EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3

[chimerahitman]

Has anyone thought about the plexiglass or the translucent plastic getting scratched after a few months of use, which then will make it even harder to read the small text?

Maybe if you have the outline of the selected plastic half-box stick out, in other words when the plexiglass is screwed in it will not touch the surface first, you could get a few months more of clear use, but eventually this product will not last as long.

Stuff gets scratched even if you take lots of care.

If I were going to buy one of these, I wouldn't mind having to pay 1-5 dollars extra in exchange for usability. There are small LiteOn 7-seg LCDs, you could even end up fitting 2 in there allowing both current and voltage simultaneous measurement. Though one could be enough. Point is, numbers plain and simple.

Being able to read both current and voltage at same time will probably do away with the need of using meters, which is the point of this small PSU, to be portable.

[free_electron]

You can't polish a turd,

that myth has been busted. yes you can. but you need tiger poo

[Blue]

Cheap - I wonder...


Hi Dave,

I saw a few moments ago that you teamed up with Adafruits (perhaps you didn't liked the postage hassle anymore).  They want $13 for shipment for the microcurrent converter.  Now, what would saving a few dollars mean in real % terms for the complete price of the usb powersupply? Right - not that much.

(Alternative -  shipment from Hongkong is regarded as quite low (as low as $2 for you mucrrent device)).

[tnt]

I'm leaning toward the same opinion as others:

Although it's a clever solution, I'm not sure the readability is gonna be adequate, especially if you have changing reading for currents for ex and the decade is changing (like oscillate between 19 and 20 for ex).

But sometimes things that seem counter intuitive at first turn out to be remarkably usable, so I'd guess making a quick prototype and see how a small sample of people get used to it ?

[tutman96]

I completely dislike the idea of the decade display. I want to be able to see what voltage the supply is on right when I look at it, not have to decode it. If you want to save money but keep functionality the same, just make a nice computer application that shows the current and voltage. I think you should do that anyways. Maybe put a switch on the board to turn on and off the display. I would buy it if I could set and read the settings from the computer and not waste any power on the leds.

P.S. Was waiting for your battery powered supply to come out. Was going to buy maybe 2, but now you are spending your time on a "toy" that powers 500 mA max and has a display that would take time to decode. GO BACK TO THE BATTERY POWERED SUPPLY!!!

[RJSC]

It's not an optimal display to get a quick reading but I can live with it.
If we still have the USB<->RS232 converter and opto isolator pads we can still get a more natural numeric reading when we have it connected on a computer.

What I think I can't live with is the fact that, as it is, it only allows to operate the PSU up to 299.9 mA
.
We have at least 2.5 Watts available on any USB port, so we can be drawing 500 mA @ 3.3V with no effort at all.

Or is the DC-DC converter only able to go up to 300 mA no mater how low the voltage?

[McMonster]

I personally think that won't be very popular. Cutting the costs is important, but a lot of people would actually be willing to spend 2-4 dollars or even more just to get a nice, readable LCD display they're used to. Too bad supplying two versions, a "DDDD" and LCD one would rise the overall cost (not to mention being a pain in Dave's ass), a direct comparison of sales would be very interesting.

Maybe just make the supply hackable? Some minimal ISP connector (or option for soldering the pins) that would actually double as an extension connector for optional SPI display? You don't like the "bargraph" display, you reflash the micro, connect SPI LCD display, glue/screw it over the old display and everyone's happy. Sure it's still the pain in the ass, but does not add any immediate costs to the device and leaves people with some choice.

[caroper]

You could save even more LED's, and board space, by displaying the value in Binary, It wouldn't be much harder to read.


If you are that determined to cut cost and quiescent current, leave off the display entirely, Just an LED for Source Connected, and LED for Output Enabled and an LED for Overload/CC Mode.


Anyone that needs a portable PSU will also have a Portable DMM if they have their priority's correct.

[Chasm]

Hmm...

A novelty toy that doesn't fart.
I think would pay $5, shipped. And thats only because I could hack it into something a bit more usable. Who cares about the BOM cost if the product is not worth the price?

The display solution is cheap but way to unreadable to be useful. If it is an adjustable power supply  use a real display.  - Otherwise go a step further and only offer a few set voltages. Yes, that means killing even more features.

If neither is an option just glue on the Arduino headers and the sales numbers should go through the roof.

[bullet308]

This is a wonderful exercise in thinking outside the box as content for the blog, and I could see a number of uses for this approach. However, as an approach for a finished product for sale...well, you can see how much pushback you are getting and there is no-doubt a high correlation between forum posters and your customer base.

Perhaps one as a decade display and one LED...

[eternal_noob]

The display solution is cheap but way to unreadable to be useful. If it is an adjustable power supply  use a real display.  - Otherwise go a step further and only offer a few set voltages. Yes, that means killing even more features.

Dave's display solution works better than most would think and the readability is not a problem AT ALL if the voltage adjustment has some kind of a vernier drive to it. Try it yourself, don't let perceptions and old habits get you.

