bi-color LEDs with mixed common lead

[Carrie]

Starting on a new PIC project using bi-color LEDs. First thought was to use
a hex inverter to change colors between RED and GREEN. However it
occured to me that if there were a bi-color LED (RED/GREEN) that had a mixed
common lead it would save me 4 74ALS04 hex inverters, the board
real estate and the board size. I can find common anode and common cathode
LEDs but, so far, no mixed common. By mixed common I mean that the
anode of one LED is internally connected to the cathode of the other LED.

Anyone heard of such an animal??

Thanks !

 

[Lostgallifreyan]

Starting on a new PIC project using bi-color LEDs. First thought was
to use a hex inverter to change colors between RED and GREEN. However
it occured to me that if there were a bi-color LED (RED/GREEN) that
had a mixed common lead it would save me 4 74ALS04 hex inverters, the
board real estate and the board size. I can find common anode and
common cathode LEDs but, so far, no mixed common. By mixed common I
mean that the anode of one LED is internally connected to the cathode
of the other LED.

Anyone heard of such an animal??

Thanks !

I've not heard of them, but if you can make a rail of half your supply
voltage (must source and sink, so op-amp driven is best), you could use
two-lead bicolour LED's, and a single resistor for each. High output would
send current through it one way, low would reverse it. You save a resistor,
each LED needs only two connections, and although it might be overkill for
a handful of LED's it might be ideal for lots of them. The main problem I
can see is the need for a very low value resistor and a very well regulated
supply if it's 5V, because half of that doesn't leave much headroom.

 

[Carrie]

Hadn't thought of that. I am trying to save having to use 4 hex inverter
chips. Each chip
has 6 inverters so I am using a total of 24 bi-color LEDs. If I understand
your suggestion
correctly I will still need 24 op-amps????

Thanks for your help!

 

[Lostgallifreyan]

Hadn't thought of that. I am trying to save having to use 4 hex
inverter chips. Each chip
has 6 inverters so I am using a total of 24 bi-color LEDs. If I
understand your suggestion
correctly I will still need 24 op-amps????

I hope not. Assuming 24*20 mA, you'll need maybe a quad op-amp with all
four stages in parallel, or better, with 6 LED's per stage, to handle that
much current.

I don't know the PIC chips, but if their outputs can source and sink 20 mA,
this will work if you choose a quad op-amp that can dissipate 1 watt. If
you can get by with 10 mA or less per LED, a dual op-amp should be enough.
All the op-amp is doing is providing a ground rail so a single logic output
can supply the LED in either state.

 

[Roger Hamlett]

Starting on a new PIC project using bi-color LEDs. First thought was to
use
a hex inverter to change colors between RED and GREEN. However it
occured to me that if there were a bi-color LED (RED/GREEN) that had a
mixed
common lead it would save me 4 74ALS04 hex inverters, the board
real estate and the board size. I can find common anode and common
cathode
LEDs but, so far, no mixed common. By mixed common I mean that the
anode of one LED is internally connected to the cathode of the other
LED.

Anyone heard of such an animal??

They are common, but becoming rarer.
Historically, the first 'bi-colour' LEDs, used this design. Latter designs
with three leads, are commonly called 'tri-colour' LEDs (though of course
the 'bi-colour' units can also generate the third colour by being fed from
AC). You can turn a tri-colour design into a bi-colour unit by just
joining the outer pins on the package, and hence these are generally
getting to be more common. Look at:
http://www.hvwtech.com/products_view.asp?CatID=99&SubCatID=225&SubSubCatID=0&ProductID=358
What you have at the moment, is normally called a 'tri-colour' design.

Best Wishes

 

[Michael Black]

They are common, but becoming rarer.
Historically, the first 'bi-colour' LEDs, used this design. Latter designs
with three leads, are commonly called 'tri-colour' LEDs (though of course
the 'bi-colour' units can also generate the third colour by being fed from
AC). You can turn a tri-colour design into a bi-colour unit by just
joining the outer pins on the package, and hence these are generally
getting to be more common. Look at:
http://www.hvwtech.com/products_view.asp?CatID=99&SubCatID=225&SubSubCatID=0&ProductID=358
What you have at the moment, is normally called a 'tri-colour' design.

Best Wishes

Wait a minute.

An old style bipolar LED has the two led's arranged so
the voltage one way lights up one LED, the opposite polarity
lights up the other (both the left and right sides are connected
together):
--->|---
--|<----

But the three lead LEDs have them arranged so (with the common lead
being either the left or the right side, I forget which):
----|<---
----|<---

If you parallel the two other ends, all you get is two different
color LEDs that light up at the same time. There is no way of
changing color.

