I am stuck with a problem I just can't seem to find an answer to, maybe
someone can shed some light.
My project uses serial comms to connect to other devices: What would be the
best way to implement little 3mm LED's to indicate the RX/TX status ?
I can't decide on a solution, because from what I have seen in the past,
driving an led from a outpin on a PIC, seems to consume all the current, so
that there is none left to drive an input on another input pin of a TTL ic
for example. So I will see my RX/TX status, but the signal will not get to
to my driver device. I thought of using something like a buffer driver, to
drive the led and to leave enough current to actually drive the rest of the
circuit. Am I supposed to use some current limiting resistor on the led?
I hope this will be clear. What will a common industrial solution be to this
problem?
My home brew PIC development board which only accommodates 18 pin devices
has an LED in series with a 330 ohm resistor for every I/O pin. There are
also dip switches in series with the LEDs so they may be disconnected.
However, it's never been necessary to disconnect an LED because of
insignificant drive to other devices. As an example, the development board
has a Seetron serial LCD display that operates flawlessly at 9,600 baud
with the attached LED in series with the 330 ohm resistor. Obviously,
disconnecting the LED would be viewed as preferable, however, it has not
been necessary. For driving most other output devices I use a darlington
transistor array IC such as the Toshiba ULN2803A which can provide up to
500 mA and is available from Jameco (http://www.jameco.com) for $0.69 as Jameco
P/N 34315.
Sincerely,
Ned Seith
Nedtronics
59 3rd Street
Gilroy, CA 95020
(408) 842-0858
>Hi Everyone,
>
>I am stuck with a problem I just can't seem to find an answer to, maybe
>someone can shed some light.
>
>My project uses serial comms to connect to other devices: What would be the
>best way to implement little 3mm LED's to indicate the RX/TX status ?
>
>I can't decide on a solution, because from what I have seen in the past,
>driving an led from a outpin on a PIC, seems to consume all the current, so
>that there is none left to drive an input on another input pin of a TTL ic
>for example. So I will see my RX/TX status, but the signal will not get to
>to my driver device. I thought of using something like a buffer driver, to
>drive the led and to leave enough current to actually drive the rest of the
>circuit. Am I supposed to use some current limiting resistor on the led?
>
>I hope this will be clear. What will a common industrial solution be to this
>problem?
>
>Conrad
>
>--
>http://www.piclist.com#nomail Going offline? Don't AutoReply us!
>email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body
Best regards,
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Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam.....interlog.com Info for manufacturers: http://www.trexon.com
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Spehro Pefhany wrote:
>
> At 12:05 PM 8/3/01 +0200, you wrote:
>
> >
> >I hope this will be clear. What will a common industrial solution be to this
> >problem?
>
> output LED
> push-pull -------|<|-----[330R]---- +5
> |/
> ----[10K]-----| 2N3904 etc.
> |\ E
> 0V
>
> Iled ~= (4.9V - Vf(LED))/330 (so 8-10mA for green/yellow/red)
> Iin = 440uA
Something i've been doing lately is using the
high-intensity leds. The 600mCd, 900 and 1600
leds are getting quite cheap, and they are
EXCELLENT for giving good light at 1mA or less,
even 250uA gives a good indicator. I keep a
drawer full of them and it's surprising how useful
they can be for indicators of special things like
very short pulse lengths (or short PWM) and
for very low current indicators, especially
when testing new circuits and you can't be
bothered to keep attaching the CRO or meter to
it, just put the led in there... Normally they
can be seen from 50uA upward and safe to 50mA.
-Roman
>Something i've been doing lately is using the
>high-intensity leds. The 600mCd, 900 and 1600
>leds are getting quite cheap, and they are
>EXCELLENT for giving good light at 1mA or less,
>even 250uA gives a good indicator. I keep a
>drawer full of them and it's surprising how useful
>they can be for indicators of special things like
>very short pulse lengths (or short PWM) and
>for very low current indicators, especially
>when testing new circuits and you can't be
>bothered to keep attaching the CRO or meter to
>it, just put the led in there... Normally they
>can be seen from 50uA upward and safe to 50mA.
Yes! The red ones are more than reasonable.
GaAlAs red super-bright LEDs are the way to go if you can
live with red. You can use a little 1206 4 resistor network
and get the parts count down to almost zero. But for some
reason the product appearance is considered by some to
be better when you have yellow and green as well as red,
and run at ~10mA. ;-) (Also there may be CE issues with
red for some applications).
