connecting a led

Robot Freak

WATCH OUT!
Not the normal way!

Because my numeric led bar (or how you call such thing) has only 2 connectors for the Vdd.
So the normal way (one of the eight pins to propeller, the Vss connector to the ground) will not work.

                       LED BAR
         -----------------------------------
         |    1     2     3     4     5    |
         |   LED1  LED2  Vss   LED3  LED4  |
         |                                 |
         |                                 |
         |   LED5  Led6  Vss   LED7  LED8  |
         |    6     7     8     9     10   |
         -----------------------------------
                          |
                          |
                          -------------------------o Vss

So my question is, how do I interface this with the Propeller.
Maybe this is an easy question after all, but we will see.

Kind regards,
Robot Freak

Mike Green

I don't understand your question. What is your LED display? Usually these are labelled as common-anode or common-cathode where the anodes of all the LEDs are connected together or the cathodes of all the LEDs are connected together respectively. Sometimes there's a decimal point that's completely separate from the rest of the display and has both leads brought out.

If it's common-cathode, you connect the common lead to ground. If it's common-anode, you connect the common lead to +3.3V. You need current limiting resistors on all the other leads and those get connected to the Propeller I/O pins. On a common-cathode display, you make the I/O pin high to light the LED. On a common-anode display, you make the I/O pin low to light the LED.

Robot Freak

I found a diagram.
See the attachment.

Post Edited (Robot Freak) : 3/10/2008 5:43:59 PM GMT

Mike Green

Well, based on the diagram, which kind do you have and how do you need to connect it? You have all the information you need.

I suggest you read the Wikipedia article on diodes (Google "wiki diode") if you're still confused.

Post Edited (Mike Green) : 3/10/2008 5:44:02 PM GMT

Robot Freak

Thanks a lot Mike, I'll give it a try. It's a common-anode by the way.
Is it also possible to connect the common ground by resistor, to use less resistors?

Thanks,
Robot Freak

Mike Green

You can do anything you want, but you need to understand what happens when you do. Compute the currents (using Ohms Law) in a simple case of two diodes connected common-anode. Try it first with the resistors in the individual leads, then with a resistor in the common lead. Try it in the two cases of one LED on and with two LEDs on and see what you get.

Harley

Robot Freak,

As a side comment to Mike's 'You can do anything you want....",

I have a design which uses 6 '7-segment' LEDs (actually 8 segments, with the DP). They are common cathode type driven by FETs. The anodes are driven via resistors (100 Ohm) from a PIC running on 5v. Green LEDs, quite visible in this configuration, even though they are multiplexed about 80 Hz rate.

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Harley Shanko

deSilva

Multilplexing is tricky when it comes to 6 digits. The common anodes should be boosted by transistors, as they have to source > 100 mA.

Multiplexing also the segments will give an excellent brightness distributen, but also make it even dimmer.

I never succeeded in an acceptable brightness as well as brghtness distribution without "true" drivers....

Robot Freak

Thank you all!

I think I'll stay with the 6 resistors, to keep it easy,
and error free.

Kind regards,
Robot Freak

Russ61

Robot Freak,

This is a schematic from an atomic clock I created a while back with an SX chip.· The circuitry may give you some ideas.

RWC

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Russell Crow

Project Manager
Cohesive Automation Inc.