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4 digit 7 segment display advice?

Started by Kaellis991, October 06, 2020, 03:44:15 pm

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Kaellis991

For the transponder build I am attempting I need a couple 4 digit 7 seven segment displays. Was going to use an 8 digit but now I am considering using 2 4 digit displays with space between for LED indicators. But I can't seem to find any with a board and IC attached. Only ones I see are on a TM1637 board set up with a colon for a clock display. Can I use those for the transponder code display or do I need to learn how to attach a bare 7 segment display to a board with the 74HC595 chip or perhaps purchase a breakout board? I'm at a loss here....another hurdle.

ame

How far down the rabbit hole do you want to go?

According to this image:


The 7-segment displays do not use decimal points. The TM1637 modules are available with decimal points, but the clock-colon model might be more common. You just have to read the description carefully to get the right one. But, since the instrument doesn't use the decimal point (or colon) it doesn't matter.

...unless you wanted the decimal points as a stand-in for the little carets?

Definitely the digits on the right can be made with a 4-digit module, and the digits on the left are a 4-digit module with the leftmost digit used only for its centre bar to make a '-' symbol. Then there are five other annunciators: FL ON ALT SBY R

I suppose the bottom segment of the leftmost digit could be FL. If the right digit module had 5 digits then the top right segment of the fifth numeral could be R. Then you could add a 6th digit and the top bar could be ON and the bottom bar could be SBY. The top bar of a 7th digit could be ALT. Unfortunately I don't think the numeric segments would be large enough to light up the text.

The next issue is size. I quickly scaled that photo to approximately actual size, and I estimate the digit height to be about 7mm. There are 7mm 7-segment modules available, but I may have made an error. Even these days a lot of components are measured in inches, so it could be a 1/4" display (6.35mm). Most of the cheap display modules use 0.36" (9.1mm) or 0.56" (14.2mm) digits. It's up to you what you want to do, and whether you care about accuracy (or how much inaccuracy you will tolerate).

It seems there are three common display drivers:
TM1637
MAX7219
HT16K33

That last one is not so common. I just put it in because it's my (current) favourite.

If you could drive 8 digits then that is enough segments to do all of the numbers, the carets, the minus sign, and the annunciators. If you could wire up your own display board that would be the best (and you wouldn't need a 74HC595), then you just need a bit of software to figure out which segments to light up to make a number, or to light an annunciator.

But probably you want to get a pre-made display module, and add a couple more LEDs for the annunciators.

Err, so, about that rabbit hole.

To answer your question, will SimVim drive the TM1637 or MAX7219? Will it drive two, or it is set up to handle only one for this instrument? How do you feel about soldering a bunch of teeny-tiny wires to a 7-segment display module?

Sorry for all of these questions. In an earlier post you said you didn't have a lot of these skills yet. However, you are definitely asking the right questions, and I think with a little practice you will get the skills you need to see this through. Apologies if all of this information is too much food for thought, but keep asking questions until everything is clear.

Oh, and I couldn't help but notice that this instrument is in fact backlit. Wouldn't that be awesome...? Sorry, I'll shut up now.

Kaellis991

The clock colon is very common. But I think every 7 segment that I have seen has the decimals. There is no need for those at all.
The indicators in the middle would just be LEDs behind an etched acrylic panel.

But I am thinking this rabbit hole is getting too deep.

The Simvim panel and the accompanying software is designed to display everything on a small monitor that can be run off a raspberry PI, both of which I have. It basically displays a picture of everything you see there on that actual XPNR. I was more interested in having the displays in a physical box rather than virtual. But more and more now I am leaning towards that simpler solution.
https://simvim.com/svpanel.html

The simvim configuration tool can connect to the TM1637 and the MAX7219 and map the pins. Simvim states "You can use LCD or 7-segment displays as well (for FL and Code), and LED based annuciators". So the configuration tool can connect to the physical transponder.

So that has been my goal, but trying to setup the (2) 7 segment displays in a way to use fewer pins on the Mega 2560 using a shift register or a breakout board is throwing me for a loop.

If the display with the TM1637 and the clock colon will work the colons would not be a big issue.
The soldering is not an issue either, but a pre-built module would be easier. I just can't find them other than the clock display.

This paragraph of yours has my head spinning...

If you could drive 8 digits then that is enough segments to do all of the numbers, the carets, the minus sign, and the annunciators. If you could wire up your own display board that would be the best (and you wouldn't need a 74HC595), then you just need a bit of software to figure out which segments to light up to make a number, or to light an annunciator.

It will take some time to wrap my head around that.

So I need to decide if I want to get one of those clock modules and accept a display with colons.
Can a 74HC595 breakout board do the same thing as a 7 segment display mounted on a board with the same 74HC595 IC?

So far I think the top of that hole is up around my shoulders....


ame

Quote from: Kaellis991 on October 06, 2020, 06:03:36 pmSo far I think the top of that hole is up around my shoulders....
So, what you are saying is that you've only just poked your head in...

I can assure you it goes a lot deeper. :)

ame

Ask, and ye shall receive:
https://m.aliexpress.com/item/32866425287.html

TM1637, 0.36", 4 digits with decimal points, available in red.


ame

Actually, this one looks interesting:
https://m.aliexpress.com/item/32889866990.html

It is based on MAX7219, with 0.36" red 7-segment displays. It looks like the displays are plugged in to sockets on the PCB. To my mind that implies that one of the displays could be unplugged and moved to one side, using fine wires to reconnect it to the PCB, thus giving the appearance of two separate 4-digit displays.

