Go Down

Topic: Segments light when they should be off (Read 3651 times) previous topic - next topic

Kreplach

Hello,

I am multiplexing a 12 volt 7 segment led using a SN74HC595 shift register and a ULN2003 darlington array.

Segments that should be off are dimly lit.

For instance if I am displaying a two A,B,D,E,G are lit as expected but C and F are dimly lit as well.

i have measured the voltage at the input pins on the darlington and pins that should be high are high and pins that should be low are low.

Any help will be greatly appreciated.

Grumpy_Mike

You need to post both your schematic and code, use the correct code tags. If you don't know what that means read the how to use this forum sticky post.

Kreplach


Kreplach

I will post the code once I have cleaned up the fluff and copious commented out lines

Grumpy_Mike

Have you not got a schematic? It is difficult to follow a physical wiring diagram. The bit round the transistor does not make any sense. What should it be?

nickgammon

The Gnd pin (8) of the UNL2003A does not appear to be connected to logic ground.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

Kreplach

When I connect ULN2003 GND to logic ground it does not light at all.

Kreplach

I have posted a new picture of the schematic.

Grumpy_Mike


When I connect ULN2003 GND to logic ground it does not light at all.

Then you have something else wrong.
You can not switch a 12V anode like that. You need to source current through the anode with a PNP transistor.
That is still not a schematic so don't say you posted one when you have not.

nickgammon


I have posted a new picture of the schematic.


A schematic looks like this:



What you have posted is not much more informative than a photo of your board.

Quote
When I connect ULN2003 GND to logic ground it does not light at all.


Not in your revised picture you haven't.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

Kreplach

Nick,

You put on the right path.

When I connected the shift register ground to the 12 volt ground path instead  of the 5 volt ground back to the arduino it works as expected.


I will work on furnishing better schematics so that the community will be able to offer help without having to guess at what it is I am trying to accomplish.

Thank You..

Grumpy_Mike

The two grounds should be connected together anyway!

Go Up