So far in this series we’ve covered an overview of setting up an in-car PC
(Setting Up an In-Car PC, Part 1)
and getting the audio from the PC to your normal car sound system
(Setting Up an In-Car PC, Part 2).
This week we’re going to concentrate on the visuals – selecting, wiring-in and
mounting an LCD screen in the cabin. As the main interface with the PC, the LCD
is important – you want it to be bright, sharp and look good.
Selecting
To give the greatest universality of PC selection, it’s best to use a screen
that takes a direct VGA input. That is, the screen can be plugged into any PC
video card and will fire up straight away without anything further needing to be
done.
The LCD that we bought also has another important function – a USB touch
screen. This allows the PC to be controlled by touching the icons on the screen,
removing the need for a mouse or other pointing device. In this first in-car PC
iteration we’re not using this function but having a screen with this capability
opens up a lot of functionality, so you keep this feature in mind when selecting
the display.
The screen that we bought was the 7-inch Lilliput XGA TFT touchscreen. The
design uses a Sharp screen, has two RCA video inputs (in addition to its VGA
input), has built-in speakers and comes with a remote control. It was purchased
through eBay from Extreme Audio Electronics of Hong Kong. The transaction was
sweet – payment through Western Union Auction payments and very fast freight.
Including freight, the total cost was AUD$408 – no GST or import duties were
charged.
Mounting
The location of the LCD is important.
- You probably want it visually accessible to as many people in the cabin as
possible. This includes the driver and front seat passenger as well as
potentially the rear passengers. However, the driver must also be able to look
at it without diverting his/her eyes too far from the road.
- You don’t want the background to the display to be bright – for example, a
front roof-mounted display sounds like it would fulfil most of the previous
criteria but on a sunny day, the screen will be hard to read because of the
bright background.
- The LCD must be located so that it doesn’t reflect the light coming in
through the windows, especially the rear glass.
- The LCD should be shaded as much as possible from direct light.
The reality is that you should hold the screen in lots of different locations
around the cabin before making up your mind.
In this car – a Toyota Prius – one of the options was to place the screen
directly behind the steering wheel. Like some other Toyotas, the Prius has its
instruments located in the centre of the dash, just below windscreen level. This
leaves the space behind the steering wheel free for the LCD screen – and the
driver can then see it beautifully. However, this location prevents anyone else
in the car being able to see the screen...
So the final spot that was chosen was in the middle of the dash, mounted
below the heater/ventilation controls and with the screen angled upwards. The
initial angle that was tried proved to reflect light from the rear window
straight towards front seat viewers, so the screen was angled a little more so
that it was reflecting light from the centre of the underside of the roof. (In
cars with sunroofs, you should always try different locations with the sunroof
fully open!)
The Lilliput screen comes with an adjustable angle mount equipped with a
large pad of double-sided sticky foam. But in this application (and in pretty
well all car uses that we can think of) there won’t be any big, flat surfaces
that this foam can get a good grip on. Initially, the sticky foam pad was
unscrewed from the adjustable-angle part of the mount and a new support made.
However, the plastic adjusting screws on the provided bracket proved to be not
up to the task and so in the end a completely new mount was made from aluminium.
This mount is screwed to a small existing shelf, with the screws hidden
inside the dash. This mount causes the top of the screen to push back against
the dash – it’s cushioned with a small piece of foam rubber. In this way, screen
vibration is kept to a minimum.
LCD Power Supply
The LCD comes with its own power supply. This is terminated in a cigarette
light plug which needs to be cut off to allow the cord to be hard-wired into
place. Make sure that you work out the polarity of the cord (the tip of the cig
lighter plug is positive) before cutting the plug off. The connections should be
made to an ignition-switched 12V supply and earth.
However, as briefly covered last week, we found that if the LCD power supply
was placed close to the FM modulator or line-level signal leads, it caused a lot
of noise in the audio system. Furthermore, the power supply (an electrically
noisy switch-mode design) proved to be also injecting noise back into the car
wiring... noise that was then picked up by the FM modulator. This was easily
demonstrated by switching off the LCD – some of the background noise in the
audio then went away.
