The resistors limit the current into the 74LS47 but also limit the
current going through the LEDs which in turn affects the brightness.
The outputs on the 74LS47 are "open collector" which means they only
sink current they do not source current. You don't need to worry
about the 45 volt specification - this I assume would be for
incandescent bulbs.
-Keith
--- In Matthieu Crapet <> wrote:
>
> Thanks for your answer Keith.
>
> So, You're right, the TS-7200 DIO header is in fact LV TTL (3.3V)
> (Low-Voltage Transistor Transistor Logic).
>
> I have found something good :
> http://www.interfacebus.com/voltage_LV_threshold.html
>
> If everything is true, I can safely connect LV TTL output to my
74LS47
> TTL input
>
> Concerning resistors (outputs of 74LS47), I don't understand:
>
> The output pin (/a, ..., /g) are sinking current, so the current
go from
> the LED to an output pin :
>
> 5V -->---[LED|>]---->--- pin /a
>
> So this resistor (located after the LED) won't affect the lighting
power
> of the LED, it's just for limiting the current entering inside
74LS47,
> am I right ?
> About voltage, how to know how many volts have we in pin /a; is
15V enough ?
>
> V Off-State Output Voltage /a to /g : 15V
> O (off)
> I On-State Output Current /a to /g : 24mA
> O (on)
>
> Matthieu
>
>
> Keith Gilman wrote:
>
> >--- 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/74LS
4
> >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
> >
> >
> >
> >
> >
> >
> >
> >
> >Yahoo! Groups Links
> >
> >
> >
> >
> >
> >
> >
> >
> >
>
------------------------ Yahoo! Groups Sponsor --------------------~-->
Get Bzzzy! (real tools to help you find a job). Welcome to the Sweet Life.
http://us.click.yahoo.com/A77XvD/vlQLAA/TtwFAA/CFFolB/TM
--------------------------------------------------------------------~->
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/ts-7000/
<*> To unsubscribe from this group, send an email to:
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
|