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901  Using Arduino / General Electronics / Re: Isolating VCC in circuit on: February 23, 2012, 12:43:40 am
By the way, should those series resistors on the TX/RX lines be 1K, or 10K?  Somewhere someone told me to use 10K, but I can't remember anymore where ...  Does it make a lot of difference, such as completely rendering those lines "dead", preventing a reprogram?
902  Using Arduino / General Electronics / Re: Isolating VCC in circuit on: February 22, 2012, 08:20:48 pm
Hey, Confucius is my middle name. smiley

Thanks for the help!
903  Using Arduino / General Electronics / Re: Isolating VCC in circuit on: February 22, 2012, 08:13:31 pm
I did, but this board will be inside of an enclosure that won't be opened too often, so if I have a slide switch that can be accessed through a thin slot, that's perfect.  The only one who would be reprogramming it (for now) would be me anyway, so I'm happy with that.

Now, for all intents and purposes, a DPDT would also work, yes?  Just connect both poles to the same thing.
904  Using Arduino / General Electronics / Re: Isolating VCC in circuit on: February 22, 2012, 08:00:22 pm
Here ya go.
905  Using Arduino / General Electronics / Re: Isolating VCC in circuit on: February 22, 2012, 07:48:43 pm
Your symbol drawing is trash.

It's SFE's symbol, not mine. smiley

The one on the left is a DPDT and the one on the right is a SPDT. A SPDT (as drawn on the right symbol) is what will work for power switching on your board. The center terminal will become the board's +5vdc bus. The two other contacts will be wired to the USB's +5vdc and the external +5vdc regulators output.

Then, what I had in my schematic earlier is correct then.
906  Using Arduino / General Electronics / Re: Isolating VCC in circuit on: February 22, 2012, 07:47:30 pm
Well to my mind the left one has two poles, it's a DPDT and the right one is a SPDT.

You're thinking is spot on with mine.

The SFE drawings are wrong ...

Wouldn't be the first time ...
907  Using Arduino / General Electronics / Re: technique for neat smd components? on: February 22, 2012, 07:40:23 pm
You need these, or similar.

For that kind of quantity on a board, I'd reflow it. Ohararp makes $25 stencils (plus shipping).  Or you can try to cut your own by hand ... been there, done that.
908  Using Arduino / General Electronics / Re: Isolating VCC in circuit on: February 22, 2012, 07:34:07 pm
Ok, what am I missing here?  On the left is SFE's SPDT symbol.  On the right is their SPST symbol.   So, why can't I use the SPST switch?  Isn't it just a flip-flop between the two poles?  If the AVR is on the left, and the two poles on the right are one to the FTDI's VCC and the other to the PSU VCC?

The SPDT seems to have more poles than I need ...

What am I missing?
909  Using Arduino / General Electronics / Re: Isolating VCC in circuit on: February 22, 2012, 07:09:21 pm
The switch you have there is an SPDT, you do need double through.
Hrm, I just realized that SparkFun's SPDT and DPDT symbols are identical, which made me grab the SPST one ... didn't think I need two contact both ways.

I guess in theory there would still be a small current drain through the 1k resistors but I could live with just using them to isolate, it's simple.
Would it be better to use 10K instead?  Or will that cause problems when I'm actually trying to program something?[/quote]
910  Using Arduino / General Electronics / Re: technique for neat smd components? on: February 22, 2012, 06:41:15 pm
I can see tape working for 0805 parts, but as you get smaller, I can't see it working too well.
911  Using Arduino / General Electronics / Re: Isolating VCC in circuit on: February 22, 2012, 06:37:09 pm
OH!  You're talking about doing something like this?

Why double throw?  Why not SPST?
912  Using Arduino / General Electronics / Re: technique for neat smd components? on: February 22, 2012, 03:36:31 pm
If you're doing it all by hand, then you're already doing some of it right.  Tin one pad first.  Put some flux on it and put the components ontop.  I use tack flux so it also holds the components in place (at least till it flows.)  You can use a pair of tweezers to hold it in place while you heat up the pad again and let the solder flow and grab the components.  Let it cool.  Now add some flux on the other side, put some tin on your iron and touch the other side.  I've never used super glue with this technique.

Of course, I have also moved away from doing SMD by hand now, I do it all by skillet relflow.  SparkFun has a tutorial on that.  It's easy, fast, and much cleaner results.

