7-segment display problem

Hi all. I'm just starting out trying to get my head round this
electronics thing and am building a few simple circuits that I find on
the Internet. I got a 7-segment display (common anode) and tried to
build this - http://www.kpsec.freeuk.com/projects/random.htm to
repeatedly count from 0 to 9. I've tinkered with the resistors and
have one as a 1meg pot so I can change the timing and it works, except
it does it in reverse. I mean instead of lighting all the outer LEDs
for a 0 it lights only the centre line, and for a 1 (when it should be
lighting the two rightmost LEDs) it lights all the others. So,
instead of each LED in the 7-segment display being on in each
configuration, it's being off. I hope this makes sense, and I'm sure
there's a very simple answer to why it's doing it, but I am very new
to this stuff!

Any help would be greatly appreciated.

 

[John Fields]

On Sun, 11 May 2008 09:55:55 -0700 (PDT), cooper.tim@gmail.com wrote:

Hi all. I'm just starting out trying to get my head round this
electronics thing and am building a few simple circuits that I find on
the Internet. I got a 7-segment display (common anode) and tried to
build this - http://www.kpsec.freeuk.com/projects/random.htm to
repeatedly count from 0 to 9. I've tinkered with the resistors and
have one as a 1meg pot so I can change the timing and it works, except
it does it in reverse. I mean instead of lighting all the outer LEDs
for a 0 it lights only the centre line, and for a 1 (when it should be
lighting the two rightmost LEDs) it lights all the others. So,
instead of each LED in the 7-segment display being on in each
configuration, it's being off. I hope this makes sense, and I'm sure
there's a very simple answer to why it's doing it, but I am very new
to this stuff!

Any help would be greatly appreciated.

---
Sounds like what you've done is connected the anode of your 7 segment
display to +9V.

If that's the case you'll need to either change the display to common
cathode and connect the cathode to ground, as shown on the schematic,
or use the display you have and invert the outputs of the driver.

If you invert the outputs of the driver you'll probably need current
limiting resistors between the inverted outputs and the segments in
order to keep from damaging the display.

You can make the inverters out of transistors, one for each segment,
like this:

(View in Courier)





+9V
|
|
+---[390R]---[<SEGa]--+
| |
C |
aIN>---[1kR]---B 2N3904 |
E |
| |
GND |
.. .
.. .
.. .
|
|
|
+---[390R]---[<SEGg]--+
|
C
gIN>---[1kR]---B 2N3904
E
|
GND


or you could make them out of gates.

JF

 

[Rich Grise]

On Sun, 11 May 2008 09:55:55 -0700, cooper.tim wrote:

Hi all. I'm just starting out trying to get my head round this
electronics thing and am building a few simple circuits that I find on
the Internet. I got a 7-segment display (common anode) and tried to
build this - http://www.kpsec.freeuk.com/projects/random.htm to
repeatedly count from 0 to 9. I've tinkered with the resistors and
have one as a 1meg pot so I can change the timing and it works, except
it does it in reverse. I mean instead of lighting all the outer LEDs
for a 0 it lights only the centre line, and for a 1 (when it should be
lighting the two rightmost LEDs) it lights all the others. So,
instead of each LED in the 7-segment display being on in each
configuration, it's being off. I hope this makes sense, and I'm sure
there's a very simple answer to why it's doing it, but I am very new
to this stuff!

Any help would be greatly appreciated.

The display in the schematic is common-cathode, while you said you have
a common-anode display. I'm kind of surprised that you're seeing anything
at all. You might need either a different display or something like a
ULN2003 to invert the signals.

Good Luck!
Rich

 

[John Fields]

On Sun, 11 May 2008 09:55:55 -0700 (PDT), cooper.tim@gmail.com wrote:

Hi all. I'm just starting out trying to get my head round this
electronics thing and am building a few simple circuits that I find on
the Internet. I got a 7-segment display (common anode) and tried to
build this - http://www.kpsec.freeuk.com/projects/random.htm to
repeatedly count from 0 to 9. I've tinkered with the resistors and
have one as a 1meg pot so I can change the timing and it works, except
it does it in reverse. I mean instead of lighting all the outer LEDs
for a 0 it lights only the centre line, and for a 1 (when it should be
lighting the two rightmost LEDs) it lights all the others. So,
instead of each LED in the 7-segment display being on in each
configuration, it's being off. I hope this makes sense, and I'm sure
there's a very simple answer to why it's doing it, but I am very new
to this stuff!

Any help would be greatly appreciated.

---
I guess appreciation doesn't go as far as acknowledgement of help
received...

JF