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Recently I have bought the Raspberry Pi 2 along with a small 3.5 touch screen. It works fine and I'm more or less happy with it (there is some choppiness in the rendering of things (even the cursor is moving in a choppy manner) but from what I've read this is due to it not using the HDMI port, which provides GPU acceleration). However now that I also want to add some extra functionality through various sensors I'm facing the issue that comes from the way its pin connector is designed - it covers all pins from 1 to 26:

1, 17                           3.3V            Power positive (3.3V power input)
2, 4                            5V              Power positive (5V power input)
3, 5, 7, 8, 10, 12, 13, 15, 16  NC              NC
6, 9, 14, 20, 25                GND             Ground
11                              TP_IRQ          Touch Panel interrupt, low level while the Touch Panel detects touching
18                              LCD_RS          Instruction/Data Register selection
19                              LCD_SI/TP_SI    SPI data input of LCD/Touch Panel
21                              TP_SO           SPI data output of Touch Panel
22                              RST             Reset
23                              LCD_SCK/TP_SCK  SPI clock of LCD/Touch Panel
24                              LCD_CS          LCD chip selection, low active
26                              TP_CS           Touch Panel chip selection, low active

Since I'm pretty new to this kind of thing (and electronics in general) I'd like to ask you guys for some help. Basically what I want to do is figure out which of those pins are indeed required by the display (and its touch module) so that I can free the rest by simply detaching the display and using jumper wires move it to a breadboard or just leave it hanging somewhere (but still able to actually use it). I'm especially interested whether it really requires 2*3.3V + 2*5V (all in all 16.6V) since I need to have as many free power pins as possible. I was unable to find any datasheet whatsoever hence I really don't know the power consumption of this thing. I have a multimeter at hand but I haven't got the slightest idea where to start (especially since I fear I might damage either the LCD display, or the RPi2, or both). I have read here (see answer by tinylcd on Tue Aug 19, 2014, 5:20AM) that such a display draws just 3.3V from the Pi. If it's indeed so - awesome! Further the answer here explains that for a 7 inch touch display by Sain Smart you require only a single power pin with 5V.

From the description in the table above I can deduce the following:

Note: I'm using this useful interactive diagram to see which pin does what.

  • It requires a GND (hence 1 GND pin is out)
  • 1 or (hope not) more power pins
  • Some of the NC pins (clock? I2C?)
  • Pins 11, 18, 19, 21, 23, 24 and 26
  • Possibly pin 22 (reset)
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The pins potentially used by the LCD module are shown at the left of the following table.

You should definitely connect everything which is not marked as 3.3V, 5V, or GND on a one to one basis. I.e. use a (female-male) jumper wire from the Pi pin to the corresponding LCD socket.

You only need to utilise one 3.3V, one 5V, and one GND pin from the Pi. The voltage supply pins and the ground pins are often referred to as rails, e.g. the ground rail, the 5V power rail, etc. All the ground pins are connected together on the Pi. Similarly all the 5V pins are connected to the other 5V pins, and all the 3V3 pins are connected to the other 3V3 pins. There are more than one of each simply for wiring convenience.

So you can choose one ground pin and connect that to each LCD socket which was connected to ground. How you do that is a wiring problem. Breadboards typically have ground and power rails so you can plug in one ground and power wire but jumper it from anywhere on the rail. Similarly choose one 3V3 pin and one 5V pin. You will probably find that you do not need both the 3V3 and 5V power connected.

By the way voltages do not add up in the way you think. You can have a hundred 5V pins on the expansion header, they all add up to 5V, not 500V, they are connected in parallel.

Pin Used        Spare
 1  3.3V
 2  5V
 3  --          gpio2 SDA
 4  5V
 5  --          gpio3 SCL
 6  GND
 7  --          gpio4
 8  --          gpio14 TXD
 9  GND
10  --          gpio15 RXD
11  TP IRQ
12  --          gpio18
13  --          gpio27
14  GND
15  --          gpio22
16  --          gpio23
17  3.3V
18  LCD RS
19  MOSI
20  GND
21  MISO
22  Reset
23  SCLK
24  LCD CS
25  GND
26  TP CS
  • Thanks! So basically after I connect one 3.3V and one 5V to the breadboard (will use 2 of the long power rails (marked with a +) to supply my breadboard with 2 different voltages) I just have to experiment by removing one of those to see if the display continues to work or not. If I remove the 5V and the display turns off then I know that 3.3V are not enough and hence I have to try with only 5V. Correct? – rbaleksandar Aug 28 '15 at 12:45
  • @rbaleksandar That is correct. There is a chance it will need both voltages but it is not hard to check. – joan Aug 28 '15 at 12:51
  • Roger that. Will check this first thing tomorrow. – rbaleksandar Aug 28 '15 at 13:35
  • I just figure that the NC means not connected (hopefully I'm not mistaken :D). 3,5 (I2C) etc. are definitely pins that are not required. So I really need to concentrate on the power output pins. – rbaleksandar Sep 10 '15 at 18:44
  • @rbaleksandar Both NC and N/C are fairly standard abbreviations for not connected or no connection. – joan Sep 10 '15 at 18:57
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The display appears to use the SPI interface pins plus 3 other GPIO 11,18,22. Combined with the power and ground this could be connected with 11 wires.

I have some reservations about the mechanical stability of attaching with jumper wires, but you could use a breadboard.

Only a single connection is required to the each of the power pins (you appear to have no understanding of parallel connections, these are not additive).

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