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What is the best choice of LED to build a large (400mm) outdoor 7-segment display?

line

Having done a reasonable amount of research (clearly not enough) I'd value some assistance in selecting LEDs to make large 7-segment displays for outdoor use.

The application is for clocks/timers, with a viewing distance of 100m, reasonable viewing angle (ideally two displays at 90 degrees to provide near 180 degree viewing angle), for a product to run from mains (240Vac) or portable (12Vdc). Using two 7-segment characters for each display module.

Can anyone suggest either some suitable 7-segment displays (ready made - I can't find any) or LEDs suitable for making some bespoke displays?

Considerations include:

Brightness: How many lumens/mcd per segment are required for 100m visibility outdoors (full sunlight)?

Forward voltage: I expect to require higher than a 12V supply for the LED segment (daisy chain), using a boost converter to achieve the required voltages from a 12Vdc power source (battery) but it (Vf) needs to be reasonable.

Forward current: anticipate using constant current drivers to drive the LED segments, again needs to be reasonable (1w max).

Cost: each 7-segment display needs to be under £50 if buying and under £25 component cost if building, for the overall product cost to be viable.

Power: a consideration for the batteries - spec is four 7-segment displays (2 modules with 2 characters each) running for 6 hours of operation.

Heat: heat sink / heat management to avoid the LED junctions overheating (eg high power LEDs)

Viewing angles: In theory 90 degrees viewing angle per segment/display but in practice would the LED viewing angle need to be this high to achieve good/acceptable results across 180 degrees with two displays fixed 90 degrees to each other?

Quantity and arrangement of LEDs per segment?

If it helps (not much) a decimal point is not required - just the 7-segment characters.

 

If anyone has experience in this area and can offer some assistance\guidance it would be greatly appreciated.

With so many LEDs to choose from - you could save me a lot of pain and headache. Thanks in advance.

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  • 0

    Hello Matthew,

     

    Have you looked at using WS2812 strips also sometimes called Neopixels?   I have seen several large 7 segment displays made from them - for example these made for a clock.  There may be 12 V versions, if not then a switching buck converter could be used to get 5 V

     

    edit: here is another one https://learn.adafruit.com/ninja-timer-giant-7-segment-display/overview

  • 0 in reply to fmilburn

    Hi Frank

    Thank you for the suggestion and a beautiful solution for indoor use but are these bright enough to be visible outdoors?

    Example: https://www.ukled.co.uk/led-strip-4-8w-m.html

    Spec quotes lumens per meter at 240lm.

    To construct a 400mm character, each segment would be 200mm long which is just 48lm.

    Sadly I don't think it would be visible in bright daylight from more than a few meters.

    I had come across the Ninja timer in my research - I think it's great (for indoors).

    I have been looking for suitable SMD devices - the viewing angles are great - but I haven't found the right combination of quantity, cost, power and brightness yet.

    What do you think?

    Regards Matthew

  • 0 in reply to line

    I think the segments that are lit will be visible if there is sufficient contrast in color and / or brightness compared to the unlit ones.  I have some and will try and do an experiment and capture the results outdoors on my phone - but it is raining here at the moment and it may be a day or so before we get good weather.

  • 0 in reply to line

    Well, finally a sunny day.  Of course it isn't exactly mid-day in July in the Bahamas here in Portland but still.  I parked my car and put the LEDs on the back window facing into the sun and paced off 100 meters.  The LEDs look like this:

    image

    There is an Arduino MKR1010 with a RGB shield on top and a small strip of 5 neopixels to the side with 15 mm spacing as that is all I had at hand.  The neopixels were driven at 100% from the 5V supply.  The RGB shield was driven at maybe 10% because the Arduino could not deliver the power at 100% without glitching.  I set them to flash alternately for about a second each with a seconds pause in between.

     

    I could still discern the strip at 70 meters but not well enough to make a claim at 100.  I think as much as anything that was due to the small size of the strip (6 cm first to last LED) and my relatively poor eyesight.  My thoughts are that increasing the density of the LEDs, having a longer strip / segment than 6 cm, maybe placing 2 strips side by side would make it visible.  A diffuser to spread the light might help as well provided it was not too opaque.  Putting a hood around it to give shade and improve contrast would help.  It isn't a direct comparison for your requirements but the RGB shield at 100% appeared to my eye to be much brighter than the rear brake lights on my car, at 20% they were dimmer.

     

    So this wasn't an entirely satisfactory experiment but I believe WS2812 can be made to work.  The links that Gerard provided would be good ideas to follow up on.  You might need to experiment a bit like I did.  In any event good luck, it sounds like an interesting project.

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  • 0 in reply to line

    Well, finally a sunny day.  Of course it isn't exactly mid-day in July in the Bahamas here in Portland but still.  I parked my car and put the LEDs on the back window facing into the sun and paced off 100 meters.  The LEDs look like this:

    image

    There is an Arduino MKR1010 with a RGB shield on top and a small strip of 5 neopixels to the side with 15 mm spacing as that is all I had at hand.  The neopixels were driven at 100% from the 5V supply.  The RGB shield was driven at maybe 10% because the Arduino could not deliver the power at 100% without glitching.  I set them to flash alternately for about a second each with a seconds pause in between.

     

    I could still discern the strip at 70 meters but not well enough to make a claim at 100.  I think as much as anything that was due to the small size of the strip (6 cm first to last LED) and my relatively poor eyesight.  My thoughts are that increasing the density of the LEDs, having a longer strip / segment than 6 cm, maybe placing 2 strips side by side would make it visible.  A diffuser to spread the light might help as well provided it was not too opaque.  Putting a hood around it to give shade and improve contrast would help.  It isn't a direct comparison for your requirements but the RGB shield at 100% appeared to my eye to be much brighter than the rear brake lights on my car, at 20% they were dimmer.

     

    So this wasn't an entirely satisfactory experiment but I believe WS2812 can be made to work.  The links that Gerard provided would be good ideas to follow up on.  You might need to experiment a bit like I did.  In any event good luck, it sounds like an interesting project.

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