Author Topic: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3  (Read 77560 times)

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Offline EEVblogTopic starter

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Dave.
 

Offline chrome

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EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #1 on: June 25, 2012, 12:47:18 pm »
I sort of like the idea of the decade LED's but I wonder how hard it will be to read depending on how small the LED's are and how bright they will be

I was wondering why you didn't try using individual LED's to make 7-segment displays, although that would require a bit more space and I'm also not sure about readability.
 

Offline EEVblogTopic starter

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #2 on: June 25, 2012, 01:06:25 pm »
I was wondering why you didn't try using individual LED's to make 7-segment displays

If you do it using the 7 seg driver approach then it's potentially 7-8 times the current.
Other arrangements can drop that current, but it's messy, and the requirement to light up more leds over a longer mux time drops the brightness.
And yes, the readability isn't as great. The best way would probably be more expensive rectangular leds and/or reverse mounting with slots perhaps.

Dave.
« Last Edit: June 25, 2012, 01:11:41 pm by EEVblog »
 

Offline free_electron

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #3 on: June 25, 2012, 01:23:44 pm »
Bweuaark. Readability : zero...
There is a reason no testequipment has used such an approach.
If i look at a powersupply i want to be able to see in one glance what the settings are. Not having to 'decode' it in my head.
Especially with the shifting location of the dot between volts and current. There should be at least an indication if we are in volts or amps mode.
And you can never see both at the same time...

Why not use a dumb 7 segment lcd display. There's cheap 8 diigits out there. You dont need a display controller. You can scan them from the cpu. 8 digit lines and 8 segment lines ( dot is also a segment ) and you can see both volts and current.

Cheap for the sake of cheap is not a good way. It's got to remain usable.
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Offline T4P

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #4 on: June 25, 2012, 01:30:41 pm »
It actually is a brilliant idea for the sake of being cheap,
being vertical and not horizontal which is what dave did is just brilliant
 

Offline notsob

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #5 on: June 25, 2012, 01:33:31 pm »
elegant solution dave, but if someone gave me a display like that to read when i was testing, I would probably toss it.
 

Offline david77

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #6 on: June 25, 2012, 01:38:17 pm »
...
There is a reason no testequipment has used such an approach.
...

Not quite true. I think HP did something like that way back in the 1940ies or 50ies, even before Nixie tubes became common.
I agree on the rest of your post.
 

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #7 on: June 25, 2012, 01:46:53 pm »
It has some merit. Given the likely use of the thing. I don't see too many times something like this would be used without a meter (or three) close by, so a quick and dirty display may be sufficient. Would I buy it? Probably not! But then I am not in the market for an every cent counts display.

An efficient isolated ultra portable USB supply selectively delivering the common 5V 3V3 etc would probably better suit the most common needs even if it lost much of it's nerd appeal.
 

Offline EEVblogTopic starter

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #8 on: June 25, 2012, 01:49:47 pm »
Bweuaark. Readability : zero...
There is a reason no testequipment has used such an approach.
If i look at a powersupply i want to be able to see in one glance what the settings are. Not having to 'decode' it in my head.
Especially with the shifting location of the dot between volts and current. There should be at least an indication if we are in volts or amps mode.
And you can never see both at the same time...

Sorry, forgot to mention that, it normally displays volts, with amps only when you press a button. Same with a single 3/4 digit LCD or LED solution.

Dave.
 

Offline Psi

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #9 on: June 25, 2012, 02:01:33 pm »
Might pay to check if, and what price, Digikey can get these things for (they don't list them but have other liteon products)
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2005497_-1

They're lite-on brand so good quality, 0.7inch so small enough, and the red ones i played with were very efficient, 0.6mA and i could see them fine sitting at my bench.  At $0.49 each in 1000 qty they're not expensive either.  Two of them together would give 14 pixel wide, enough for four 3bit x 5bit characters.  I/O wise, if you parallel the 7 columns you'd need 7 +5 +5 = 17 I/O

But i've not seen them for sale at many stores, so i'm not sure if they were a special run that Jamaco did or just a non-stocked product (hence needing to check their availability).
« Last Edit: June 25, 2012, 02:13:50 pm by Psi »
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Offline free_electron

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #10 on: June 25, 2012, 02:21:26 pm »

Sorry, forgot to mention that, it normally displays volts, with amps only when you press a button. Same with a single 3/4 digit LCD or LED solution.

