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1111  Forum 2005-2010 (read only) / Interfacing / RX pin 0 or 1? on: April 11, 2010, 09:09:08 am
Hey guys,

I was reading this tutorial on board design on Sparkfun:
http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=109

And I saw this:
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Routing TX and RX correctly? What does this mean? I can't tell you how many times I've heard newbies say 'well my PCB would have worked, but the manufacturer swapped the TX and RX pins'. No, the manufacturer did not swap them, the newbie neglected to actually read the datasheet (RTFD!). Sometimes an RX pin is an input. Sometimes an RX pin is an output. If you get it wrong, you'll look like a dunce. Read the datasheet and verify that everything is kosher. In the case of our FT232RL breakout, the TX pin is an output and RX pin is an input (pretty standard). I'm going to change the silkscreen indicators to read 'RX-I' and 'TX-O'. This should remove any doubt in my mind when I'm using the board a year from now - and so I don't have to go digging up the datasheet.

Which led me to check my Pro Mini, and sure enough, what I thought were RX1, TX0 indicating the pin numbers, were actually RXI, TXO indicating input and output.

This led me to question whether I may have wired my circut incorrectly.  So I went into board data section here and I found this:

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Serial: 0 (RX) and 1 (TX). Used to receive (RX) and transmit (TX) TTL serial data. These pins are connected to the TX-0 and RX-1 pins of the six pin header.

Which only served to confuse me more.


So which is it?  Is RX pin 0, or pin 1?
1112  Forum 2005-2010 (read only) / Interfacing / Re: Can 4017 handle this? on: May 18, 2010, 09:16:39 pm
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But if it was commercial or on display then I'd recommend using a buffer.

When you say 'buffer' you mean like a darlington array or transistor, right?
1113  Forum 2005-2010 (read only) / Interfacing / Re: Can 4017 handle this? on: May 18, 2010, 08:15:25 pm
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4017 is a pretty old, low-tech part.  There are much better options today such as TPIC6B595
Shift register in, and high-power MOSFET outputs. It seems to be everything you want in a single package.

http://www.sparkfun.com/commerce/product_info.php?products_id=734

That chip looks interesting.  Requires three pins like the 74HC595 I take it?  
http://www.arduino.cc/en/Tutorial/ShiftOut

Hm... I may be able to spare a couple more pins.  Might be a bit cheaper than buying two chips and two ice sockets to go with them.  Would definitely take up less space and space is at a preminum on this board.

Definitely something I'll consider.
1114  Forum 2005-2010 (read only) / Interfacing / Re: Can 4017 handle this? on: May 18, 2010, 07:53:00 pm
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The LED wizard can only tell you about the power dissipation of the LED it knows nothing about the driving circuit. So how much power are you dissipating in the package.

I guess I don't understand the question.

The 4017 has one output that goes high at a time.  If I have two leds on each pin, then wouldn't the power it needs to dissipate always be equal to that which is required to run two leds?
1115  Forum 2005-2010 (read only) / Interfacing / Re: Can 4017 handle this? on: May 18, 2010, 07:41:56 pm
I'd still like more info on the 4017's capabilities, but I thought I should let ya know that I'm now looking at using the ULN2803A (or its 7 input cousin whose part number I forget) in conjunction with the 4017.

The reason for the change is I realized that if I was gonna have to use two 4017 chips anyway if I wanted to run two displays with a total of three LEDs, that I might as well make one of those chips a darlington array and shift the load onto that.  

If I do that, I won't need to muck about with running leds in serial, and I can still run different colors without having to use more than three resistors.
1116  Forum 2005-2010 (read only) / Interfacing / Re: Can 4017 handle this? on: May 18, 2010, 07:25:59 pm
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The point is you asked if this chip was up to it. My answer is NO. It was not designed as a driver and was not meant to be used in the way you are planing to use it. If you don't like the advice then pay no attention to me, I have only been in electronics 40 years what do I know.

Mike, don't get me wrong.  I really appreciate the advice, and I'm taking what you say seriously.  I just want more information.  I need to understand the 'why' of things.  Answering 'Can the 4017 handle this?' with 'No.' isn't useful to me.  If it can't handle 20mA, I need to know what it can handle, so I know when I can use it and when I can't.

