Dear forum viewers,
As a newbie to this forum AND to PilotAware I thought I’d share some thoughts with you .
I received my box of bits earlier in the year and have now had several good flights with a fully working system.. I thought I would let you know how I got on.
I’m a (retired) design engineer, I’ve been designing electronic products all my life; my aircraft is a pusher with a fibreglass nosecone, ideal for forward upward and downward radio reception with no horrid engine in the way; so between us I though we should be ideally placed for an optimum installation. Central to my philosophy is that I wanted something that would work first time, I’m not interested in experiments and successive approximations! Here are my thoughts.
1. The enclosure
I’m sure you all agree that the PiAware system is a bit of a meccano kit – too many bits! Scarcely anything has caused me more grief in my career than poor connectors. And here we have a box full of them! So the first thing I wanted to do is put everything in one box, including the power supply and amplifier (see later) and all the dongles and antennas. So out went the supplied enclosure and I found one in my junk box about the right size (but I later wished I’d found one a bit bigger), So at least they won’t get knocked about in flight. The box is permanently installed and wired straight into the aircraft 12V supply. Sadly, not much we can do about the micro-USB connector that is used for the power supply to the raspberry pi –it’s a pity that raspberry pi had to choose the most unreliable connector ever devised by mankind, but at least it’s all inside my box.
2. The antennas.
It’s a pity that Pilot Aware don’t see fit to supply alternative sets of antennas, one for metal-skinned aircraft and one for fabric/plastic, as the pair they provide are good for neither.
The end-feed dipole supplied for the pi-aware board is great for plastic planes, but should be mounted well clear of metal and certainly not on a metal-skinned aircraft, which need a monopole quarter-wave antenna.
The monopole antenna supplied for the receiver dongle I can’t see as being any use for anybody. It needs a ground plane, but it’s only means of attachment is the magnetic base! Throw it out! For myself, needing another end-feed dipole, I cut the wire, stripped back the screen about 80mm and folded it back along the wire and taped it down, then inserted the whole in a bit of plastic tube about 160mm long, and glued it to the side of my box. Job Done, zero cost. For the rest of you, I don’t know what you are supposed to do.
3. The Power Supply.
Those of you in the know will be aware that there are two fundamentally different types of voltage regulator: Switch-Mode and Linear.
Switchmode regulators are small, light, efficient, cheap, so are the universal choice these days, and probably the only ones you can buy. But they have the unfortunate property of emitting electrical noise, both conducted and radiated, so be prepared for problems. Of course, some are worse than others, so you must stick to the tried and tested types.
Years back, when installing my radio, I was getting complaints that my transmissions were almost drowned by a high-pitched whine, which after much head-scratching was traced to the power supply. A change to a linear power supply fixed it. Some years later, when running moving-map software on my android tablet I found that the power supply that came with the tablet was blocking the GPS reception. I tried using a power-bank battery instead, but these too have a switchmode power supply inside, and this did the same. So my tablet now also has a linear power supply – problem solved. So you’ll understand why I wasn’t going to have a switchmode power supply inside my box right up close to the GPS and radio dongles!
If you want a linear power supply, you have to build your own. The chip I used is and LM1084 – you’ll find some data sheets telling you how to use it. But the biggest problem with linear regulators is that they need a big heatsink, and central to the design is an accurate figure for the PiAware power consumption.
You’ve read the instructions about the wire length and the “approved” 2Amp 5V regulator. But I measured a mere 0.7 amps average consumption, but with microsecond-long peaks up to more than 3 amps occurring every 29 microseconds, ringing horribly and causing disturbance to the 5V rail ! I put this down to the piaware transmission bursts (possibly wrongly), but whatever the cause it seems to me that the instructions about the wire length and 2amp regulator are the wrong solution . What it needs is more energy storage at the piaware to feed the peak currents. Looking at the PiAware board, and also the Raspberry Pi itself, I see no sizeable capacitors to perform this role, so it is left to the power regulator at the wrong end of a very long wire! A very poor bit of design I think. I did not bother to try it, because my power supply lead was only 2 inches long, but if anybody suspects they are having power problems, they should try fitting a capacitor on the Pi Aware board. There are some “expansion” solder pads on the board, even labelled with 5V and ground, which are perfect for the job. A few hundred microfarads will be fine. Then maybe, if you need to, you can use a longer power supply lead.