Also on the fan, don't have it blow onto your project.  You'll be cooling both your iron as well as the parts as you're trying to solder them on.  Flip it around and have it suck the fumes away.  I have a bench ATX power supply from an old computer with two fans connected to it.  They both sit close to my helping hands so they can suck the fumes away.
913  Using Arduino / General Electronics / Re: Isolating VCC in circuit on: February 22, 2012, 03:31:23 pm
Understood. The Seeeduino does this by having a manual slide power switch which determines how the rest of the board is powered. In the external position the Vcc comes from the on-board +5vdc regulator which gets it's power from the external power connector. If placed in the USB position the whole board is powered by the USB +5vdc. This could just as easy be a 3 pin sip jumper clip. So no diode drop concern, no complex auto-voltage circuit required, just a users choice with a on board switch or jumper clip. Elegance and reliability through simplicity.

I did consider the switch option which, as you said, would be reliable and simple.  I'm not sure how to make that work though because in one configuration (USB), both the FTDI and AVR need to be powered, nothing else on the board.  And in the other (PSU) mode, only the AVR and the rest of the board need to be powered.  And if BOTH are plugged in, then everything is powered (since the FTDI gets its power from USB anyway).  Basically, the AVR is the "overlap" between the two circuits.

And then there's the issue of the TX/RX lines of course.

Maybe I'm just asking for too much then.  Maybe I should just forget about saving current consumption and just power the whole dang thing at all times.  Gah.
914  Using Arduino / General Electronics / Re: Isolating VCC in circuit on: February 22, 2012, 01:49:38 pm
Basically yes that would do it but with two reservations:-
1) You get a 0.7V droop from a diode so you will be powering your arduino with 4.3V and not 5V.
2) Why do you want the FTDI not to be powered? It will have inputs connected to arduino through 1K resistors and connecting signals to unpowered chips is never a good idea.
1) Okay, I was aware of that, sure.  4.3V would still be enough to download a sketch.  The FTDI chip requires 4V to 5.25V (with internal clock generator), and the AVR will also be happy with that.  So technically that shouldn't be a problem, unless I am overlooking something here.  Is there even something else that won't cause a voltage drop?

2) Because I'm trying to not burn an additional 15mA. smiley  So how would I isolate the TX/RX lines as well?

If you have the 328's Tx signal connected the FTDI will be powered through that signal. You have to isolate that as well.

See #2 above.  Can it even be done?

The Seeeduino line of arduino compatible boards (both 328p and 1280 based) actually do what you wish. They do not power the FTDI chip from the board's +5vdc Vcc bus. The Vcc pin of the FTDI wires only to the USB's +5vdc bus coming from the PC. So if the board is being powered by external power Via external DC power connector and not plugged into the PC's USB then the FTDI remains powered down.

Right, I see that.  However, that part I already figured out, it's what comes afterwards.  USB needs to also provide power to the AVR, but not what the AVR itself is driving (being the LED drivers and LEDs themselves.)  The idea here is that one can program the AVR with the USB connected, REGARDLESS of whether the external PSU is connected or not.


build a board with the 328 and none of the other stuff you don't want. Nothing very complex about doing that and it can be done on a small protoboard with a socket for the 328.

That's exactly what I'm doing.
915  Using Arduino / General Electronics / Isolating VCC in circuit on: February 22, 2012, 02:30:56 am
Still trying to figure this out.  Let's say I have an FTDI, AVR, and a LED driver and LEDs in a circuit.  Following the FTDI datasheet, I figured out how to build the circuit so it also powers the AVR.  However, here's where I'm stuck.  The AVR and LEDs can (and will be) also be powered from an external PSU.  So, how can I isolate the VCC lines so that:

a) the external PSU will only provide power to the AVR and LEDs.
b) a USB cable plugged in will only power the FTDI and AVR.

Basically, if ONLY the external PSU is connected, I don't want (or need) the FTDI IC powered up.  And if ONLY the USB cable is plugged in, I don't want any of the LEDs to turn on.  If BOTH are plugged in, everything turns on and hopefully I don't hear a snap, crackle, pop and see blue smoke rising ...

I *think* all I need is a diode on the VCC line from the USB connector that will prevent current coming from the PSU to reach the FTDI, and on the VCC line from the PSU, another diode that will prevent USB power to reach the LEDs.

In the simplest of simple forms, I believe the circuit would look something like what I've attached with D1 and D2 taking care of the flow.  Ignore the fact that there are many other wires and components missing, I did say "in the simplest of simple".

So, is there something wrong with that?  And if it would work that way, can I get some suggestions of what diode to use for this?  It's all going to be 5V.
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