Dot Matrix - question

[PhillyP]

Resolved the issue myself - thanks

 

[nick12ab]

Hello, I am using a 7 x 5 Dot Matrix display (Common Anode columns) with a PICAXE 28X1

Circuit diagram: http://gyazo.com/e113619e3666e8e3fb9495e04e14de2a

A PICAXE-28X2 would be better suited for this application.

To light any LED, I will use Q7 (ROW) configured as output and D7(COLUMN) configured as output for my example in which I need a bit of confirmation, I am not talking about a specific LED at the moment just in general.

Qx and Dx are GENIE terminology! Please don't use such blasphemy without at least stating what pins on the PICAXE they will be equivalent to.

In terms of lighting the LED corresponding to Q7.

Would Q7 need to be 0 or 1?
Would D7 need to be 0 or 1?

If the display is column anode like you say, it won't work because the transistors connect the columns to ground.

It would also help if someone could tell me how I would need to program Q7 and D7 if the scenario was switched and it was a Common Cathode display.

Thanks,

To light the top left LED, both Q7 (output 7) and D7 (output c7) would have to be 1.

 

[hippy]

The anodes need to go to Qx so Qx would need to be high, the cathodes need to go to the Dx transistors which pass current when Dx is high so Dx need to be high.

Using the same circuit and a common cathode display you would need to wire it the same, cathodes to Qx, anodes to Dx transistors.

 

[nick12ab]

Another note, the download circuit has not been implemented properly and the PICAXE would likely be damaged if the RS232 download cable is used.

Also there's no decoupling capacitor - its absence will come back to bite you if you try to extend the display later.

 

[AllyCat]

Hi,

Yes, as nick said earlier, that type of drive circuit is intended for a common cathode display. The "common" (anode or cathode) pins carry much higher currents, which is where/why the transistors are needed.

For a common anode display, you need to use "high side" auxiliary transistor swtches (driving the anodes), either PNP transistors or NPNs in an "emitter follower" configuration.

Then, directly-driving the cathodes from the PICaxe pins needs a "0" to activate the segment(s). PNP drivers will also require "active low" (0) signals to their base, but emitter followers will still need a "1" to activate the segment(s).

Cheers, Alan.

 

[hippy]

Note that in a LED matrix display there is no real distinction between common cathode and common anode; multiple cathodes and multiple anodes all join together, the cathodes have to go to the cathode drive lines the anodes have to go to the anode driver lines.

 

[AllyCat]

Hi,

Yes indeed, so strictly the driver transistors are not required at all (since the column and row currents are potentially similar).

However, since PIC(axe) pins are better at "pulling down" than "pulling up", if I were using drivers, I would still put the transistors in a "high side" position (even with 5 anodes and 7 cathodes per pin).

Cheers, Alan.

 

[srnet]

Hello, I am using a 7 x 5 Dot Matrix display (Common Anode columns) with a PICAXE 28X1

Circuit diagram: http://gyazo.com/e113619e3666e8e3fb9495e04e14de2a

The 9V supply will destroy the PICAXE...........................