cuuddii
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Raspberry Pi and LCD

Tue Jul 14, 2015 10:00 pm

Hello,

I am currently doing a project which requires a Raspberry Pi and 7" LCD display to be powered with a battery pack. It's all going into a picture frame that will be for alzheimer's patients so size is a bit of an issue. Here is a link to the LCD I plan on using:
http://www.sainsmart.com/7-inch-tft-lcd ... a-2av.html

I plan to power the Raspberry Pi with a 5V 2.1A battery pack, and then I can power an RFID reader and speaker directly from the Raspberry Pi. But, I read the Raspberry Pi can only supply 100mA and the LCD display requires a 5V 2A. Does anyone know of a small battery pack that would be capable of supplying both a LCD display and a Raspberry Pi? Is there a clever way I can go about this using a single 5V 2.1A battery pack? Thank you for your time.

klricks
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Re: Raspberry Pi and LCD

Wed Jul 15, 2015 12:23 am

cuuddii wrote:Hello,

I am currently doing a project which requires a Raspberry Pi and 7" LCD display to be powered with a battery pack. It's all going into a picture frame that will be for alzheimer's patients so size is a bit of an issue. Here is a link to the LCD I plan on using:
http://www.sainsmart.com/7-inch-tft-lcd ... a-2av.html

I plan to power the Raspberry Pi with a 5V 2.1A battery pack, and then I can power an RFID reader and speaker directly from the Raspberry Pi. But, I read the Raspberry Pi can only supply 100mA and the LCD display requires a 5V 2A. Does anyone know of a small battery pack that would be capable of supplying both a LCD display and a Raspberry Pi? Is there a clever way I can go about this using a single 5V 2.1A battery pack? Thank you for your time.
I am not sure were your 100mA figure came from but it is not true.....
The RPi (B+ and 2B) can provide up to 1.2A to the USB ports. The RRi + whatever is connected to the USB and/or GPIO can not be allowed to exceed 2A. So you will need either 2 power sources or a single source using some kind of 'Y' cable arrangement.
Are you sure the screen can operate on 5V? Most screens like that are made for 12V.
There are many battery packs with dual 5V outputs and some with 12V and 5V.
Before you can decide on a battery, you need to decide how many minutes of run-time you need.... Then add up, or better measure, the actual current needed.
With that information you can choose a battery with enough mAH to get the run-time you want.
Note that the RPi requires amplified (powered) speakers. A USB powered speaker will work as long as it has a separate audio connection to the 3.5mm audio out from the RPi (not USB only).
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

ame
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Re: Raspberry Pi and LCD

Wed Jul 15, 2015 12:36 am

Are you sure the screen consumes 2A at 5V? I couldn't see that specification on the product webpage.

Since you have a power supply (PSU) with only one output you'll need to split it somehow to power the two devices (Pi and display).

One suggestion is to power the Pi from the PSU and then power the display from the 5V output pins on the GPIO header.

Another suggestion is to power the display from the PSU and tap into the 5V line on the display somewhere and route a wire to the Pi's power input.

Your project sounds interesting. I suggest you get it working on the bench first then figure out how you're going to put it into the enclosure. It's certainly possible.

By the way, what is the capacity of the battery pack?

cuuddii
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Re: Raspberry Pi and LCD

Wed Jul 15, 2015 1:08 am

Are you sure the screen consumes 2A at 5V? I couldn't see that specification on the product webpage.

Since you have a power supply (PSU) with only one output you'll need to split it somehow to power the two devices (Pi and display).

One suggestion is to power the Pi from the PSU and then power the display from the 5V output pins on the GPIO header.

Another suggestion is to power the display from the PSU and tap into the 5V line on the display somewhere and route a wire to the Pi's power input.

Your project sounds interesting. I suggest you get it working on the bench first then figure out how you're going to put it into the enclosure. It's certainly possible.

By the way, what is the capacity of the battery pack?
I was looking at the specs on amazon, link below:
http://www.amazon.com/SainSmart-Display ... MI+VGA+2AV

But after looking a bit more it does say "Please connected ACC to 12V." So it seems that I will need to use a 12 V supply for the LCD and a simple voltage divider will get me the 5V I need for the Raspberry Pi. I just have to make sure that the battery pack can handle the required current output, 2A for the LCD and 2A for the Raspberry Pi. Although neither of them are going to be using the full 2A, so I might be able to get like a 12V 3A supply or something.

