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PICList Thread
'HELP with an LED Driver'
1996\12\16@155859 by Scott Horton

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I am using a PIC16C84 to drive some 7 segment LED displays.  My problem is
that the LED's are those BIG (3 inch) ones and each segment needs 40mA.  I
was going to use a 2803 to drive them but with all 7 segments on, it looks
like the power was too high.

Can some of you suggest the best (simplest/cheapest) way to drive these
LED's.  Right now, I'm planning to have a 4511 decoder switch individual
2N2222's for each segment.

Thanks for any help/suggestions.

Scott

1996\12\16@172219 by Zhahai Stewart

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> I am using a PIC16C84 to drive some 7 segment LED displays.  My problem
> is that the LED's are those BIG (3 inch) ones and each segment needs
> 40mA.  I was going to use a 2803 to drive them but with all 7 segments
> on, it looks like the power was too high.

> Can some of you suggest the best (simplest/cheapest) way to drive these
> LED's.

You could use multiple ULN2803's or ULN2003's, keeping the package
dissipation within bounds on each;  (ie: Don't use all channels per pkg).
They're cheap.  {For any reader who is unfamiliar, there are 8/7 channel
DIP packaged darlington low side drivers.)  This may be closest to your
current design (no pun intended) and thereby simple in that way; also
cheap.

You could consider the the MAX7219, which will handle everything for up
to 8 zeven-segment-plus-decimal LED displays.  It has high side drivers,
low side drivers, BCD-7 segment decoders, oscillator, current limiters,
RAM for the BCD, intensity modulation via PWM, shift register input
(fewer pins).  Around $8 in small quantities, US.  I believe that
Harris/Intersil have a similar parallel interfaced chip in the ICL7218.
Being N-way multiplexed (for N digits), this may be less bright than you
wish, tho you can probably up the peak current some to compensate.  This
may be the simplest, in that one chip and a single current setting resistor
handles up to 8 digits; you don't even need current limit resistors per
segment.  (You can also use the MAX7219 in direct undecoded mode, with
individual control of each segment).  Check the Maxim web pages.

Or you could substitute the TI TPIC6B595 8 bit SIPO latched shift register
with low side drivers; I think these can handle your 40ma / segement
continuous current, unlike the ULN2803.  Again, fewer pins needed, but
you need to handle multiplexing the high side, or use one per digit
unmultiplexed.  This is back to your earlier design, but with a shift
register/latch with more drive.  Check the Texas Instruments web pages.
   Zhahai

@ Zhahai Stewart       spam_OUTzhahaiTakeThisOuTspamhisys.com
@ A Meme Gardener      http://rainbow.rmii.com/~hisys/zhahai.html
@ Standard Disclaimer  YMMV - Your Maya May Vary

1996\12\16@173602 by verhage

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Not only to drive an LED segment, I'd like to hear suggestions for
using a controller to ignite rocket ignitors.  As I understand, I'll
need at least an amp to do that.

thanks

Lloyd Verhage

{Quote hidden}

1996\12\16@204122 by Byron A Jeff

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>
> Not only to drive an LED segment, I'd like to hear suggestions for
> using a controller to ignite rocket ignitors.  As I understand, I'll
> need at least an amp to do that.

That's easy enough: use the pic to drive a relay and wire the relay circuit
to the ignitors... I'm using a 16C84 and a couple of 12V 10A relays to drive
6 floodlights (200W each) around the perimeter of the house. When I had only
one relay (for the back lights) I wired the typical 2N2222 driver. When I added
a second relay instead of adding another 2N2222 I switched the 2N2222 driver
to a 5V relay and drove the 2 12V relays from the 5V one. Speed was an
absolute non issue since the lights only go on and off twice a day.

I suggest using a relay.

BAJ

1996\12\16@213855 by Dwayne Reid

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>I am using a PIC16C84 to drive some 7 segment LED displays.  My problem is
>that the LED's are those BIG (3 inch) ones and each segment needs 40mA.  I
>was going to use a 2803 to drive them but with all 7 segments on, it looks
>like the power was too high.

Scott - are you going to multiplex the displays?  If not, 2803 (or its less
expensive 7 wide brother 2003) will work just fine.  Each channel is good
for a maximum of 500 mA, total package current is ok for 600 mA or so.  Be
sure to take the aprox. 1.2 V saturation voltage when calculating your LED
segment resistors.

PS: I've been talking up TPIC6B595 power o/p shift registers for several
months now.  Each channel is good for 150 mA and all channels can be used at
full current without any concern.  You can drive multiple displays
(non-multiplexed) using only 3 output pins on your pic (2 pins if you use a
RC delay on the clk line to drive the latch pins.  My current project uses 3
seperate 32 bit bidirectional serial chains using only 5 i/o pins.  My
eeprom shares 2 of those lines and takes one more i/o for a total of 6 i/o
pins for ALL of my digital inputs, outputs and configuration storage.

Anyways, hope this helps.

Dwayne

1996\12\16@220403 by Sarunas Cepulis

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Dwayne Reid wrote:
{Quote hidden}

Use Current limiting resistors !!!!!!!!!!!!!
       saras

1996\12\17@121720 by Scott Horton

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Zhaahi (and everyone),

Thanks for the pointer.  The TI chips look like they have the power
capacity I need.  I followed your suggestion and found not only the
TPIC6273 power shift register but also the TPIC6273 Power Octal D latch.
They have similar capacities and the latter looks like it would meet my
needs in my current design.

However, your suggestion of using the shift register looks better only I'm
afraid I'm not sure how to interface to it.  Any chance you (anybody)
could give me some pointers?  I've got the data sheet but I'm not sure how
to handle talking between my 16C84 and it.  I think I can handle the SI
but I'm not sure what to do to the G, RCK, SRCLR, SRCK inputs.

If it's not already clear, please assume I'm a dummy when you provide any
help <g>.

Thanks very much for the help.

Scott

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