Darren New wrote:

> Cute. Is there a name for when you have a red LED in one direction and a 
> green LED in the other and you use AC to make it yellow? I want to use 
> that around my colleges and sound like I know what I'm talking about. ;-)

Most people call that a tri-colour LED - but sometimes that term is used 
to refer to an LED with three different-coloured dies inside it (and 
hence 3 pins). Which is obviously a different component.

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scott wrote:
>> I always thought of it as logic high meaning "connected to the +V 
>> rail" and logic low meaning "not connected to anything".
> 
> Logic "high" and "low" are just names given to a certain agreed range of 
> voltages.  The difference between "not connected to anything" (floating) 
> and "logic low" is very important when working with data busses.  If you 
> have several devices all with their outputs connected to a data bus, 
> then only one device at a time must be driving the data lines to logic 
> low/high, the rest must set their outputs to floating.

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.

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".

Is that right?

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Am 19.05.2010 16:16, schrieb Invisible:
> While we're here, does anybody comprehend what VCC, VSS, VDD and related
> symbols mean?

According to JEDEC,

VBB = substrate power voltage (bias voltage), a voltage < VSS/GND.

VCC = logic power voltage, serving as a reference for high signal levels 
(often sharing a common rail with VDD, in which case they may be used 
interchangeably).

VCCQ = output stage logic power voltage, separate (positive) power rail 
to drive output loads.

VDD = drain power voltage. separate (positive) power rail to drive the 
chip's internals.

VDDQ = output stage drain power voltage. Effectively the same as VCCQ.

VEE = emitter power voltage. Only used for ECL devices.

VHH = special function enable voltage. An extra-high voltage to enable 
certain special functions of a device.

VPP = programming voltage. An extra-high voltage to supply the power for 
the programming function of a device (e.g. EPROM)

VREF = reference power supply, serving as a special reference voltage, 
e.g. for ADC inputs.

VSS = GND = ground reference / source power voltage, serving as both 
(negative) power rail and low signal level reference.

VSSQ = GNDQ = output stage source power voltage / ground reference, 
serving as both a (negative) power rail and low signal level reference 
specifically for output signals (should always be at the same level as 
VSS=GND).

(This is just a subset of all the symbols used; for instance, 
digital/analog devices such as programmable amplifiers or ADCs/DACs 
often use separate voltage rails for the analog signals.)

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clipka wrote:

> According to JEDEC,
> 
> VCC = logic power voltage, serving as a reference for high signal levels 
> (often sharing a common rail with VDD, in which case they may be used 
> interchangeably).
> 
> VDD = drain power voltage. separate (positive) power rail to drive the 
> chip's internals.
> 
> VSS = GND = ground reference / source power voltage, serving as both 
> (negative) power rail and low signal level reference.
> 
> VSSQ = GNDQ = output stage source power voltage / ground reference, 
> serving as both a (negative) power rail and low signal level reference 
> specifically for output signals (should always be at the same level as 
> VSS=GND).

Suddenly the picture becomes even more complex than you feared...

> (This is just a subset of all the symbols used; for instance, 
> digital/analog devices such as programmable amplifiers or ADCs/DACs 
> often use separate voltage rails for the analog signals.)

Thank god I don't have to deal with anything analogue yet! ;-)

-- 
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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? ;-)

-- 
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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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