The best ones are (expensive) but visible in a dark room
with literally a few microamps, none of this ~1mA offset
current that early GaAs LEDs had.
You've got a drawer full? That's a lot! I've only got about
10K of the 3mm ones on hand at the moment and it doesn't take
up much space. ;-)
Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..." "The Journey is the reward" @spam@speffKILLspaminterlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
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Spehro Pefhany wrote:
>
> At 09:18 PM 8/3/01 +1000, you wrote:
>
> >Something i've been doing lately is using the
> >high-intensity leds. The 600mCd, 900 and 1600
> >leds are getting quite cheap, and they are
> >EXCELLENT for giving good light at 1mA or less,
> >even 250uA gives a good indicator.
> Yes! The red ones are more than reasonable.
> GaAlAs red super-bright LEDs are the way to go if you can
> live with red. You can use a little 1206 4 resistor network
> and get the parts count down to almost zero. But for some
> reason the product appearance is considered by some to
> be better when you have yellow and green as well as red,
> and run at ~10mA. ;-) (Also there may be CE issues with
> red for some applications).
I get them from http://www.jaycar-electronics.com and
they stock yellow and green (and blue and white)
all in about 5000mCd, although I prefer the cheaper
green 350mCd and yellow 160mCd which are still
SO much better than a standard 10mCd 5mm led...
> The best ones are (expensive) but visible in a dark room
> with literally a few microamps, none of this ~1mA offset
> current that early GaAs LEDs had.
>
> You've got a drawer full? That's a lot! I've only got about
> 10K of the 3mm ones on hand at the moment and it doesn't take
> up much space. ;-)
Touche! I have a small plastic drawer about 1/3
full, in my parts drawers, a couple of hundred.
Is that better?? ;o)
-Roman
PS. Am I the only person who thinks they should
put a coloured spot or something on the leds??
It's a pain having to power them up to see the
colour and brightness. Maybe we need a new
"led colour code"...
>. Am I supposed to use some current limiting resistor on the led?
YES. The problem you are seeing is that the voltage on output pin is
dropping to the level of the LEDs forward voltage (Typically 1.5V-2V). A
suitable series current limiting resistor will ensure that the output pin
voltage remains at 5V, which can then be used to drive the next stage
succesfully.
>I get them from http://www.jaycar-electronics.com and
>they stock yellow and green (and blue and white)
>all in about 5000mCd, although I prefer the cheaper
>green 350mCd and yellow 160mCd which are still
>SO much better than a standard 10mCd 5mm led...
Roman, I don't see these on the web site, just the
350mcd green and 2000mcd yellow (@0.81/1.05 AUD
respectively)
Is there a trick to finding them?
Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..." "The Journey is the reward" RemoveMEspeffTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
> PS. Am I the only person who thinks they should
> put a coloured spot or something on the leds??
> It's a pain having to power them up to see the
> colour and brightness. Maybe we need a new
> "led colour code"...
Very ocassionally I come across clear LEDs with a
small coloured dot on the anode (+ve). Strangely,
these LEDs are made differently. You know when you
look through a LED the anode is much smaller than the
cathode ? These funny ones actually connect the other
way, with the large electrode to +ve, yet they look just
the same inside as any other LED. I've never seen them
on sale, the ones I have are envulturated from membrane
keyboards that come my way from time to time
>Very ocassionally I come across clear LEDs with a
>small coloured dot on the anode (+ve). Strangely,
>these LEDs are made differently. You know when you
>look through a LED the anode is much smaller than the
>cathode ? These funny ones actually connect the other
>way, with the large electrode to +ve, yet they look just
>the same inside as any other LED.
All super-bright LEDs that I've come across are made
that way.
Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..." "The Journey is the reward" RemoveMEspeffEraseMEEraseMEinterlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
> >way, with the large electrode to +ve, yet they look just
> >the same inside as any other LED.
>
> All super-bright LEDs that I've come across are made
> that way.
The HP SB whites I've got aren't ???
The ones I talked about before are unusual in that they
produce just a pin-point of light inside. Definitely not SB.
In fact not very bright at all. They're flat-top waterclear
3mm, ie no lens
Spehro Pefhany wrote:
>
> At 09:42 PM 8/3/01 +1000, you wrote:
>
> >I get them from http://www.jaycar-electronics.com and
> >they stock yellow and green (and blue and white)
> >all in about 5000mCd, although I prefer the cheaper
> >green 350mCd and yellow 160mCd which are still
> >SO much better than a standard 10mCd 5mm led...