I'm not sure what the capacity of the battery pack will be because I haven't decided which to get. I was hoping someone on the forums could help me out with that. This is one of my first major projects, I'm still trying to figure alot of stuff out.

cuuddii
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Re: Raspberry Pi and LCD

Wed Jul 15, 2015 1:26 am

I am not sure were your 100mA figure came from but it is not true.....
The RPi (B+ and 2B) can provide up to 1.2A to the USB ports. The RRi + whatever is connected to the USB and/or GPIO can not be allowed to exceed 2A. So you will need either 2 power sources or a single source using some kind of 'Y' cable arrangement.
Are you sure the screen can operate on 5V? Most screens like that are made for 12V.
There are many battery packs with dual 5V outputs and some with 12V and 5V.
Before you can decide on a battery, you need to decide how many minutes of run-time you need.... Then add up, or better measure, the actual current needed.
With that information you can choose a battery with enough mAH to get the run-time you want.
Note that the RPi requires amplified (powered) speakers. A USB powered speaker will work as long as it has a separate audio connection to the 3.5mm audio out from the RPi (not USB only).
Is it 100mA from the GPIO pins? I just searched the raspberry pi voltage/current output on google, it may have been the incorrect information. But it's good to know it can supply 1.2A from the USB ports. Thank you for including that link, I'm going to check it out now.

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bucker
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Re: Raspberry Pi and LCD

Wed Jul 15, 2015 1:38 am

you could use a mobile phone power bank charger also.
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ame
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Re: Raspberry Pi and LCD

Wed Jul 15, 2015 1:51 am

The 2A supply current seems rather high. Also, the voltage input range (according to Amazon) is 5-12V, which means it will run on 5V.

The ACC pin is used as a signal, not a supply. It allows the module to detect that the vehicle ignition is turned on. Probably the unit has a power-saving mode where it consumes little power if the ignition is off (because the engine is not running, so the battery is not charging).

Since the screen will run on 12V there is probably a 5V regulator in there somewhere. If you are lucky, you can probably power the Pi from that too.

Finally, does this device have to be portable? If it's going into a picture frame won't it hang on the wall? Can't you run a USB power cable to it from a nearby socket? If you want to run it on a battery pack, how long do you want it to run between charges? This will help to calculate the capacity you will need.

cuuddii
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Re: Raspberry Pi and LCD

Wed Jul 15, 2015 2:21 am

The 2A supply current seems rather high. Also, the voltage input range (according to Amazon) is 5-12V, which means it will run on 5V.

The ACC pin is used as a signal, not a supply. It allows the module to detect that the vehicle ignition is turned on. Probably the unit has a power-saving mode where it consumes little power if the ignition is off (because the engine is not running, so the battery is not charging).

Since the screen will run on 12V there is probably a 5V regulator in there somewhere. If you are lucky, you can probably power the Pi from that too.

Finally, does this device have to be portable? If it's going into a picture frame won't it hang on the wall? Can't you run a USB power cable to it from a nearby socket? If you want to run it on a battery pack, how long do you want it to run between charges? This will help to calculate the capacity you will need.
It's going to be a picture frame that sits on the table, it won't hang on the wall. I'm collaborating with a design student for this, I'm trying to figure out the technical aspects.

Basically I'm just going to put RFID tags onto pictures, then when the user puts a picture into the frame it will read the RFID tag and display text on the LCD, which is on the back, based on the picture. Reminiscent of when people write things on the back of photographs.

This way family members can write things about the pictures and it will display it on the back to help the Alzheimer's patient with memory, it will also play a song (something meaningful to the patient) based on the picture. It will only need to store 10-15 sets of data from the picture frame and play 2-3 songs, so I think the Raspberry Pi will be able to store all the information no problem.

I don't think it will need to run too long, also just displaying text and playing a song shouldn't take too much power. I think just like 1-3 hour run time should be sufficient between charges.

cuuddii
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Re: Raspberry Pi and LCD

Wed Jul 15, 2015 2:37 am

bucker wrote:you could use a mobile phone power bank charger also.
Not a bad idea, I was looking into this onehttp://www.amazon.com/RAVPower-Portable ... ne+charger

It seems like it would be perfect for what I'm doing, the only thing is both of the USB ports are on the same side. Not quite sure how I would power the devices internally and have one on the outside of the frame for charging, but I'm sure that's something I could figure out, or I could just look at a different charger. Thanks for the idea.

