Taking the Raspberry Pi over hill and dale

The Raspberry Pi requires three components for operation as a standalone and independent computer. These include a keyboard, a mouse, and a display screen.

With respect to power, the Raspberry Pi does just fine with 5V of supply voltage. Thanks to the regulatory bodies that have approved universal standards for USB power supplies, there are ample accessories available for powering mobile phones and other small devices. This means that Raspberry Pi fans have a large selection from which to choose. Power packs that supply mobile phones via an external battery are also suitable for the Raspberry Pi. In order to guarantee an adequate reserve, you should choose a pack that can handle at least 2A of current.

Remote desktop programs can provide I/O devices for the Raspberry Pi if there is an Internet connection available. The current Raspbian image comes with VNC Connect, which includes VNC Server for controlling a computer's desktop from a computer or mobile device running VNC Viewer. If needed, you can find 10-finger mobile keyboards that fold up to a fraction of their full size for easier typing from a mobile device.

Let There Be an Image

The optimal solution for using a display for a Raspberry Pi happens to exclude mobile operation. Monitors that have an HDMI input are usually too heavy for mobile use, and they consume too much power. Touchscreens are appealing because they replace the mouse. Even so, they often require their own interface, which can mean extra work on driver installation. Luckily, there are other small analog displays in the marketplace that cost just a few dollars. These displays are often found in cars for showing backup camera images. The trick to using these economical devices with a Raspberry Pi involves transmitting the composite video signal to the display screen and securing a reliable source of power.

The BAS/composite signal is found on the fourth pole of the jack socket on newer Raspberry Pi models. An adapter plug (Figure 1) splits the four contacts for the jack plug into two audio cinch connectors (red and white) and also a video cinch connector (yellow). The ground contact deserves special attention since there are two different standards: OMTP and CTIA. Viewed from the tip, the ground contact for the OMTP lies on the last ring. But with the Raspberry Pi, this is the second to last ring, making CTIA the correct choice here. Practically speaking, therefore, you will need to know the pinout when purchasing an adapter cable.

Figure 1: The pinout for a CTIA jack plug.

Since the cheap displays are typically used by the automobile industry, they are powered by a voltage between 9V and 12V. Even so, the screens can be retrofitted so they perform with the 5V that the Raspberry Pi uses for its working voltage. This means you will not need to include any items for additional supply voltage in your project plans. Once retrofitting is complete, you can either continue to operate the monitor with 12V, or you can choose to use 5V provided by a USB power device.

The two outer contacts of a USB plug provides the power supply. Sometimes the (black) grounding line is difficult to see. Figure 2 illustrates the polarity. Figure 3 shows an analog monitor's drive that has been opened up. The power supply line leads to the ground contact. Figure 3 shows the contact with the blue and black cable. You will have to search a little bit in order to find the secondary side of the 5V voltage converter.

Figure 2: The location of the voltage connector on a USB plug.

Figure 3: The electronic drive for an analog monitor. The blue and black cables lead to the ground contact.

Switched-mode power components require a large electrolytic capacitor on the secondary side. The price of a capacitor depends on how voltage-proof it is. Naturally, manufacturers select the most economical components. Only one of the three capacitors on the drive has been constructed for lower voltage. This capacitor is marked 220[µF]/10V/VT. The black side indicates the negative pole. Usually the capacitor contacts are continuous to the underside. This is where the 5V lead for the USB power cable can be soldered. If in doubt, you should use a multimeter to measure the voltage.

Modifying Software

If you leave the configuration file as is, the Raspberry Pi will check at startup whether it recognizes an analog device. If the test is negative, it will connect the HDMI output. Even resetting after boot results in a dark analog screen. The same goes for most monitors. They do not turn on if they don't recognize a signal. This holds true independently of the power voltage that is connected.

The composite monitor's limited resolution requires you to modify the level of detail. To this end, the Raspberry Pi offers two possibilities for making adjustments. The first of these is found in the /boot/config.txt file. You should use root rights in a terminal window to edit the file via:

sudo nano /boot/config.txt

The 640x480 pixel resolution in the example below:

framebuffer_width=640
framebuffer_height=480

can be set with the corresponding variables. Afterwards, you should save the file with Ctrl+O, Enter, and Ctrl+X. Finally, close the editor before rebooting the Raspberry Pi.

On the composite display, you can now call the terminal using the graphical interface. However, not much is recognizable on the display. If you reduce the resolution, for example, to 320x240 pixels, then it is much easier to read the text in the terminal window. The downside here is that this comes at the expense of the icons in the graphical interface.

Therefore, the second setting concerns the /etc/default/console-setup. Like the config.txt file, this gets opened with root rights in the editor. Here is where you insert the following:

FONTFACE="Terminus"
FONTSIZE="16x32"

After restarting, you should use the keyboard combination Ctrl+Alt+F2 to get to a virtual terminal where text can be read easily but naturally where there are just a few letters per line. Figure 4 shows the welcome screen on the test monitor. I have plugged in a measuring device between the power supply and the display. The device indicates the current for the power pack as ~5V, and the power consumption as 180mA.

Figure 4: The Raspberry Pi with the little analog display screen. The measuring device indicates the attached power supply and the consumption at 5V and 180mA respectively.