I'm only going by the photo. I don't have one on my desk.

Far too many ways to skin a cat...

ame

Quote from: Kaellis991 on October 06, 2020, 06:03:36 pmThis paragraph of yours has my head spinning...

If you could drive 8 digits then that is enough segments to do all of the numbers, the carets, the minus sign, and the annunciators. If you could wire up your own display board that would be the best (and you wouldn't need a 74HC595), then you just need a bit of software to figure out which segments to light up to make a number, or to light an annunciator.

It will take some time to wrap my head around that.

So I need to decide if I want to get one of those clock modules and accept a display with colons.
Can a 74HC595 breakout board do the same thing as a 7 segment display mounted on a board with the same 74HC595 IC?

Sorry, that paragraph you quoted might be misleading. I said you wouldn't need a 74HC595, but I should have added that you would need a driver chip of some description.

To answer your question about the 74HC595, I think the answer is yes.

Basically, the driver chip has segment outputs and digit outputs, set up as a matrix or grid. But the segments are just LEDs, so you have a matrix of, say, 4x8, which could be wired as four 7-segment displays (a 7-segment display is 8 LEDs including the decimal point). Inside the chip is a tiny amount of memory that holds the pattern of LEDs to light up. The chip then cycles through the digit outputs and for each digit it drives the desired pattern on the segment outputs. It does this fast enough that you don't see any flickering. If you want a new pattern you update the contents of the chip's tiny memory.

The difference between the driver chips is how many LEDs they can handle (one digit is a group of 8 LEDs), how the interface to the microprocessor works, and what extra features they have (such as dimming, for example). Fundamentally they are all doing the same thing, which is lighting up a pattern of LEDs. If the LEDs are arranged in the familiar 7-segment display manner then humans see numbers.

What I was getting at was using some segment LEDs for the annunciators, but it's really hard to break out the pins on an assembled module.

ame

Just for larks I decided to model the KT76C in OpenSCAD to see how hard it might be to make one. The design goals are:

1 Accurate dimensions
2 All switches present and operable
3 Accurate display and annunciators
4 Backlighting

I spent a couple of hours and came up with a model of the housing, but with no rebates for switches or indicators yet. I don't have a real one, so I am relying on photos from the internet, which I can scale and convert to measurements for my model. I have attached a rendering of the model, which I would fabricate by 3D-printing.

Two of these 0.25" 7-segment display modules could be used:
https://www.aliexpress.com/item/32799427729.html

The display doesn't have decimal points in all positions, but they are not required. It is a common-cathode module (2453AS), which I would need for an HT16K33 driver, but it is available in common anode (2453BS) if you need it.

I propose to illuminate the carets and annunciators with standard 3mm LEDs, driven by the HT16K33. The carets will shine through small triangular apertures. The annuniciators will be made from laser-printed decals and will probably need a diffuser.

The switches will be based on these:
https://www.aliexpress.com/item/32902907884.html

The yellow version has an amber lens, and might look authentic. I would use additional switches behind the rotary switch to illuminate the front panel decal in the same colour as the switches, but you wouldn't be able to press them. The rotary switch labels need to be illuminated, and "XPDR", but not "BENDIX/KING" or "KT 76C TSO".

The switch would be this one:
https://www.aliexpress.com/item/4001165694966.html

It is a 45 degree switch with 5 positions. The switch would have a 3D-printed knob and skirt.

In terms of construction, the body would be 3D-printed in black. The front face of the deco ring would be painted white. A piece of 2mm smoked acrylic would be dropped into the rebate as a display filter, and a full-width laser-printed decal/label would be stuck on to cover the edge of the filter and provide a way to include the artwork and lettering. The button actuators would be printed in white, so that the yellow/amber LED light from the switches would light them up from behind.

Nothing particularly clever, but I think it would look cool.

I'm not sure how far I'm going to go with this, but it's fun and interesting (to me).

ame

I think the pushbutton switches are too large and will be slightly cramped in that layout, so instead I will design in these:
https://www.aliexpress.com/item/32760444535.html

I don't know how 'yellow' the LEDs are. Based on my research it seems the buttons are backlit with a more 'amber' light.

ame

Ok, I ordered these (0.25" 4-digit red common cathode):
https://www.aliexpress.com/item/5pcs-0-25inch-7Segment-4-bits-Clock-LED-Display-Timer-RED-4-Digital-Numbers-LED-Signs/32799427729.html

And these (yellow LED):
https://www.aliexpress.com/item/10Pcs-6-6mm-Through-Hole-Micro-Push-Button-Tactile-Momentary-Switch-With-LED-Sample-Green-Yellow/32760444535.html

And one of these (1 pole 5 position):
https://www.aliexpress.com/item/4000582837052.html

I intend to drive everything with an HT16K33 chip. However, the 7-segment displays are common-cathode, so they could be driven by a MAX7219. The advantage of the HT16K33 is that it will handle switch inputs as well as LED outputs, and communicates over I2C.

Once the design is settled it could easily be re-worked for any other interface or modifications. However, the first step is to get something working.

ame

Ok. The little switches arrived. They are nice, and the yellow colour of the LED is acceptable.

I have the rotary switch.

Still waiting for the red 7-segment displays.

And a lot of other things to do...

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