Michael

 

[Homer J Simpson]

But the three lead LEDs have them arranged so (with the common lead
being either the left or the right side, I forget which):
----|<---
----|<---

If you parallel the two other ends, all you get is two different
color LEDs that light up at the same time. There is no way of
changing color.

But if you connect to one, the other or both you get red, green or yellow,
right?

 

[Carrie]

I am confused as well. I looked at the link provided and it showed a 2 lead
LED. Description says it is a RED/GREEN LED. I will assume then that you
are guaranteed a color will light up
no matter what as long as one is positive and the other is negative. This
will give you either a RED lite or a GREEN lite. Nice but once in curcuit
there is no way to change the color. Expensive
way to avoid having to figure out which is the anode or the cathode of as
single color LED.

No, what I need is a LED that will lite up RED OR GREEN, under a PIC
control. It will never have to go off. It will either be RED or GREEN.

Thanks for your help.

 

[Homer J Simpson]

I am confused as well. I looked at the link provided and it showed a 2
lead
LED. Description says it is a RED/GREEN LED. I will assume then that you
are guaranteed a color will light up
no matter what as long as one is positive and the other is negative. This
will give you either a RED lite or a GREEN lite. Nice but once in curcuit
there is no way to change the color. Expensive
way to avoid having to figure out which is the anode or the cathode of as
single color LED.

No, what I need is a LED that will lite up RED OR GREEN, under a PIC
control. It will never have to go off. It will either be RED or GREEN.

Thanks for your help.

Either connect them back to back and drive the control lead above and below
zero or connect the two non-common leads to two output pins on the chip and
get 3 colors (which is better).

 

[Michael Black]

But if you connect to one, the other or both you get red, green or yellow,
right?

Now I'm confused.

What I read in the post I replied to was the suggestion that old
style two lead bipolar LEDs could be created by taking three lead
bicolor LEDs and connecting the two uncommon leads together.
The point of my post was that it wouldn't create a two lead
bipolar because the three lead package starts with a common cathode
or anode.

What confuses me in reading it now is what the purpose of that was.
The original poster wants an easy way to switch an led between
two colors. A two lead bipolar requires both sides to be driven,
one by a buffer (or directly if the original output is strong
enough) and the other side through an inverter. The three
lead LEDs would just need that same inverter, just arranged
differently.

Michael

 

[Lostgallifreyan]

[email protected] (Michael Black) wrote in

Now I'm confused.

What I read in the post I replied to was the suggestion that old
style two lead bipolar LEDs could be created by taking three lead
bicolor LEDs and connecting the two uncommon leads together.
The point of my post was that it wouldn't create a two lead
bipolar because the three lead package starts with a common cathode
or anode.

What confuses me in reading it now is what the purpose of that was.
The original poster wants an easy way to switch an led between
two colors. A two lead bipolar requires both sides to be driven,
one by a buffer (or directly if the original output is strong
enough) and the other side through an inverter. The three
lead LEDs would just need that same inverter, just arranged
differently.

Michael

Carrie was trying to avoid inverters! That was the point of the original
post.

 

[Michael Black]

I am confused as well. I looked at the link provided and it showed a 2 lead
LED. Description says it is a RED/GREEN LED. I will assume then that you
are guaranteed a color will light up
no matter what as long as one is positive and the other is negative. This
will give you either a RED lite or a GREEN lite. Nice but once in curcuit
there is no way to change the color. Expensive
way to avoid having to figure out which is the anode or the cathode of as
single color LED.

No, what I need is a LED that will lite up RED OR GREEN, under a PIC
control. It will never have to go off. It will either be RED or GREEN.

Thanks for your help.

But it really seams that you want something that does away with
an inverter.

Whether the LED has two leads or three leads, you need the inverter (unless
you go with the scheme of resistors that someone suggested).

With the two lead LED, you drive one side directly (or via a buffer
if more current is needed) and the other side via an inverter. When
the side directly connected to the buffer is high, that pin will be
high while the other pin will be low (because of the inverter). Switch
the buffer output to low, and that pin of the LED will be low while
the other side is high (because of the inverter). You are changing
the direction of the voltage flow, and hence can switch the LED
between red and green.

With the three lead LED, you connect the common anode to the positive
supply, and then connect one of the non-common leads to your buffer.
Then also connect an inverter to that buffer, and connect the output
of the inverter to the other non-common lead.

