Multiplexed signals to drive large 7 segment leds

[djvsmarkviii]

I have a up/down counter kit I built that utilizes a microcontroller to drive 4small 7segmenet leds to display the time count. I want to use the multiplexed signal for the small 7 segment leds to drive much larger 10" led displays that I will be constucting for a large sports scoreboard for my sons Lacrosse team. Each segment will have 8 leds which will end up being 56 leds per digit, * 4 digits= 224 leds for display. I have a manual remote control that will house the counting circuit and all the various switches for the scoreboard. Cable length from remote to display panel will be about 10'. My question is how can I use the multiplexed signal to drive the large display and can I send the multiplexed signal that far? Any help would be appreciated.

 

[ericgibbs]

I have a up/down counter kit I built that utilizes a microcontroller to drive 4small 7segmenet leds to display the time count. I want to use the multiplexed signal for the small 7 segment leds to drive much larger 10" led displays that I will be constucting for a large sports scoreboard for my sons Lacrosse team. Each segment will have 8 leds which will end up being 56 leds per digit, * 4 digits= 224 leds for display. I have a manual remote control that will house the counting circuit and all the various switches for the scoreboard. Cable length from remote to display panel will be about 10'. My question is how can I use the multiplexed signal to drive the large display and can I send the multiplexed signal that far? Any help would be appreciated.

hi,
Can you post the circuit diagram that you have for the kit.?

 

[djv2008]

Here are the drawings

Here are the drawings

 

[Pommie]

The assembly manual is available here.

Mike.

 

[Pommie]

My initial thoughts are that the supply for the displays would have to be upped to something like 12V (some large displays that I have require 8V per segment). T1-4 would need to be able to handle the higher current. The base resistors (R9-12) would have to have 8V zeners in series with them and resistors R1-8 would have to be replaced by NPN transistors. All of this could be done on the new board that would be required for the new displays.

Mike.

 

[djv2008]

I was planning on supplying the large display with +12 volts. Possibly by using a computer power supply. I want to be able to use the existing small 7 segment displays in the hand held controller. So would I just tap into the multiplexed signal to the small displays and run a cable to my large display and add larger transistors at the common anode for each large display segment and add a trasistor for each segment line?

 

[Pommie]

That would work. You would have to take the signals before the resistors and you would need the 8V zeners so the PNP transistors aren't turned on by 5V.

Mike.

 

[djv2008]

Do I put the Zeners on the emitter of the BC557's or the base?

 

[Pommie]

The base in series with a current limiting resistor.

Mike.

 

[djv2008]

Thanks Mike, I really appreciate your help and quick responses. You said you had large displays. Did you make your own? Do you have any suggestions on my idea on making my 10" 7 segment displays that would have 8 leds per segment? I'm a backyard tinkerer and I've been engineering this project from the microcontroller to the large display, thinking the large display would be the easy part. I bought a bunch of 8" x 10" x .062" double sided circuit boards with the intention of etching the circuits for the led segments. I work in a machine shop and have already cut out the the face of the large display on a laser. Do you have any thoughts or ideas for the circuit or led layout for the segments?

 

[Pommie]

The displays I have have 4 LEDs per segment and so require 8V. If you put 8 in series you will need around 16V which is not very practical. I suggest two strings of 4 LEDs per segment with each string containing a current limiting resistor. This, of course, doubles the current required and if you choose bright LEDs you could end up requiring 100mA per segment and so your common anode transistors have to be able to switch at least 800mA.

There is a thread on here somewhere about making big displays similar to your requirement but I can't find it at the moment. <edit>found it</edit>

Mike.

 

[djv2008]

That post, along with the 12' long clock were my inspirations to even attempt trying this endevor. The Led segments in the 12' long clock would be great but they are expensive when you add up 28pcs at $15 a pc. I'm on a very low budget. I bought 600+ pc of 5mm ultrabright red leds and the boards along with various ics, switches for under $150. I dont mind etching and drilling boards as long as I know my design is do-able. I'm gonna try to mock up a prototype design for the segments and I will repost. Thank you again for your help so far.

Joe

 

[ericgibbs]

That would work. You would have to take the signals before the resistors and you would need the 8V zeners so the PNP transistors aren't turned on by 5V.

Mike.

hi Mike
He could use a 74LS07 o/c buffer ic for the remote display digit driver to the 10K's

 

[Pommie]

Yes, that would work or 4 NPNs driving the PNPs.

Mike.

 

[ericgibbs]

Yes, that would work or 4 NPNs driving the PNPs.

Mike.

hi,
I was concerned about the 'drive sense' if intermediate npn's are used.?

 

[djv2008]

Would this be close?

 

[Pommie]

Yes, it would be the wrong way around. Must remember to engage brain before replying.

Mike.

 

[Pommie]

Would this be close?

Close but not quiet right. The NPNs shouldn't be connected to 12V and the PNPs need a resistor and zener in the base connection.

Mike.

 

[ericgibbs]

Would this be close?

Sorry, no way.
Would you like a sketch.?

 

[djv2008]

A sketch would be great. I really appreciate both your patience with my noobie status. I must admit this is the best forum I've seen in a long time!