Dave.

 i woud make it display current by default.. when powering up an 'unknown' that's what you typically anxiously keep an eye on ...

if you go for a 31/2 digit passive lcd there are versions that come with V and A symbols sou you could show what is going on.
what happened to the rotary encoder ? gone ?

hammond has translucent enclosures... displays will sine straight through them.

i would have gone with a 2x8 alpha display with led backlight. 5mA for the backlight and plenty of displat real estate to show both set voltage and current and real voltage and current.

something like the DOGM modules ( mouser )
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Offline EEVblogTopic starter

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #11 on: June 25, 2012, 02:27:20 pm »
i woud make it display current by default.. when powering up an 'unknown' that's what you typically anxiously keep an eye on ...

if you go for a 31/2 digit passive lcd there are versions that come with V and A symbols sou you could show what is going on.
what happened to the rotary encoder ? gone ?

hammond has translucent enclosures... displays will sine straight through them.

i would have gone with a 2x8 alpha display with led backlight. 5mA for the backlight and plenty of displat real estate to show both set voltage and current and real voltage and current.

something like the DOGM modules ( mouser )

I've used those before, I like them, but not that cheap.
Two 3-4 digit 7-segment LCD displays like shown would be cheaper, and easier to read than dot matrix. But harder to drive of course.

Dave.
 

Offline MikeK

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #12 on: June 25, 2012, 02:34:33 pm »
I was wondering why you didn't try using individual LED's to make 7-segment displays

If you do it using the 7 seg driver approach then it's potentially 7-8 times the current.

There's no reason to light all of the necessary segments of the digit at the same time, though.  Segments can be multiplexed just as the digits are, which brings current back down to that of a single segment.
 

Offline Psi

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #13 on: June 25, 2012, 02:37:20 pm »
You don't get something for nothing. If you have 64 segments and only 1 on at a time you have to put quite a lot of current into the one segment to get the brightness back up. High efficiency leds lose their efficiency at high current.

There's only so far you can trick the human eye before it starts to notice the brightness dip  :D
« Last Edit: June 25, 2012, 02:40:06 pm by Psi »
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Offline firewalker

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #14 on: June 25, 2012, 02:39:48 pm »
I was wondering why you didn't try using individual LED's to make 7-segment displays

If you do it using the 7 seg driver approach then it's potentially 7-8 times the current.

There's no reason to light all of the necessary segments of the digit at the same time, though.  Segments can be multiplexed just as the digits are, which brings current back down to that of a single segment.

This means larger pin count for the mcu. So the price will go up.  Also there will be a 25xDelay_Time_for_Adequate_Brightness.

Alexander.
« Last Edit: June 25, 2012, 02:42:16 pm by firewalker »
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Offline Stephen Hill

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #15 on: June 25, 2012, 02:40:48 pm »
I like the ingenuity of the solution but like others I don't think it's very user friendly. How many devices have you ever owned or used that have this type of display?

I'd personally pay the extra for two 4 Digit LED display.

I work around if you wanted to get close to the 500mA current would be to have a switch to toggle the displays on and off, letting you measure the current with a multimeter. You could also add a two pin header with a removable jumper to measure the current from.

Cheers
Ste
 

Offline EEVblogTopic starter

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #16 on: June 25, 2012, 02:42:10 pm »
There's no reason to light all of the necessary segments of the digit at the same time, though.  Segments can be multiplexed just as the digits are, which brings current back down to that of a single segment.

