart, which can vary quite widely, depending on how cheap it is.)

There is 5v available coming out of a USB cable from a PC, so when you have your Arduino connected to the PC by the USB cable, the 5v in the Arduino may well be coming from the PC.

Because a regulated 5v isn’t always easy to come by, many Arduinos and clones have a little corner of the PCB dedicated to making 5v from variable slightly higher voltages… 7 volts… 9 volts… even 12 volts… but those higher voltages go almost nowhere on the board, do almost nothing apart from supplying the “regulator” circuit… which creates the 5v from the higher voltage.

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So far so good.

You should also be aware, though, that some parts of Arduino related boards, and some Arduinos and clones, and sensors people use with them, run at 3.3 volts, often written 3v3. Don’t let it “worry” you, but keep the fact at the back of your mind for the day it becomes relevant to something you are doing.

The question said exactly so here goes:- If you feed any voltage into the power jack the regulator cuts this down to 5V so the arduino is getting 5V.

Now the voltage on the output pins can be anything above 4.2V. It will stay above 4.2V providing you don't draw more than 20mA. That is in the data sheet.

Anything outside those limits all bets are off. So draw more than 20mA from the pin and it is likely you get less voltage out.

If you power the arduino with 3V on the +5V pin you will only get a minimum of 2.3V on a high output pin.

Thank you pekkaa, tkbyd and Grumpy_Mike.

Grumpy_Mike: Now the voltage on the output pins can be anything above 4.2V. It will stay above 4.2V providing you don't draw more than 20mA. That is in the data sheet.

If you power the arduino with 3V on the +5V pin you will only get a minimum of 2.3V on a high output pin.

This is interesting! Now I am tempted to ask, how do you calculate that ? 5v will output 4.2v....so just minus 0.8v from main supply and one will get the output pin voltage ?

But that would be wrong, because 3v will give 2.3 volt on the output and not 2.2v.

What is the correlation/math/formula here ?

And also, supplying a 3v ? Where would I put the 3v in ? The Vin ? I though the "operating voltage for Vin is 6 to 20v ? 3v would be useless for Vin and the other input requires a regulated 5v, so....where does you 3v input go ?

Now I am tempted to ask, how do you calculate that ?

You might be but there is no way of calculating it unless you are the chip manufacturer. It is just a given that you look up in the data sheet. As you say there is no simple relationship.

And also, supplying a 3v ? Where would I put the 3v in ?

I said:-

If you power the arduino with 3V on the +5V pin

So with no other power you can put 3V into the pin marked +5V : it might work, but might not, because the data sheet says it is outside it's specification to run at 16MHz when powering it with 3V. But it is a bit like over clocking you will find some chips that will work that fast with that little voltage. That is why lower voltage arduino boards run at 8MHz.

I guess this means that using a cheap USB wall charger to power the Arduino might be a very bad idea? Seems to me if it is regulated then there'd be a voltage drop.

Also I see in some posts that 12V to Vin is better than 9V. This depends on power drawn from I/O or just that 9V batteries don't stay 9V forever? I ask because it seems there is no problem running on USB power and I would expect that true 9V at Vin should always be at least as strong given that 7V works to give 5V under light load.

Hmmmm, does that completely make sense?

I guess this means that using a cheap USB wall charger to power the Arduino might be a very bad idea? Seems to me if it is regulated then there'd be a voltage drop.

As a USB charger is a regulated +5V then just feed it into the +5V pin. Not the Vin or power jack.

Also I see in some posts that 12V to Vin is better than 9V

Without context it is hard to say but I would go along with your idea, because a 9V battery is not well suited to powering an arduino due to its lack of capacity.

Hi, BBT...

This stuff is not intuitive. Don't worry, just keep working through it.

This may help: http://yourduino.com/pins-bits-1-0-.htm

Almost everything Arduino does is "in between the rails" where the lower rail is ground and the upper rail is 5.0 Volts.

Arduino just connects an output pin to +5V (HIGH, ONE , or 1) or connects it to Ground (LOW, ZERO, 0) ..

You can use this inexpensive wallwart http://www.dipmicro.com/store/DCA-07510 and plug into the barrel jack.

Or use this one, cut the connector off, solder on a couple of 0.025" square pins, and connect it to the 5V and GND pins. http://www.dipmicro.com/store/DCA-0520 This one will give you lots of extra 5V current if you want to connect other devices to the 5V pin also.