Display Bezel Idea

image copy copy copy copy copy copyOne of the issues to be solved in developing the Wake Turbulence Timers was bezels for the red and yellow 3 digit 7-segment LED displays.  The old-school traditional type clamped to the panel and provided long 4-40 threaded rods to support the printed circuit board in the proper position.  There are two problems with these - they only have red filters and they are over twenty bucks a piece!

 

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Digi-Key has some bezels by Proud Industries.  They bezels snap into a panel cutout but don't provide any means of mounting a printed circuit board.  Lenses are available in a range of colors, including red and green.  For my yellow displays, they recommended their smoke lenses.  These lenses were more appropriate for welding goggles than being in front of LED displays!  Proud Industries will make custom colors...for a thousand piece minimum order.  I'Ve written an article here on dying the clear lens in dye made for glasses....it's basically boiling the lenses in dye for 15 minutes.  It's doable, but not practical on a large scale.

 

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Hack-A-Day touched on an  alternative bezel idea.  Three 7-segment voltmeters and ammeters are available dirt-cheap on ebay.  For 0.86" digits, complete meters in bezels are less than two bucks!  Even if you throw out the existing electronics, the cost is less than half the alternative.

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The Hack-A-Day article suggested building your own microcontroller controller board to fit the bezel where you need a 3 digit readout.  I think the genius solution is to design a circuit with a 3 digit serial or I2C LED driver.  This would work perfectly for the Wake Turbulences timers, and for other applications.  It looks like many of these display housings are made by the same vendor.

 

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Posted: 4 months 2 weeks ago by Jon Chandler #17912
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I received 3 of these meters today. The board is held in by a tab on each end and can be pried out without any problem.


This photo shows the housing, the lens and the removed meter circuit board. Wow, there is almost no clearance around the 3 digit display on the circuit board! This is a multiplexed display, so few traces are needed on the circuit board. I have to use 3 individual digits - the second one is rotated so the decimal points form a colon. M:SS



Here are the dimensions of a board that will fit. I need to verify that the digits I have will actually fit!

Posted: 4 months 2 weeks ago by Jon Chandler #17913
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So in my ever-so-slow progress, I located 3 of the digits I use for the Wake Turbulence Timers; they do fit with perhaps a millimeter to spare in each direction.



The boards originally in these units used a 3 digit display, meaning there are just 11 connections to be made to the driver chip. For my application, I need to use individual digits, as I rotate the second one to use the decimal points as a colon. That means 8 connections in parallel across the digits and the three common connections for the digits.

My plan is to use a MAX7219 as the driver. I need a 6 pin header for the I2C signals and ideally a second one to daisy-chain at least 2 modules.

The MAX7219 has the ability to decode the digits you send. For example, if you send an ASCII 7, the chip will decide what segments need to be illuminated to display the digit 7. To make this work, I need to remap the pinout for the rotated digit. Segment G and the decimal points swap positions when the digit is rotated. I had hoped to include some solder jumpers to select a normal or rotated digit but the tiny board size may preclude this. The display will still be usable with the digit in either orientation, but in one position I can't use the built-in decode feature.

I'll see if I can squeeze everything in.
Posted: 4 months 2 weeks ago by Jon Chandler #17915
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Here's a picture of the board so far with most of the connections between the MAX7219 driver chip and the 3 displays complete. Just minor details like power and ground and i/o left :) It's a via fest!

Note - the inner black outline is the board shape. The connector isn't on the board yet.

Posted: 4 months 1 week ago by MMcLaren #17917
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Oh my goodness, why aren't you using a multiplexed three digit display?

Oops! Never mind. I just read why...
Posted: 4 months 1 week ago by Jon Chandler #17918
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Here is the almost complete board. I may shift everything slightly to the right to give the connector a bit more room. It's a 2mm pitch header.


Posted: 4 months 5 days ago by hop #17919
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My that is pretty! I love how you can create what you need on a board. I still have not crossed over to that world yet. I really need to.

EDIT: Forgive my ignorance Jon, you lost me with rotating the 2nd digit to use it as a colon. I get why they have to be individually addressed to achieve that, but I might better understand what you are doing if I saw a rendered, drawing even, simple image of the expected output.
Posted: 3 months 4 weeks ago by Jon Chandler #17922
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Here's a picture showing the rotated middle digit. Instead of all 3 decimal points being on the bottom, there is a "good enough" colon between the first and second digits.




My boards arrived this morning. Here is one inserted into the housing.


Posted: 3 months 4 weeks ago by hop #17923
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Very nice! I get it now thank you. Two bad they do not make a digit with decimal places on the top and bottom. But then that would be 9 segments, not 8. You would have to go alpha-numeric.

Just curious, why didn't you go alpha-numeric? Maxim makes great controllers for that too.

EDIT: Nevermind. Even with alphanumeric digits you can't make a colon. You would have to use dot matrix, 7x5 probably. 17 or 18 segments versus 35.
Posted: 3 months 4 weeks ago by Jon Chandler #17924
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I soldered one of the displays together and it works.

This picture shows the back of the display. I forgot to order 2mm headers, so I soldered in a 6 pin pigtail.



Notice the position of the cable/connector. It's squeezed right against the side of the housing. This is where I made a small mistake. To ensure the header would fit, I shifted everything over slightly - 0.5mm or 1mm.

Notice the top rightmost segment - it's just under the edge of the bezel! My slight shift to provide room for the connector clipped the edge of this segment. Other digits in the same package might be a touch smaller and this wouldn't be an issue at all.



The display is entirely usable despite this small error, and it looks sharp.




I don't know if I'll use these displays for the next batch of timers, but it's an attractive option. With the acrylic panels, I laser-cut a rounded rectangle for each display and put a piece of 3M Vikuti Light Control film behind the panel to act as a filter. This film is polycarbonate and roughly 1/16 inch thick. I cut it to size on a paper cutter, peel off the protective film from both sides and adhere it to the panel with double-backed tape, all while ensuring I don't get any finger prints on the film. It's a rather tedious, time-consuming process done as part of the final assembly. A stack of pre-built modules would make the job easier.


Posted: 2 months 3 weeks ago by MMcLaren #17945
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Hi Jon.

I bought a couple of those volt meters and I think it's amazing that three 0.56" displays just barely fit. Anyway, I just ran across these three digit 0.39" displays on AliExpress that I think may be a better fit.

Cheerful regards, Mike

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