And I need to understand why what you're saying about it's power dissipation seems to conflict with the datatsheet because obviously I don't understand something important.

Also, why does it matter what it was designed to do, as long as I run it within safe parameters?  If it can handle 5-10mA and I'm fine running my LEDs with that level of current, then what's wrong with using it in that instance?  Current is current, right?
1117  Forum 2005-2010 (read only) / Interfacing / Re: Can 4017 handle this? on: May 18, 2010, 09:36:20 am
Speaking of the LED wizard, I found this page on that site which shows the nonlinear relationship between led brightness and current:
http://led.linear1.org/how-is-led-brightness-related-to-current/
1118  Forum 2005-2010 (read only) / Interfacing / Re: Can 4017 handle this? on: May 18, 2010, 07:25:50 am
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No the 80% is below the recommended operating level not the absolute stress maximum.

I've never seen a datasheet include a 'recommended operating level'.  The only thing I've seen that might be considered a reccomendation is when the manufacturer lists a 'test coniditon' for a part, like an LED where they usually list how it performs at 20mA.

And besides, why would you run at 20% below the operating level which is reccomended?  That doesn't make sense to me.  Why reccomend an operating level if it's not actually reccomended you run at that level?  

So in the absence of any reccomended operating conditions, and with the only metric I have to go off of being that absolute maximum rating, how am I supposed to dtermine what's safe to run at?


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Also, according to the LED wizard, the leds in this configuration will dissipate 84mW, which is well below the 500mW which another CMOS 4017 I found said it could handle.

Don't mix up the power dissipation in an LED with the power dissipation in a package. Remember to multiply the power dissipation per pin by the number of LEDs.

That calculation was multiplied by the number of LEDs.  

http://led.linear1.org/led.wiz

Enter 5, 2.05, 20, and 2, and the wizard states:
"together, the diodes dissipate 82 mW"
1119  Forum 2005-2010 (read only) / Interfacing / Re: Can 4017 handle this? on: May 18, 2010, 05:56:50 am
Okay, so based on what's been said...

Let's say I connect the 4017 to the Arduino pin via a 100ohm resistor, and connect two LEDs with a forward voltage of 2.05 in serial to each of the 4017's outputs, proving each pair with around 5v at 10mA.

Will THAT work?

(I'll also be connecting individual LEDs with a forward voltage of 1.7 to a second 4017, and connecting that 4017 to another pin on the Arduino via a 330ohm resistor.)
1120  Forum 2005-2010 (read only) / Interfacing / Re: Can 4017 handle this? on: May 18, 2010, 05:49:07 am
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scswift
The other option is to use a ULN2803 which is a darlington driver.
This has an open collector, which can sink 500mA.

But that won't save me pins.  I'll still have to use a 4017 to drive it.  

And the thing is, I have to drive a display with 7 leds on it as well.  So I gotta use a 4017 for that too.  I don't want to have to use four chips in this prop just for the sake of making the leds a little brighter.

Besides, I just tested the leds at 10mA, and you're right.  I cannot see a difference.  That extra 10mA made a big difference in my led array, but those leds were kinda dim and being multiplexed.  These are super bright ones.


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OR use transistors to drive the LEDs.

No thanks!  I'll stick with darlington arrays.  Too much soldering required for discreete transistors.


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This would also allow connection to a higher voltage for the LEDs, rather than loading your 5v supply.

What is with you people and multiple power sources!


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Personally I always stick to a around 10mA, as there is very little visible difference in the light output (except for different colours)

What do you mean by "except for different colors"?
1121  Forum 2005-2010 (read only) / Interfacing / Re: Can 4017 handle this? on: May 18, 2010, 04:49:00 am
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Now, the 4017 can handle 25mA max

No it can't. That is the absolute maximum stress rating

Yes, and I was referring to the absolute maximum.  That's why I said 'max'.  I'm aware I should not actually run that much current though it for any period of time.  I quoted that figure because that was my starting point for determining what a safe current to run through it is.  I'm not sure how else I'm supposed to calculate what a safe current is.