When that buffer is low, it will switch on the first LED (by
grounding it), while the inverter output will be high and thus
the second LED can't turn on. Drop the buffer output to high,
and the first LED will have both sides at the same potential,
and hence not be on, while the second LED will see a low, and thus
turn on.

(It would be a lot easier with a diagram, but I can't be bothered
trying to draw it here).

Michael

 

[Lostgallifreyan]

No, what I need is a LED that will lite up RED OR GREEN, under a PIC
control. It will never have to go off. It will either be RED or GREEN.

I already explained a way. I since looked up PIC chip outputs, they can
source AND sink, up to 50 mA either way. All you need to do is supply the
PIC with 5 volts, and make a 2.5 volt rail to use as LED common connection.
Feed each two-lead bipolar LED through a resistor selected for a 2.5V
supply. High level gives one colour, low level gives the other.

Take care to work out the power dissipation fpr the op-amp you'll need to
make the 2.5V rail, use a quad op-amp with all 4 stages controlled as
voltage followers by one pair of series resistors of equal value. If you
don't need much LED current, you might get by with an 8-pin dual op-amp IC.

 

[Lostgallifreyan]

[email protected] (Michael Black) wrote in

Whether the LED has two leads or three leads, you need the inverter
(unless you go with the scheme of resistors that someone suggested).

That 'someone' was me. There was a tad more to it than resistors though.

With the two lead LED, you drive one side directly (or via a buffer
if more current is needed) and the other side via an inverter. When
the side directly connected to the buffer is high, that pin will be
high while the other pin will be low (because of the inverter).
Switch the buffer output to low, and that pin of the LED will be low
while the other side is high (because of the inverter). You are
changing the direction of the voltage flow, and hence can switch the
LED between red and green.

I think Carrie's first post left no room for doubt that she knew how the
inverters worked for this. What she wants is to avoid them, they take too
much space on the board.

 

[Homer J Simpson]

With the two lead LED, you drive one side directly (or via a buffer
if more current is needed) and the other side via an inverter.

I guess you could tie each of two pins to pull up resistors and pull down
one or the other pin - but it seems perverse.



















....

 

[Byron A Jeff]

I already explained a way. I since looked up PIC chip outputs, they can
source AND sink, up to 50 mA either way.

20 mA. Not 50 mA.

BAJ

 

[Lostgallifreyan]

[email protected] (Byron A Jeff) wrote in

20 mA. Not 50 mA.

BAJ

True, it seems. I had the misfortune to first bump into this:

"PIC pins will sink (drive to ground) or source (output to a ground-
connected load) 50mA."

http://tinyurl.com/fqfq5 Near the bottom of that page...

20 mA is enough though, assuming it can sink as well as source.

 

[Byron A Jeff]

[email protected] (Byron A Jeff) wrote in

True, it seems. I had the misfortune to first bump into this:

"PIC pins will sink (drive to ground) or source (output to a ground-
connected load) 50mA."

http://tinyurl.com/fqfq5 Near the bottom of that page...

The page is incorrect.

20 mA is enough though, assuming it can sink as well as source.

The PIC can sink and source @ 20mA.

BAJ

 

[Lostgallifreyan]

[email protected] (Byron A Jeff) wrote in

The page is incorrect.


The PIC can sink and source @ 20mA.

BAJ

Ok, while we're in the mood for heavy pedantry, I'll annouce formally and
disticntly, I KNOW IT WAS INCORRECT. Happy?

 

[James Thompson]

Starting on a new PIC project using bi-color LEDs. First thought was to
use
a hex inverter to change colors between RED and GREEN. However it
occured to me that if there were a bi-color LED (RED/GREEN) that had a
mixed
common lead it would save me 4 74ALS04 hex inverters, the board
real estate and the board size. I can find common anode and common
cathode
LEDs but, so far, no mixed common. By mixed common I mean that the
anode of one LED is internally connected to the cathode of the other LED.

Anyone heard of such an animal??

Thanks !

I'm not sure if have it figured out by now, but here is a simple way to
figure it.

Consider the led's red or green has a forward drop of about 2 volt, and its
equivalent resistance at 20 ma will be 100 ohms. Now take 2) 100 ohm
resistors to make a voltage divider ( + 5 volt to resistor a with resistor b
in series then connecting to ground). now with the back to back led
connected to the center of your divider being 2.5 volt and the other end of
the led to the Pic output, A high or a low will give roughly 2.5 volt
difference on the led. You may or may not even need a resistor to drop the
extra .5 volt but if you do, you end up with 3 resistors and the led instead
of the extra opamp's. I would still consider at least a transistor buffer on
the Pic outputs to drive the leds.
JTT