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mikronauts
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Re: Raspberry Pi and LCD

Wed Jul 15, 2015 3:03 am

1) You CANNOT "just use a voltage divider" to get 5V from 12V! You need a switching regulator, or UBEC.

2) to run 3-4 hours:

- Pi will need USB battery pack of >5000mAh, based on my experience
- 12V 1A display would need 12V battery with 4-6AHr capacity
cuuddii wrote: I was looking at the specs on amazon, link below:
http://www.amazon.com/SainSmart-Display ... MI+VGA+2AV

But after looking a bit more it does say "Please connected ACC to 12V." So it seems that I will need to use a 12 V supply for the LCD and a simple voltage divider will get me the 5V I need for the Raspberry Pi. I just have to make sure that the battery pack can handle the required current output, 2A for the LCD and 2A for the Raspberry Pi. Although neither of them are going to be using the full 2A, so I might be able to get like a 12V 3A supply or something.

I'm not sure what the capacity of the battery pack will be because I haven't decided which to get. I was hoping someone on the forums could help me out with that. This is one of my first major projects, I'm still trying to figure alot of stuff out.
http://Mikronauts.com - home of EZasPi, RoboPi, Pi Rtc Dio and Pi Jumper @Mikronauts on Twitter
Advanced Robotics, I/O expansion and prototyping boards for the Raspberry Pi

cuuddii
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Re: Raspberry Pi and LCD

Wed Jul 15, 2015 5:47 pm

mikronauts wrote:1) You CANNOT "just use a voltage divider" to get 5V from 12V! You need a switching regulator, or UBEC.

2) to run 3-4 hours:

- Pi will need USB battery pack of >5000mAh, based on my experience
- 12V 1A display would need 12V battery with 4-6AHr capacity
Originally I was going to just use a voltage divider but implement a zener diode as well which would be similar to a regulator. But I think because I'm using a 1A display I'll have to use a switching regulator, which is no big deal because they're quite cheap. Thanks for the reply.

PiGraham
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Re: Raspberry Pi and LCD

Wed Jul 15, 2015 6:18 pm

1. I have one of those Sainsmart 7" LCDs and it does work OK on 5V.

2. I have to ask, what's wrong with writing on the back of the photo? OK, that won't play music.

3.RasPi is overkill for what you describe. You could get MUCH better battery life if you use a low power microcontroller (Pro Trinket, Pro Nano, Gemma or similar) with an eInk display. Not as large as you may want:
http://www.dx.com/p/waveshare-4-3-e-ink ... aahXfmm3ao

RFID and SPI drive to display..

Take a look at electronic shelf labels. A growth market for e-paper labels that could be close to your functionality.

You might also consider putting a QR code on the photo and decoding that with a camera. A super cheap tablet could do that, which gives you an all-in one solution.

cuuddii
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Re: Raspberry Pi and LCD

Wed Jul 15, 2015 9:02 pm

PiGraham wrote:1. I have one of those Sainsmart 7" LCDs and it does work OK on 5V.

2. I have to ask, what's wrong with writing on the back of the photo? OK, that won't play music.

3.RasPi is overkill for what you describe. You could get MUCH better battery life if you use a low power microcontroller (Pro Trinket, Pro Nano, Gemma or similar) with an eInk display. Not as large as you may want:
http://www.dx.com/p/waveshare-4-3-e-ink ... aahXfmm3ao

RFID and SPI drive to display..

Take a look at electronic shelf labels. A growth market for e-paper labels that could be close to your functionality.

You might also consider putting a QR code on the photo and decoding that with a camera. A super cheap tablet could do that, which gives you an all-in one solution.
Using a tablet is actually a very good idea, I can just make an app to do exactly what the LCD display was going to do. Also the designer wants it to read the code automatically. Would there be a way to set the tablet to read the QR code automatically? Or does anyone know of a way I can connect an RFID module to a tablet, I did some research but I couldn't really find anything that fits my needs.

cuuddii
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Re: Raspberry Pi and LCD

Fri Jul 17, 2015 2:13 pm

I would just like to note that I have decided to use an android tablet and implement NFC (near field communication) in order to achieve my desired results. Thank you to everyone who took the time to reply to my question.

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