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Dilemma:

I have a project in which I am trying to use a Raspberry Pi Zero to display static content through a laptop screen. I am trying to make my unit as thin as possible and use as little power as possible. Previous searches have found solutions that would work, but are less than ideal for what I am trying to do. I would also like to mention that I am no electrical engineer and I struggle at reading datasheets and making sure I won't blow anything up, which is why I need your help.

An example of a previous solution: Some solutions mention using an LCD controller from eBay that had input and output in the form of HDMI, VGA, DVI and LVDS. However, these boards require something like 9V minimum, where I am trying to limit my build to a small 5V DC power supply. They are also massive in size and more importantly, height, which is also something I am trying to avoid.

Goal:

My goal is to be able to power the Pi from a 5V 2.4A DC wall adapter, and power the screen through the micro USB port (micro USB > USB A Hub > micro USB cable) so that only one small external power cable is required. Eventually I would like to see if battery power is an option, but as far as this question goes, I am concerned only about the 5V power supply.

My idea is to use an HDMI cable out from the Pi that goes to a 40-pin TTL breakout board that is typically used to power small TTL TFT displays and takes a micro USB in for providing power. From that board, I would take the TTL and run it through an LVDS converter board to try and output something that can communicate to the screen that I have available from an old laptop.

The screen datasheet can be found at: http://www.datasheetspdf.com/datasheet/B156XW02-V0.html
I ran out of links, sorry I'm new...

And the laptop screen model is: B156XW02 V.0
This is an LED backlit 15.6 inch screen that uses a single 40-pin LVDS connector to the motherboard.

Some Info/My Attempt:

I figure that from the datasheet about the screen, the LCD has a max Voltage of 3.6, and a max current of 400 mA (0.4 A) and the backlight has a max Voltage of 3.4 and typical forward current (no max listed) of 20 mA (Seems really low?). It also lists the LED power consumption at 4.4 Watts max, which leads me to believe if the Volts are at 3.4 max, the current would be 1.29 amps max?

It seems to me that the max Volts required is 3.6 and max current required is .4 A or 1.29 A (depending on what is correct)? Since the Pi can output 5V and .5 A from the micro USB out (after the hub), or 5V and 2A from a special PiHat, I would say this is feasible.

My question to you fine folks is: Will my idea work? If not, where does this breakdown, and can I add anything to make this work?

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    \$\begingroup\$ Are you going through all this hassle just because of the 9V requirement? Why not use a small boost converter? Or use a 9V PSU and a buck converter for the RPi? \$\endgroup\$ – Wesley Lee Dec 27 '16 at 0:46
  • \$\begingroup\$ You could also consider a lower power display. If black-and-white is sufficient, an ePaper display might work. \$\endgroup\$ – Mitch Dec 27 '16 at 4:30
  • \$\begingroup\$ the pi0 already has an LVDS display connector, \$\endgroup\$ – Jasen Dec 27 '16 at 8:25
  • \$\begingroup\$ @WesleyLee I am trying to keep the build as small as possible on the power requirements, as I may eventually make this battery powered. I feel like 9V would not help me in that regard. \$\endgroup\$ – tbauer516 Dec 30 '16 at 7:58
  • \$\begingroup\$ @Mitch I like the idea of an ePaper display, and I might use your suggestion. I am trying to reuse a display that I currently own. \$\endgroup\$ – tbauer516 Dec 30 '16 at 8:00
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For a start - the only useful figure you can get from the screen datasheet so far is its power consumption, which is a nice figure you can use to know what to expect in terms of battery life etc. Other than that, nothing of interest there unless you plan to interface with I2C of the panel (which you likely won't).


I vote for the "get a HDMI-to-LVDS board" way. Those boards do the task you want without TTL in between (possibly, having TTL-to-LVDS on the board, but any possible problems with connecting two modules together are already sorted out). I encourage you to do just that - they're cheap enough. What's with the 9V then?

Those boards mainly use 5V and lower inside, having a regulator of some sorts to produce the 5V/lower for the display logic. You can remove that regulator, power the 5V parts from the Pi and, if anything on the board actually uses 9V (say, backlight driver), you can then use a boost converter, which won't consume much energy since the bulk of current should be taken by the 5V line you'll connect directly to the Pi without conversion losses.

As for the height concerns - you can desolder unnecessary connectors, remove some capacitors and replace some with the ones laying flat. Inductors are kinda harder to minify but they're likely to belong to the power regulator parts and if you're going the "remove regulator" route, you can remove them. The connectors can be replaced with angled ones (which lowers height) or bypassed altogether by soldering to the board (think power-menu-button board which typically comes with those HDMI-to-LVDS boards and that you won't need to remove).

For the size concern - you can find a board without the audio amplifier (which is often populated on those boards), or maybe HDMI-only. In some circumstances, boards can even be cut (provided you can see there are no traces that are important for you - or you can jumper those with small wires).


Oh, and the link you provided to "TTL-to-LVDS converter" is not really a converter - it's a PCB with two connectors soldered together, there are no active components to make the conversion. The actual converter is somewhere lower in "Suggested items", I guess.

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  • \$\begingroup\$ Or a mini hdmi to vga adapter, and a vga to LVDS converter. \$\endgroup\$ – Passerby Dec 27 '16 at 6:11
  • \$\begingroup\$ @Passerby Yes, a good idea. Cheaper and with a lower footprint (those adapters are quite small). \$\endgroup\$ – Арсений Пичугин Dec 27 '16 at 8:55

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