Not really. As I said after that line, and as PSI said, the brightness will drop with the increase in mux number. You can't get something for nothing.

Dave.
 

Offline Psi

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #17 on: June 25, 2012, 02:47:19 pm »
hm.. yeah, i like the circle idea. It is easier to read as people are more use to reading a clock.

Another idea is side mounted LEDs that illuminate a silkscreen number (maybe inverted, white box, text=nosilkscreen to reflect the light better).
That would make it instantly obvious what the voltage/current is, rather than having to scroll your eye over to a number and read it.
You could also have the same side illumination with the word Voltage/Current to make the mode obvious too.

The disadvantage is that unless the text is really close to the led it can requires more current to illuminate than have the led shine directly at the person.

Maybe if side mounted leds were placed next to each other, on their long side, with the numbers between them each led would shield the light from the one before it. This would help to concentrate the light right at the text.


Or maybe a transparent overlay could somehow be placed over the leds. So they can instead shine upwards as normal but through sometimes with the number printed on it.
« Last Edit: June 25, 2012, 03:00:37 pm by Psi »
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Offline _Sin

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #18 on: June 25, 2012, 02:54:52 pm »
I reckon you could have a human readable display in only 5 leds, but it would involve waving the board around to be able to read it...
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Offline all_repair

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #19 on: June 25, 2012, 03:07:24 pm »
Maybe a proper display that only turns on for a short while when a button/knob activity.
Or move the display consumption before the isolation converter.

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Offline T4P

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #20 on: June 25, 2012, 03:10:24 pm »
Or maybe a transparent overlay could somehow be placed over the leds. So they can instead shine upwards as normal but through sometimes with the number printed on it.
Alignment ... this is the reason he didn't want to use individual leds to make a segment
 

Offline amspire

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #21 on: June 25, 2012, 03:44:21 pm »
Here's an even crazier idea to save micro pins, and probably more money.

No display at all. If you need an accurate output, you attach a meter.

However, the supply has a piezo speaker so it can output voltages and currents in Morse code.  It would mean out is still fully useable without a meter, and you get to learn some Morse. The speaker can be replaced by a led when you needed silent operation.
« Last Edit: June 25, 2012, 03:47:59 pm by amspire »
 

Offline free_electron

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #22 on: June 25, 2012, 03:45:47 pm »
Two 3-4 digit 7-segment LCD displays like shown would be cheaper, and easier to read than dot matrix. But harder to drive of course.
Dave.

take a look at this :

67-1795-ND (digikey)

Drive from a pcf21xx display driver. All you need is I2C ... and that you can bitbang. if the dacs in the psu are I2C ... you could get away with an 8 pin CPU. power, ground , i2c and 4 pins for the buttons...
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Offline bxs

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #23 on: June 25, 2012, 03:51:58 pm »
I really think you gone too far, let's see:

- you show something with knobs, them put buttons, ok I can go with that, I don't like, but we need to understand that it need to be small and portable,

- you show the worst LCD case, it only have a common pin, this is crazy, you will almost never use something like that because of the number of pins required, you need to a proper LCD, see the if the price is viable and only then go for a micro, so the search you made for the microcontroller didn't count,

- you show something with independent displays for V/A, then you end up with one, just by that you almost lost me,

- then that ugly thing with the leds, that you have to stop for a minute or tow to decode the thing, men, I could never buy it.

I really think you gone too far...  :(

But you are in a excellent position, your work don't have to respond to anyone, you are your boss, you can do what you want  ;D
 

Offline Bored@Work

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Re: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3
« Reply #24 on: June 25, 2012, 04:00:46 pm »
You can't polish a turd, and that display is one in my book.

But you can alter the shades of brown. What about using bi-color LEDs? One color when amps are displayed, the other when volts are displayed.
« Last Edit: June 25, 2012, 04:07:15 pm by Bored@Work »
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