And why is 20mA pushing it?  I thought the rule of thumb was to stay below 80% of the max rating.

I don't have to use 20mA.  I would just like to so that my LEDs are as bright as possible.  

What do you think would be a safe amount of current to put through it?  What rule of thumb should I be using?


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There is also package power dissipation issues that are difficult to address as this chip is not designed for driving LED loads.

This chip says it can handle dissipating 500mW:
http://www.st.com/stonline/books/pdf/docs/1960.pdf

And 20mA at 5v is way way less than 500mW.  So what's the issue?
1122  Forum 2005-2010 (read only) / Interfacing / Can 4017 handle this? on: May 18, 2010, 02:01:23 am
Hey guys,

I just wanted to run this idea past you, and make sure I haven't overlooked something...

For the circuit I'm building, I have 14 LED's of which I need to illuminate two at a time, then cycle back to the first.

To do that, I'm looking at using this 4017 IC:
http://focus.ti.com/lit/ds/symlink/cd74hc4017.pdf

Now, the 4017 can handle 25mA max.  And 80% of that is 20mA.  So in theory it should source 20mA at 5v from each pin, in sequence, without a problem.

I'd like to power my LEDs with 20mA each, so the plan is to connect the 4017 to the Arduino's pin via a current limiting resistor, then connect two LEDs in serial, directly to each of seven pins, and then to ground... the idea being to make full use of that 5v coming off the pin to power both leds with 20mA without exceeing the 4017's output capability, and to reduce the number of resistors and arduino pins needed to just one.

What do you think?  Everything I know so far tells me this ought to work without any problems, but I'm still learning so I want confirmation. :-)


PS:
I plan to wire the eigth pin in the 4017 sequence to the 4017's reset pin, which I assume will trigger the first led in the sequence to illuminate the moment I hit the 8th clock in my sequence.

PPS:
The current limiting resistor I've calculated I'll need for the 4017 is 47 ohms, based on a 5v source, a 2.1v drop, and 20mA of current, powering two leds per pin in serial.  Also, according to the LED wizard, the leds in this configuration will dissipate 84mW, which is well below the 500mW which another CMOS 4017 I found said it could handle.  (The TI one doesn't list the power dissipation from what I could tell, but I assume its similar.)
1123  Forum 2005-2010 (read only) / Interfacing / Re: Anyone use molex flat/ribbon jumper cable? on: May 17, 2010, 07:55:58 pm
But it is tinned isn't it?  

What I need is something I can just pop right into the holes on a PCB and then solder without stripping, and preferably without having to tin it either.

[edit]

I found this picture of it.  It sure looks like its tinned:

1124  Forum 2005-2010 (read only) / Interfacing / Re: Anyone use molex flat/ribbon jumper cable? on: May 17, 2010, 02:34:34 pm
To the fellow that suggested using an IDE cable...

The point of using cable like this is to avoid the need to strip the wires.  

I just went through the process of stripping a ribbon cable and it's way too much work for the number of boards I need to build.  I spent an hour to get the exposed wires into the holes on my boards, until I realized I could tin the ends to keep them from fraying.  But even then it was still a pain in the ass, partly because it's impossible to get the wire insuation to all be the same length.


Btw, I also just found this:




It's called AMP flat flexible cable by Tyco Electronics.  And digikey will attach the connector to it for you.  

Unfortunately, they don't offer it in the 7 conductor variety I need.
1125  Forum 2005-2010 (read only) / Interfacing / Anyone use molex flat/ribbon jumper cable? on: May 17, 2010, 05:07:29 am
I'm talking this stuff:
http://www.molex.com/molex/products/datasheet.jsp?part=active/0250010702_CABLE.xml&channel=Products&Lang=en-US

I need to know two things:

How flexible is it?  Does it bend like standard ribbon cable?  Does it hold it's shape?  Does it not like to bend when in short lengths?  (I'm looking at getting 2" lengths.)

Is it easy to seperate?  Could I take a 9 conductor cable and just tear it to make a 7 and a 2?  Or will I need to use an exacto blade?

[edit]

Actually, one more thing.  This is solid wire right?  Is it sturdy enough/suitable for plugging straight into a header like those found on the edge of some Arduinos?
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