--- In Matthieu Crapet <> wrote:
>
> Greetings,
>
> I need electronic help :) As you will see, I am at my very early
stage (like
> some kernel...!)
>
> I want to display a number on a digit (7-segment led), and I use a
74LS47
> My digit is a "LTS-546AP" (common anode).
>
> Here are the datasheets:
> http://www.liteon.com.tw/OPTO/SPEC/DATABOOK.NSF/PASN/DS-30-98-055/
$file/S546AP.pdf
>
http://dragon.herts.ac.uk/facilities/stores/datasheets/logic/ls/74LS4
7.pdf
>
> a
> +---+
> f| g |b
> +---+
> e| |c
> +---+ .
> d dp
>
>
> 6 +-----+
> g f A a b------\ /e -| |- GND (pin 8)
> | | | | | | /d -| 7 |- A
> --------- | /c -| 4 |- D
> | _ | \----- /b -| L |- /RBI
> | |_| | /a -| S |- /(BI|RBO)
> | |_| | /g -| 4 |- /LT
> | *| /f -| 7 |- C
> --------- (pin 16) Vcc -| |- B (pin 1)
> | | | | | +-----+
> e d A c *
> 1 3
> | (anode)
> +5V
>
> What I want to do:
>
> - connect 74LS47 pin 16 (Vcc) and LTS546AP pin 3 (anode) to TS-
7200 +5V (LCD
> header)
> - connect 74LS47 pin 8 (ground) to TS-7200 ground (LCD header)
> - as partially drawn : pin a, b, c, d, e, f, g to
74LS47 /a, /b, /c, /d,
> /e, /f,
> /g
> - connect 74LS47 pin 7, 1, 2 and 6 (A, B, C, D) to TS-7200 DIO
header
> pin 1, 3,
> 5 and 7 (DIO_08, DIO_09, DIO_10, DIO_11)
>
> That sound too easy, my questions are :
>
> - Should I need to put resistances between a,..,g => /a,...,/g,
how
> many, why ?
> According to the datasheet, the 74LS47 can sink up to 15V. How
many
> current is
> there on the pin /a when lit ?
>
> - Should I need to put resistances between my DIO header and the 4
> inputs A, B,
> C, D ? My DIO can only source 4ma is that enough ?
> In the datasheet I don't understand this line :
> I = On-State Current /a to /g : 24ma <= ?????
> O (on)
>
> - I am confused with TTL input, sometimes I read it is 0-5V and
> sometimes 0-3.3V ?
>
> - Can I connect the ground (needed by the 74LS47) to the TS-7200
ground
> (DIO header and not LCD header), is it the same ground ?
>
> - In the 7-segment datasheet, I don't understand this line :
>
> Peak Forward Current Per Segment : 60 mA
> (1/10 Duty Cycle, 0.1ms Pulse Width)
>
> If all segment are lit, I will need 7*60 = 420mA ? That's quite
much ?
> (read that the "5V power output pin on the LCD header has 750mA
Poly-fuse
> to prevent excessive current)
>
>
> Thanks by advance !
>
> Matthieu
>
Matthieu,
I'll try to answer some of your questions starting from the last.
1) The peak forward current is the maximum you can apply to the LED
with a 10% duty cycle. The current you will need is probably much
lower than this. Typically on the newer LEDs you will only need to
drive it at about 10 mAmps per segment.
2) The grounds are the same.
3) The TTL levels depend on which logic family you are using. 5v TTL
levels are not the same as 3v TTL levels. I'm not an electronics
engineer so what I would do is just try it. As long as you don't
connect a 5v logic output to the TS-7200 inputs you should be OK
(although I can't guarantee it will work :-)
4) The output lines from the TS-7200 should be able to drive the
inputs to the 74LS47 (A, B, C, and D).
5) Yes you WILL need to add resistors from the outputs of the 74LS47
to the LED segments. This is what sets the current for each segment.
You may need to try different resistance values to get the
brightness you need. I would start with 470 ohm resistors giving a
current of about 10 milliamps. If all segments were lit you would
then be drawing 70 milliamps.
6) Everything looks ok to me. The only concern I have is can the 3.3
volt logic levels drive the 74LS47 (you won't need resistors on
those data lines). Since the LCD port is used to drive LCDs with a
5v logic input I would assume that it will work.
Good luck. Hope I answered some of your questions.
-Keith
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