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

[ruku]

My vote's for the 8x2 if you've got to do it... but I really like the idea of the LEDs.

Why not do this? Change the display to a bar graph. (We'll say only one LED on at a time.) The default mode is to show current consumption. How often do you need to change the voltage on your power supply? How often do you wonder what the voltage actually is? Most times, after all, I fix it at a constant value. You regain "easy" functionality, and still keep cost and power usage low.

Put a beeper or a latched LED on the system to let the user know when there's an under/over voltage condition. I think that'd be a winner right there!

Also... this still lets you run variable voltage supplies. You just have to hold a button to set the voltage, or something.

Perhaps a decade style system... Four LEDs reading 1/10/100/1000, and then a 0 to 100% graph.

[dcel]

I think it's coming down to:

standard 8x2 text display:


The 8x2 text display is about half the cost of the Nokia one.

Snap poll please!

Dave.

Funny, I have that exact LCD sitting on my keyboard, reflective no backlight.
Now if I can just figure out how to use it.

Its got my vote!

Chris

[Bored@Work]

Funny, I have that exact LCD sitting on my keyboard, reflective no backlight.
Now if I can just figure out how to use it.

Likely http://en.wikipedia.org/wiki/Hitachi_HD44780_LCD_controller compatible.

For every self-respecting microcontroller there are typically multiple driver implementations out there.

[kolbep]

For the display, how about going retro, and using a Moving Coil Meter. U could have both the V and I on the scale. The Micro can then use PWM to generate the correct voltage needed to move the meter.

P

[rr100]

Not enough resolution especially with such a small scale, quite nasty to calibrate and set up, not able to display both values at the same time, possibly fragile and expensive.

[PedroDiogo]

I vote for the 8x2 display. You could display the voltage and current at the same time and you have space for a simple bar graph with the remaining segment like so:

V:05.00#
I:0.250#

Where # could be a custom character of a bar chart.

[RJSC]

It's a damn shame that the reflective or transflective character LCD's are much more expensive than those crappy white on blue backlight one can find for an euro on ebay.

[dcel]

Funny, I have that exact LCD sitting on my keyboard, reflective no backlight.
Now if I can just figure out how to use it.

Likely http://en.wikipedia.org/wiki/Hitachi_HD44780_LCD_controller compatible.

For every self-respecting microcontroller there are typically multiple driver implementations out there.

Thank you sir! I did manage to find a datasheet in chinese that has the pin-outs on it.
And I have been to New Haven and d/l'd that datasheet. I didn't think of Wiki.

Thanks
Chris

[Zad]

If I were making it purely as a standalone commercial product, I would go for the 2x8 display. Nokia displays are great for hacking stuff together in 1-offs, but in commercial products, unless they have a separate bezel/mask they look rather naff and something a student would make. The Nokia display also requires a separate socket, whereas the 2x8 uses standard 0.1in header pins.

However, in this instance it isn't quite so simple. It is being aimed at the hacker and Open Hardware market where using a Nokia screen might actually be a selling point, especially if you want to get the design published in a magazine (great advertising).

Do I remember correctly that the local Maker group Dave visited has a laser cutter? I'm sure for a modest fee they would allow you to cut up a load of acrylic sheet. That would make custom marking available at no extra financial cost, just time.

[NiHaoMike]

As for the 500mA vs. 1A/2A part, what about design it to allow extra DC/DC converters to be added? Maybe have a "standard" version that has only one DC/DC converter and a "deluxe" version (or provision for a user upgrade) that has a second DC/DC converter and an isolated FTDI interface.

I really like the idea for the LED display. You could potentially further cut the cost by making it BCD, but then the analog engineers would hate it...

[EEVblog]

As for the 500mA vs. 1A/2A part, what about design it to allow extra DC/DC converters to be added? Maybe have a "standard" version that has only one DC/DC converter and a "deluxe" version (or provision for a user upgrade) that has a second DC/DC converter and an isolated FTDI interface.

It's not that easy. The DC-DC converter changes, the regulator changes, and their associated thermals change. Bigger and more expensive in both cases.

Dave.

[pickle9000]

Hey Dave,

So if I remember correctly you wanted to use a single sided board, tactile switches that extend through the front case.

If you used the 2x8 module then you will already have a sturdy base to work from. If you cut a hole in the pcb big enough for the lcd holder (the aluminum thingy) , slip it in from the back side then that would be pretty clean and keep the entire assy nice and thin. You would need to use a bit of adhesive to ensure that it would be solid. The adhesive would be out of view on the back side of the board, easy to assemble.