>
> Roman, I don't see these on the web site, just the
> 350mcd green and 2000mcd yellow (@0.81/1.05 AUD
> respectively)
>
> Is there a trick to finding them?
Yes! I look in my catalogue. :o)
Some are 5mm and some 3mm, the red and yellow
are both in 3mm 5000mCd, the 3mm blue is
4000mCd, in 5mm the red is 6500mCd, yellow
5000mCd, green and white 6000mCd (the white
6000 is excellent!) blue 4000mCd. Also cheap reds
at 500 and 1000mCd.
-Roman
> >I get them from http://www.jaycar-electronics.com and
> > >they stock yellow and green (and blue and white)
> > >all in about 5000mCd, although I prefer the cheaper
> > >green 350mCd and yellow 160mCd which are still
> > >SO much better than a standard 10mCd 5mm led...
> >
> > Roman, I don't see these on the web site, just the
> > 350mcd green and 2000mcd yellow (@0.81/1.05 AUD
> > respectively)
> >
> > Is there a trick to finding them?
I couldn't get that URL to work. Is there a trick to getting to the URL?
Jinx wrote:
>
> > >way, with the large electrode to +ve, yet they look just
> > >the same inside as any other LED.
> >
> > All super-bright LEDs that I've come across are made
> > that way.
>
> The HP SB whites I've got aren't ???
Yep the SB whites have the chip mounted on the cathode,
the SB coloured leds have it on the anode.
> The ones I talked about before are unusual in that they
> produce just a pin-point of light inside. Definitely not SB.
> In fact not very bright at all. They're flat-top waterclear
> 3mm, ie no lens
They're just cheap and nasty low-power clear leds.
Very common in faxes and consumer appliances. :o)
-Roman
I've got a friend with a Harley Davidson chopper who doesn't want to spend
$119 on a LED tail light. The vibration breaks the filaments in his brake
light way too often. I bought a cheap 3rd light, brakelight off Ebay to
modify. It has the super bright LED's (probably the 5,000 mCD type) with
different value current limiting resistors for the two brightness levels.
The LED's are really, really tiny with a plastic focusing lens on each to
give the proper visual effect. When I hooked it up to my PTT line for fun
on my ham rig, it hurt the eyes it was so bright. I recommend to experiment
with the value of resistor to get the proper effect.
Mike
{Original Message removed}
> They're just cheap and nasty low-power clear leds.
> Very common in faxes and consumer appliances. :o)
Figures. I think they came from copier keyboards. They
aren't bright, but still use a fair whack of power. I use them
as "oh, just grab anything" components
--Message-Boundary-3573
Content-type: text/plain; charset=US-ASCII
Content-transfer-encoding: 7BIT
Content-description: Mail message body
> My project uses serial comms to connect to other devices: What would be the
> best way to implement little 3mm LED's to indicate the RX/TX status ?
I have just designed a circuit where I needed to do exactly this,
schematic diagram attached.
I put the LEDs directly on the RS232 lines, with a 2k2 current limit
resistor and a small signal diode to ground. As the RS232 levels will
be about +/-10V the diode was needed because the LEDs can only
handle about 5V reverse bias.
The current will only be about 3.5mA per LED. A quick test on my
PC showed the TxD data was typically +/-12V no load and dropped
to about +9V / -12V with my LED circuit attached. I think this will
work quite well as there is still plenty of amplitude according to
RS232 voltage specs.
Ask me again in a week when I should be testing the prototype!
Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail: spamBeGonebrent.brownKILLspamclear.net.nz
Mike Kendall wrote:
>
> I've got a friend with a Harley Davidson chopper who doesn't want to spend
> $119 on a LED tail light. The vibration breaks the filaments in his brake
> light way too often. I bought a cheap 3rd light, brakelight off Ebay to
> modify. It has the super bright LED's (probably the 5,000 mCD type) with
> different value current limiting resistors for the two brightness levels.
> The LED's are really, really tiny with a plastic focusing lens on each to
> give the proper visual effect. When I hooked it up to my PTT line for fun
> on my ham rig, it hurt the eyes it was so bright. I recommend to experiment
> with the value of resistor to get the proper effect.
Again I recommend http://www.jaycar.com au
for their 6000mCd red leds, these are incredible
if you have never seen one. Many of those superbright
ones in car taillights are 900mCd or 1600mCd.
Jaycar carry good stocks, price is good and have
a good website with online ordering. :o)
-Roman
mitvma.mit.edu with SET PICList DIGEST in the body
