Going RGB with 7 segment displays

We can order seven segment displays in red, green, yellow, or blue all day long. One thing we haven’t seen is an RGB segmented display, so [Markus]’ project is really interesting. He took a stock seven segment display and modded it into an RGB display.

After taking a Dremel to the back of the stock display, [Markus] was left with a seven segment light mask. A few SMD LEDs were purchased through the usual channels. The RGB LEDs were epoxied into place on the back of the light mask one at a time. Thankfully, the LEDs came with magnet wire already attached – helpful, since these LEDs are only 1.6mm x 1.2mm big.

With 32 pieces of magnet wire, [Markus] needed some sort of socket. A small piece of perfboard and some .100″ headers handled the job very nicely. [Markus] still has to work on some way to drive the 24 cathode lines his LED display. He’d like an I2C interface, but with something like an individual seven segment display, the footprint of the circuit should be pretty small. If you’ve got any tips, drop them in the comments section. [Markus] is sure to catch them there.

22 thoughts on “Going RGB with 7 segment displays

  1. Not so fast on slagging the random repost if it’s a rewritten version. and the CONCEPT of modding a display’s shell to such a result is damned worthy Hack Cred. More so if there’s NOT a common/off the shelf display that has RGB 7segs.

    One Hack worth looking into if you use I2C:

    http://www.harbaum.org/till/i2c_tiny_usb/index.shtml

    His methods have defined limits, but for many interfaces his board is quite “Good enough” to prevent wheel reinventing.

  2. First of all, that’s a “plug”, and not a “socket”.

    Secondly, I’d look into using something like a PIC or similar processor mounted on that plug/socket. You’d only need 10 I/O lines, plus the serial and power. Less if you use a multiplexer.

    Looks cool, but really, I don’t see that much use for rainbow colored displays. Unless maybe you can display say, time, and “relative” temperature at the same time (though color).

    The human eye is able to pick out more information from a color display. That’s why they can use one RGB LED to display voltage or frequency with one of those “probes”. It’s not super accurate, but hey… (EE Times had the article I think, sorry, no link)

  3. You can HALVE the number of pins required to drive the display by wiring it to have all the anodes(or cathodes, if you prefer) of a single color tied to a single pin.
    You can then use multiplexing to display different patterns in different colors.
    Treat it as 3 separate displays in one housing.

    1. I’m not the best at this, and I’ve never done any REAL work with this, but could you draw a diagram of them laid out so I could understand the concept better? Multiplexing still has me confused. :(

      1. Dunno if I can link straight to the image…

        But it’s here on this page

        https://www.alibaba.com/product-detail/0-80-inch-led-7-segment_60610106217.html?s=p

        The page says they’re RGB LEDs. They’re not, maybe they’re available in red OR green, but they’re only ordinary 1-colour displays. China, t’sk!

        Anyway, The diagram shows the pin connections, a load of LEDs. It’s for a 2-digit display. To save pins, the cathodes of each digit are connected to the same pin. So the cathodes for, say, the top segment, are connected to pin, say, 3. Bottom segments are both connected to, say, pin 5. I dunno which exact pins, doesn’t matter.

        So each individual segment of the paired displays share a cathode. How then to light up just one at a time? Because the left display’s LEDs are all connected to one anode, the right display’s to another.

        Imagine it like a 2×8 grid. For each LED, if it’s anode is connected to +V and it’s cathode to gnd, then it’ll light up. With one cathode each segment, and one anode per display, by connecting as many cathodes up at once as you like, to make a number, and the relevant left or right anode, you get a number on that display.

        How to make both displays light at the same time? Well, you can’t! Well you could as long as they’re both showing the same digit. But since pins are shared, if you want separate digits on each display, you have to first connect the left anode, and the relevant segment cathodes, to power. Then disconnect left, and do the same with right. Flick between the 2 digits very fast. A human eye won’t notice. Although some cheap LED alarm clocks, you can see the flicker.

        And that’s how multiplexing works. The diagram should give you most of the idea.

  4. I have built a few things using RGB seven segment displays before, you can be spared the difficulty of trying to shuck a conventional 7-segment LED if you can find an incandescent 7-segment display instead. Incandescent 7-segment displays usually have screws that can be removed to open them, to change the light bulbs inside; it’s easy enough to replace the bulbs with LED’s instead. Try looking for “Dialco Display” on Google to see an example.

  5. an easy method of cutting down the pins would be to simply charliplex the cathodes and tie all the anodes together leaving the center pins the only ones left induvidual. if done correctly you could reduce 24 pins down to 12 or so pins, which can be made even less with a single shift register. but if you are looking to use multiples or scroll text over these then you are going to kneed to use 2 or more shift register at a rate of 2 registers per RGB 7 segment piece if you want full color controll and the option to power certain parts of the display as kneeded. in other words things will get complicated fast this way ( im talking about a 3 pin RGB Led version of this mod)

  6. well, i’m inspired. One day when I get a surface mount PCB making abilities, it would be neat to try to see how many shift registers can be crammed onto a small board with the LEDs on the other side, within the footprint of the 7 seg display as close as possible.

  7. Hmmm, invent a game that uses this display and you’ve really got something…see how long the red segment can avoid the yellow segment that chases it? When either segment reaches the decimal point it can stall the other for one turn. “Eat” green segments for more energy. Just throwin’ out ideas here…

  8. There are some RGB 7-segment LEDs you can buy on the web. Still well done on the craftwork.

    To cut down the number of pins, might be sensible to do as some of the commercial ones do. Common anode all of the reds, all of the greens, all of the blues (or common cathode, either way). Then you’ve 1 pin each for R, G, B, and the remaining 8 for the segments. Only 2 extra pins.

    It’d mean you’d have to time-multiplex to get more than one colour at once, but that’s pretty normal for LED displays anyway.

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