I don't know about your component count but I wanna see you spell "BLOG" with some of the components. It would be very novel, hide components and basically I would get a kick out of it. It may also cause premature baldness.

...mike

[EEVblog]

If you used the 2x8 module then you will already have a sturdy base to work from. If you cut a hole in the pcb big enough for the lcd holder (the aluminum thingy) , slip it in from the back side then that would be pretty clean and keep the entire assy nice and thin. You would need to use a bit of adhesive to ensure that it would be solid. The adhesive would be out of view on the back side of the board, easy to assemble.

I considered that, but it would take up a huge chunk of the board space, unless you mount some components between the two PCBs and mount the LCD offset. But even then it's a big chunk gone.
And then there is the problem of connecting the 0.1" header. Can't use through hole if you use the PCB as the front panel like the uCurrent. Not enough room for a SM header I suspect, and you could bodge it with pads and solder paste reflow, but that's nasty.
A better solution for this would likely be those compact 16x2 LCD like I used on my uWatch. At least then you have a flat flex ribbon cable that can be mounted some distance away.

Dave.

[Rutger]

My vote would be for the Nokia display as you can splash the EEVBlog logo when you start it up. Also my guess is that the Nokia display is a lower current display, but that is just a guess. I would buy both and compare the current.

Rutger

[MagFlux]

 Using a 16 x 2 instead of the 8 x 2 might give you more space to have added functionality.
You may think adding such things as over current, finer current resolution(microcurrent), or preset
voltage and current settings to name a few as excessive.
If this is the case then the 8 x 2 display would be a better fit for the case size that you're
contemplating over the larger Nokia.

[ggchab]

The 7 segments LED displays might not be more power hungry than the bar graph if not only displays are multiplexed but also the segments per display. That's easy to do with a microcontroller and most probably still bright enough to be easily readable. I think it's easier and faster to read a number than having to interpret a bar graph. But that's only my opinion.

I discovered your site not that long ago and I really like it 

[Short Circuit]

Been said before, No, doesn't work; muxing is optically equivalent to reducing the current.
Want to keep brightness, then you must increase the current.

[ejeffrey]

Been said before, No, doesn't work; muxing is optically equivalent to reducing the current.
Want to keep brightness, then you must increase the current.

This is not true at all.  A muxed display will look considerably brighter than a steady-state display of the same power.  Your eye is a non-linear detector and detects something between peak brightness and average brightness.  The real behavior is quite complicated, and of course there is a limit to how much you can win.  At a certain point you simply don't have enough photons, but muxing/pulsing can get you a long way.  This is even a nice trick for single indicator LEDs in low power electronics.  If you flash an LED at ~200 Hz with a ~10% duty cycle it will look like it is on steady but have much greater apparent brightness than a constantly lit LED with the same average power.

[george graves]

^That's really interesting actually.  Thanks for that low power tip!  I'll have to remember that one.

I wonder what PWM does to led life?

[free_electron]

there's another effect at play as well. as the led heats up its emission decreases.

so , by pulsing the led you get lots of photons but the die doesnt heat up as much. it heats very shortly , then has a relatively long period to cool down. so by the time the next pulse comes along you are cold enough to have a gain lots of photons.

so a pulsed led emits more photons than a constant on one  . there are led controllers that run a fixed current to keep the led at low heat while still having emission and then spike short pulses that do not casue noticable heat increase but pump up the emission of the led.

[Psi]

That's what i said on page 1
This thread has come full circle.

There's only so far you can trick the human eye before it starts to notice the brightness dip 

[HackedFridgeMagnet]

If you flash an LED at ~200 Hz with a ~10% duty cycle it will look like it is on steady but have much greater apparent brightness than a constantly lit LED with the same average power.

Can you guys quantify this, or point me to somewhere where they do quantify this. I wonder if it's worth incorporating the effect into a design.

[mathk]

Donno if the idea have been propose and I don't want to read the entire thread so here is my proposition to improve the  Dave's Decade Digit Display.
Instead of having the figure print on the slick screen you can have a reverse silk screen print on a "transparent pcb" for each led. So each led get his own figure. I don't know if that is doable or not (to have a transparent pcb) but that just an idea.

[MikeK]

I wonder what PWM does to led life?

Probably no effect, as long as you're below the spec'd current.

[tom66]

Dave - how about a display like this? http://www.ebay.co.uk/itm/0-96-128x64-OLED-LCD-LED-Display-Module-white-/221030175657?pt=LH_DefaultDomain_0&hash=item33766cd3a9

Pretty pricey for one - but you see those cheap OLEDs in a lot of products. So they must be available cheaper somewhere else. Then again, I wouldn't mind paying a little extra to have a cool OLED display.