Driving 7 segment LED display

Archiver · 2003-06-06 14:11

I am driving a LN518RK Panasonic LED display from the BS2. Specs in the DIGIKEY
Catalog list the I forward at 20 mA and VF Typical at 2.2. Can I drive the
display without a current limiting resistor, one resistor in the common cathode
lead or with a current limiting resistor for each segment?

Thank you for your advise.

Larry

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Archiver · 2003-06-06 15:03

In a message dated 6/6/2003 6:13:09 AM Pacific Daylight Time,
kb0bmp@y... writes:

> I am driving a LN518RK Panasonic LED display from the BS2. Specs in the
> DIGIKEY Catalog list the I forward at 20 mA and VF Typical at 2.2. Can I
drive
> the display without a current limiting resistor, one resistor in the common
> cathode lead or with a current limiting resistor for each segment?
>
> Thank you for your advise.
>
> Larry
>

You are best off with a current limiting resistor in each segment. You should
also have the stamp drive a transistor to provide a ground for the LED
resistor combination.....the reason is that will all segments on, 20mA * 7
segments
will exceed the current capabilites of the stamp......You can use a ULN2803
Darlington array that will easily handle this job.

Or consider a serial shift register to reduce the number of Stamp outputs
needed. Or.....if I an not mistaken, there is an LED driver IC, 4 output lines
of the Stamp will control your 7 segment LED.

Write back if more detail is needed

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Archiver · 2003-06-06 15:14

I looked at the datasheet for this part, but I don't understand the pin-out.

Seems to me a 7 segment display should have 7 anodes and 7 cathodes, but
this also has two decimal dots, so should have 9 of each... Usually, the
datasheet shows everything you need to know, but this one...

I also understand that each may have a common anode or cathode, but the
table on the datasheet seems to list two assignments for each pin! I'm
curious how this works out as well...

If it was one LED, 2.2 V and 20 mA, a series 140 ohm resistor would be the
choice, but for this?

Steve

Original Message

From: "larry vandyke" <kb0bmp@y...>
To: <basicstamps@yahoogroups.com>
Sent: Friday, June 06, 2003 3:11 PM
Subject: [noparse][[/noparse]basicstamps] Driving 7 segment LED display

> I am driving a LN518RK Panasonic LED display from the BS2. Specs in the
DIGIKEY Catalog list the I forward at 20 mA and VF Typical at 2.2. Can I
drive the display without a current limiting resistor, one resistor in the
common cathode lead or with a current limiting resistor for each segment?
>
> Thank you for your advise.
>
> Larry
>
>
>

> Do you Yahoo!?
> Free online calendar with sync to Outlook(TM).
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> [noparse][[/noparse]Non-text portions of this message have been removed]
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>

Archiver · 2003-06-06 15:15

This part is a common-cathode, single character,
7-segment LED number display with two decimal points.

1. You MUST have a current limiting resistor of some
kind, to prevent burning out LED segments, and
possibly burning out the BS2 driving pins.

2. If you only have one resistor in the common
cathode lead, then the brightness of the display
will depend on how many segments are lit --
probably not what you want.

3. Recommended you have a resistor for each
segment of the display, and tie the common
cathode to ground. They make 'DIP' form
factor resistors for this purpose.

NOTE: The 'typical' entry for this devics says
5 mA per segment. 20mA per segment is the
MAX you can use, above that segments start
burning out. Don't go there.

I'd drive at 5 mA per segment for
power savings, which gives you:
500 ohms (470 ohms is close enough).
220 ohms gets you 12 mA, which is
probably the max current you should use.

--- In basicstamps@yahoogroups.com, larry vandyke <kb0bmp@y...> wrote:
> I am driving a LN518RK Panasonic LED display from the BS2. Specs
in the DIGIKEY Catalog list the I forward at 20 mA and VF Typical at
2.2. Can I drive the display without a current limiting resistor,
one resistor in the common cathode lead or with a current limiting
resistor for each segment?
>
> Thank you for your advise.
>
> Larry
>
>
>

> Do you Yahoo!?
> Free online calendar with sync to Outlook(TM).
>
> [noparse][[/noparse]Non-text portions of this message have been removed]

Archiver · 2003-06-06 15:28

The datasheet is showing one table for two parts.
The left box is for the 'common anode' part.
the right box is for the 'common cathode' part.

They do seem to have a lot of 'common cathode'
pins, don't they? I count 5 of them.

Since I don't know how the inside is wired, and
since they all have the same name, I have to
assume that all the 'common cathode' pins are
common inside the module, should
be wired together outside the module,
and tied to ground. You could check
this with an ohm-meter.

Then you'll wire your 470 ohm (220 ohm for
more brightness, but PLEASE don't go lower
than that, 20 mA is a MAX before destruction)
resistors in-line between the BS2 and the
'anode' pins.

An external driver IC (like an SPI 74HCT595)
would save you pins on the BS2. AND, you
should look at the max current available from
the BS2 -- 470 ohms should drive 7 segments
without overdoing it, but 220 ohms might take
too much current if directly driven by the BS2.

--- In basicstamps@yahoogroups.com, "Steve Ziuchkovski"
<zman97211@y...> wrote:
> I looked at the datasheet for this part, but I don't understand the
pin-out.
>
> Seems to me a 7 segment display should have 7 anodes and 7
cathodes, but
> this also has two decimal dots, so should have 9 of each...
Usually, the
> datasheet shows everything you need to know, but this one...
>
> I also understand that each may have a common anode or cathode, but
the
> table on the datasheet seems to list two assignments for each pin!
I'm
> curious how this works out as well...
>
> If it was one LED, 2.2 V and 20 mA, a series 140 ohm resistor would
be the
> choice, but for this?
>
> Steve
>
>

Original Message

> From: "larry vandyke" <kb0bmp@y...>
> To: <basicstamps@yahoogroups.com>
> Sent: Friday, June 06, 2003 3:11 PM
> Subject: [noparse][[/noparse]basicstamps] Driving 7 segment LED display
>
>
> > I am driving a LN518RK Panasonic LED display from the BS2. Specs
in the
> DIGIKEY Catalog list the I forward at 20 mA and VF Typical at 2.2.
Can I
> drive the display without a current limiting resistor, one resistor
in the
> common cathode lead or with a current limiting resistor for each
segment?
> >
> > Thank you for your advise.
> >
> > Larry
> >
> >
> >

> > Do you Yahoo!?
> > Free online calendar with sync to Outlook(TM).
> >
> > [noparse][[/noparse]Non-text portions of this message have been removed]
> >
> >
> > To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@yahoogroups.com
> > from the same email address that you subscribed. Text in the
Subject and
> Body of the message will be ignored.
> >
> >
> > Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/
> >

Driving 7 segment LED display

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