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On 5/19/2010 12:22 PM, Orchid XP v8 wrote:
>
> Thank god I don't have to deal with anything analogue yet! ;-)
>
I spent hours trying to figure out why an op-amp wouldn't give me the
output I was expecting. It finally clicked when I read the warning on
the data sheet that pulling an input within 0.5v of V- would cause it to
latch-up and force the output to near V+ no matter the state of the
inputs. I went through a few chips thinking they were defective.... or
that I toasted one (got V+ and V- reversed, which is apparently bad for
op-amps...)
--
~Mike
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>> Yes, that's the other fun thing. Depending on which type of gates
>> you're using, they supply about 20 mA. But some of the LEDs I'm
>> looking it draw 40 mA...
>
> What you mean is, they draw *up to* 40 mA? How much they actually draw
> depends on what value series resistor you put in there. Using Ohm's law
> and the V-I curve of the LED from the datasheet you can work out exactly
> what resistor size you need for a given current.
See for yourself:
http://www.maplin.co.uk/Module.aspx?ModuleNo=35702
"Forward current: 25 mA"
If you can figure out what the hell that means, be my guest...
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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On 19-5-2010 13:07, Invisible wrote:
> Ah yes. That's the fun thing about doing digital electronics. You can't
> just *buy* a 7400. No, you have to decide whether you want a 74LS00 or a
> 74HC00 or a 74HCT00 or...
>
> Reading the datasheets is like walking into another world. (Especially
> since the datasheets are usually poorly-scanned grainy PDFs that don't
> match the part number you actually asked about!)
>
> However, I just found the following pages:
>
> http://www.kpsec.freeuk.com/components/ic.htm#logic
> http://www.kpsec.freeuk.com/components/74series.htm
> http://www.kpsec.freeuk.com/components/cmos.htm
>
> Suddenly everything seems so much clearer. For example...
>
> "74LS series: Inputs 'float' high to logic 1 if unconnected."
>
> Oh crap. Well that would explain a thing or two! >_<
>
> (I foolishly assumed that being unconnected would register as 0V - i.e.,
> logic 0. Damn, this is going to make interactive testing *so* much harder!)
>
> Also, it's nice to have some indication of the power requirements,
> switching frequencies, and what the hell all the letter codes mean!
I think I resist any urge to comment on this. Other than "RTFM" and "buy
any 'electronics for dummies book'" before embarking on such a project.
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andrel wrote:
> I think I resist any urge to comment on this. Other than "RTFM"
I don't think electricity comes with a manual. Neither do ICs, usually...
> and "buy
> any 'electronics for dummies book'" before embarking on such a project.
Hey, what do you *think* I've got a dad for? ;-)
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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On 19-5-2010 22:59, Orchid XP v8 wrote:
> andrel wrote:
>
>> I think I resist any urge to comment on this. Other than "RTFM"
>
> I don't think electricity comes with a manual. Neither do ICs, usually...
They do, they are called datasheets.
>> and "buy any 'electronics for dummies book'" before embarking on such
>> a project.
>
> Hey, what do you *think* I've got a dad for? ;-)
Hard to say, other than the obvious task of a father ;) . But I guess
you mean that he is the actual dummy that didn't know that a floating
input is seen as a '1'?
Have you ever looked at e.g.
http://en.wikipedia.org/wiki/File:TTL_npn_nand.svg ?
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On Wed, 19 May 2010 20:38:21 +0100, Orchid XP v8 wrote:
> See for yourself:
>
> http://www.maplin.co.uk/Module.aspx?ModuleNo=35702
>
> "Forward current: 25 mA"
>
> If you can figure out what the hell that means, be my guest...
http://wiki.answers.com/Q/What_is_forward_current
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andrel <byt### [at] gmail com> wrote:
> I think I resist any urge to comment on this. Other than "RTFM" and "buy
> any 'electronics for dummies book'" before embarking on such a project.
A good place to start would be Don Lancaster's two books, TTL COOKBOOK and CMOS
COOKBOOK. Wonderfully easy to read and comprehend. My copies are *old*,
dog-eared and marked up with my comments--I basically taught myself digital
electronics and logic from them, starting from scratch. The best books on these
subjects that I've ever run across. (I see that they are still available at
Amazon.)
Ken
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> OK, so if I'm understanding this right... The 74126 (go look it up)
> contains 4 "buffers" - gates who's output is logically equal to their
> input. But each gate also has an "enable" pin. When the enable pin is
> high, the gate works like normal. When the enable pin goes low, it's like
> the output pin isn't connected to anything any more.
Yes exactly.
> And it seems that this allows you to connect several outputs together,
> forming a kind of wired-OR configuration, provided that at all times only
> one gate is "enabled".
Yep, you could make a simple address decoder logic circuit (ie have a 2 bit
binary input to select which of the 4 buffers to enable) - that way you
would guarantee only 1 would be active at a time. If you were to connect a
2-bit counter to the 2-bit address lines you would then have a 4-bit
parallel to serial converter :-)
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> See for yourself:
>
> http://www.maplin.co.uk/Module.aspx?ModuleNo=35702
>
> "Forward current: 25 mA"
>
> If you can figure out what the hell that means, be my guest...
Load up the actual PDF spec that is linked from that page, it has more
detail, just stating the forward current like that means nothing.
On page 3 there is the "Absolute max" table, the continuous current here is
listed as 20/25/30 mA depending on the colour. That is the maximum, above
that they do not guarantee the LED will work.
On page 2 you see they've done all the spec values at 20 mA forward current.
Now look at the lower part of page 3, some interesting charts there
(assuming the Bright Red colour, the others are on the next pages).
The one in the top right shows "brightness" against current. You see it's
linear up to 10 mA and then flattens out. This is saying there isn't much
point using more than 10 mA, it won't be very efficient.
The one in the top left tells you what current will be drawn for a
particular voltage across the LED. If you want 10 mA then you're going to
need to give the LED 2.1 V. You also see here that even adding just 10% to
that voltage makes the LED current almost 3x higher!
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>> Why is that weird? The LED doesn't care which leg is connected to the
>> logic output pin, it's just a voltage. You could even connect the LED
>> between two output pins*, then it would only turn on when one was high
>> and the other was low.
>
>
> There's even a name for that trick: Charlieplexing.
Cool - is there also a name for when you connect a tiny speaker to two
output pins and give one pin a square wave, and the other the inverse of the
square wave - to make your speaker 2x louder than everyone elses? :-)
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