The technical advice of Silicon Chip magazine’s John Clarke was sought
and it was his suggestion that rather than noise suppression techniques be
applied to the existing power supply, a whole LCD new power supply be built.
This step isn’t as daunting as it sounds – it takes about 15 minutes and less
than AUD$10 to build a new power supply.
While the LCD is happy working on 11-13V, the problem in a car use is that
there can be all sorts of nasty voltage spikes surging around in car wiring.
(That’s presumably why Lilliput provide the extra power supply.) So the primary
requirement of a new power supply isn’t to precisely regulate voltage but to
make sure that an over-voltage situation cannot occur.
To build the simple power supply you’ll need:
- 1x 16V zener diode – Jaycar cat no ZR-1416
- 1x 5W 1 ohm resistor – cat no RR-3220
- 1x 10uF 25V electrolytic capacitor – cat no RE-6070
- 1x 100uF 25V electrolytic capacitor – cat no RE-6140
- 1x 12V 7812 voltage regulator – cat no ZV-1512
- Small amount of punched board – cat no HP-9562
- UB5 box – cat no HB-6015
You’ll also need a small heatsink or piece of scrap aluminium.
This is the circuit. The resistors prevents large current flows, the zener
diode acts as a safety valve bypassing any voltages that exceed 16V, and the
voltage regulator drops the voltage to a max of 12V. (The capacitors stabilise
the action of the regulator.)
The circuit can be physically laid out on the punched board exactly as shown
here, making it easy to construct. Note that the voltage regulator, capacitors
and zener diode are all polarised – have a close look at this pic of the
built power supply (click on the image to enlarge) before starting construction.
Here’s the underneath view of the completed power supply. Easy, huh?
Both the resistor and the voltage regulator will get warm – that’s why a
heatsink is needed on the voltage reg. The one here was salvaged from old
consumer equipment and then cut down to suit. Once constructed and mounted in a
box, the power supply can be tucked from sight under the dash.
The LCD Cable
Unless you find a spot to mount the PC that’s within the cabin of the car,
it’s a certainty that you’ll need to run an extension cable for the screen. VGA
extensions are commonly available in 1.8-metre lengths but you’ll usually need
something more like 4-5 metres long. This is stretching the capability of the
cable – so you’ll need to pick a high quality design with good shielding and
ferrite rings at each end. We used the 5-metre Jaycar WC-7588 XVGA Monitor
Extension Cable, which cost AUD$19.95. A female/female adaptor was also needed
(cat no PA-0876 at AUD$5.80).
Unfortunately - for reasons that we’re not sure of – when we got it home, the
5 metre cable proved to actually be 10 metres long! This length introduced some
noise into the signal – when using this ultra-long cable, the LCD display showed
a slight double image. Since only about 4 metres was needed, the long cable was
cut and shortened. This is a fairly tricky operation requiring rejoining
internal braids as well as the 15 wires – see the breakout box ‘Joining Cables’
for more.
It’s almost certain the screen image won’t be as good with the extension
cable in place as without it. That’s just the way it is – but the effects can be
minimised if you pick the default screen displays and colours carefully – more
on this later.
When doing the cable runs, make sure that there’s sufficient room under the
trim to hide the connectors. As with the audio cables, tape the connectors up to
prevent inadvertent grounding and make them more secure.
The Results
So how well does the screen work on the road? The main problem is that it
cannot be easily seen if it’s in direct sunlight. Positioned low on the dash,
the LCD is mostly shielded from direct light, but at times it can still be
flared out. A solution might be to place a short shield around the sides and
upper surface – even a shield only 10mm long would remove the problem most of
the time. Other than this issue, the screen works very well.
Next week: A custom suspension mount for the PC, and finishing off the
system.
Joining Cables
Cables for both audio and video often are of an electrically shielded design.
This places a braid or aluminium foil around a central conductor that’s
insulated from the shield. As the name suggests, a shielded cable protects the
signal from interference.
If you are shortening a shielded cable, it’s important that you maintain the
shield. The easiest way of doing this is to join the conductors conventionally
(eg bind them together and then solder the connections), insulate the centre
conductor join with tape, and then wrap aluminium foil around the braid across
the width of the join. That way, the shield is continued.
If the cable that you are working has multiple shielded internals, you need
to do each one individually.
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