[lk]

Hi Dave,
Its a very intersting idea, for me its no use that i would have to decode that as well. And its already sometimes an issue for me, not being able to decode the amp/milliamp dot on my power supply which causes weird smells.

If the 7 segment displays cause power consumption problems, cant they be driven from a PWM setup, to reduce the power consumption?

Generally i wont have a problem paying 4$ more for a proper display.


-lk

[hlavac]

Even an analog meter would be better than this weirdness.

[IanJ]

Hi all,

Yup I don't like the bargraph idea either.

Quick idea: So why not drive the 7-segment LCD's direct from your PIC/uP.......you don't necessarily need one with a built in 7-seg driver. 3 digits would need 7+3=10 digital outputs or thereabouts.
And if you don't have the I/O then you could also use a BCD to 7-segment IC.......so you could drive the 3 digits (minus DP's) with just 7 total digital outputs (4+3=7).
A nice we routine in your code could scan the LCD accordingly (backplane).

Ian.

[nitro2k01]

I don't know if anyone has mentioned it, but color coding!! The complex variant of this is to have one color for each of the 9 digits, somewhat matching the resistor colors used on hole mount resistors. The less complex option is to give (for example) even digits red LEDs and odd digits green LEDs. This should reduce the "seek time" when having a quick look at the readout.

[SeanB]

I will add my few pence of idea as well.

Use a thinner board, and make the red solder mask on the topside have clear sections where the led displays are going to be. Place a white screenprint around the areas where the digits are going to show, and a grid of white to delimit the digits. Instead of placing the leds on the front, place them on the back so they show through the clear pcb material, using the material as a diffuser. The white mask will help with digit reading, and will also enable you to hide the traces that you will need to common the columns. Rows can be routed on the back side, so that the front panel is bigger display wise, has easy to read masks and has no visible display when off aside from the legends. If you do not want to have multiple vias under the soldermask and screenprint, then you can just use 0R jumpers on the back side to provide the required crossings. Might be cheaper than vias, but more parts to lay out.

Just a thought, I have used thin clear pcb material as a display cover for HP led displays that run on 5mA, they are visible through it when on, and off is a milky white. The on is somewhat diffused, but perfectly legible. It will mean using a thinner board material, or at least a clear one with a reasonable light transmission.

[DavidDLC]

Fugly solution.

As somebody mentioned, I will pay a couple of extra dollars to have 7 segment displays.

David.

[armandas]

I'd love to hear what this guy would have to say about this display idea

[rr100]

Frankly I like these exercises and maybe I could use the concept, in the end anything that works works.
And yes, I know every cent counts but if you're going to turn your product around for some tens of cents and potentially cripple it this is getting ridiculous. To put things into perspective: the ucurrent is like 5 or 10 bucks more expensive on adafruit (which only makes sense of course, as they get their share as well). Then the cheapest shipping to Europe (with no tracking or anything) is $12.63 (next one is $41.20, and is still only USPS). Then of course you're "hitting the limit" and you need to pay VAT on it (=whole value, including shipping), let's say 20% on average (that is if there are no extra fees for electronics). So we're looking at something like $30-$40 extra, assuming no serious bank charges (or overinflated exchange rates), no customs (only VAT), cheap shipping and considering you can do the customs within walking distance (which usually isn't the case of course). Your fifyish ucurrent (or USB power supply or whatever) might easily get intro triple digits when we draw the line. Anyone still willing to cut serious corners for 2$?

[McMonster]

I'd love to hear what this guy would have to say about this display idea

I miss this kind of rant videos. Also the video about the topic mentioned around 9:25 is way overdue.

[FenderBender]

How about concentric circles (or semicircles) of LEDs? Like the ones you see on light up rotary encoders? I would find this more intutitive than reading a bar graph. I made something quickly in powerpoint. It would be something like reading an analog panel meter...."I really kind of like it"

[PA0PBZ]

[Bored@Work]

It would be something like reading an analog panel meter....

But an analog meter with five needles, brrrr

[free_electron]

Use a thinner board, and make the red solder mask on the topside have clear sections where the led displays are going to be. Place a white screenprint around the areas where the digits are going to show, and a grid of white to delimit the digits. Instead of placing the leds on the front, place them on the back so they show through the clear pcb material, using the material as a diffuser.

There's even special led's for this kind of stuff.
go to digikey and serch for TOPLED BOTTOM ENTRY. those ae made for pcb backside mounting and shine through the PCB. ( with or without holes.
if you use a homogenised compound pcb ( and not FR4 ) you can shine straight through them. these led's are often used in instrument clusters for cars or test equipment. if you make a 32 mils thick board this stuff sill shine perfectly through.
you can use blue soldermask to act as a filter. just make openings in the soldermask in the shape of the segments and off you go ...

[G7PSK]

My personal preference would be an LCD and pay a few pounds more simply for the ease of reading without having to think to hard as when I am concentrating on some project or problem I don't want or need extra effort in deciphering the various readings that I am looking at. Choice of equipment is not down to the cleverness of design and cost cutting but how well it serves my needs and ease and accuracy of use.