Even though less than one amp consumption, a linear regulator needs a sizeable heatsink. Dropping 14-odd volts down to 5V at (say) 1Amp needs to dissipate about 10 Watts, which needs quite a big chunk of finned aluminium. To get round this I used a fan-cooled heatsink, you can buy them very cheaply from china, designed for cooling computer graphics cards. The fan increases the effectiveness of the heatsink ten-fold. Best look for ones with 5V fans. Mine is 40mm square and 10mm high (including the fan) , and when I tested it gave only 30 degree rise for 10 watts dissipation – perfect!. I have arranged airflow in my box so that the cool air entering the fan first cools the Raspberry Pi and Pilot Aware, and that dongle that runs so hot, which should improve their reliability also.
4. The audio output.
Even the few flight I have made since installation have convinced me that the audio output is the MOST USEFUL benefit of PiAware, as you are never going to be always looking at your screen. However, I bet most of you haven’t got a satisfactory audio installation yet. Partly because you haven’t got a spare input to your intercom system, and partly because the audio output from the Raspberry Pi is a low amplitude signal from a high impedance, not really capable of driving very much. It needs an amplifier. I measured a peak-to-peak signal amplitude of about 400mV, and the source impedance (from memory) is some tens of thousands of ohms.
Some of you may have looked at amplifier modules using the LM386 chip, very cheaply available from China. This chip has a fixed gain of 20, but the modules I have looked at claim a gain of 200, much too high. If you run these modules from 5V, you can only have a peak to peak voltage swing of less than 4V, so a gain of more than 10 would be bad, so buy the version with a pot and turn it right down. I used the LM380 chip, which has the advantage of being able to run from the raw 12v supply, giving much more power if you need it, but I couldn’t find any ready-made modules using this chip so you have to do your own thing.
The other issue relating to the audio output, and possibly more important, is the creation of ground loop. Your audio output will create an additional ground path to your intercom and radio, and in my opinion the chances of being able to get a decent audio feed without introducing the inevitable buzzes and burps created by digital electronics is very low. It’ll probably interfere with your radio transmissions as well. You can buy ground-loop filters, how effective they might be is anybody’s guess, and I am not interested in experiments, so I went for the only possible certain solution of transformer isolation of the audio signal. Searching through my junk box I found a telecom line isolating transformer from an old computer modem card, absolutely perfect for the job and only a 20mm cube. These old modems turn up on Freegle frequently so you should be able to get one for free.
But having said that the audio out is the most useful feature pf PiAware, you are going to need the capability to turn it off. As I have found, your ears are full of dire warnings of imminent collisions when you are taxying around an airfield and trying to communicate with ATC. Hopefully, PiAware will at some time be able to turn off the warnings when you are on the ground, but they will remain a nuisance when on approach unless you can turn them off. I am planning to fit a mute switch, and maybe mod my intercom box so it is automatically muted for (say) 30 secs after I have pressed the PTT button, to allow for communications with ATC.
A friend has suggested that it would be just as effective to have a flashing light on the dash instead of audible warning, and I agree. I am sure it would be easy for PiAware to code this, as a simultaneous warning with the audio output, I am inclined to ask for it. There are spare outputs from the Raspberry Pi which are routed to an expansion port on the Pi Aware board, it would be easy to connect a cheapo relay board to this port and much easier to implement a flashing LED than the audio solution. If they wanted to be really helpful, they could have a low flash rate for the low priority alerts and a high flash rate for the important ones. And no interference with your important radio calls. How about it?
5. Software upgrades
With a permanently installed box (albeit still only fixed with Velcro) I was wary about the ease of software upgrades. The “network” method is not practical. But the USB Dongle method worked a treat. The “spare” USB socket is out of reach, and I was intending to put a socket on the side of my box, but I ran out of room. In the end I just left a short flying lead with the USB socket on the end.
6 The “Radar” display screen
Do you like it? I think it’s great fun, you can run it on your phone at the same time as your mapping on your tablet. Being only black-and-white display, I thought it would be amusing to see if it would run on a Kindle with e-ink display (and a web browser), which has the perfect sunlight-readable display for monochrome. But my Kindle won’t connect to the WiFi hotspot. I wonder why? Please try it out and let me know if yours is different.
So I have my system all working; range is great, now all I need is the rest of the light aircraft fleet to fit their own. I have to say that some of the “meccano” aspects of PiAware need to be resolved before this is likely. But I’ve done my bit. Look out for me on your screens G-MYLN. Safe flying..
PS if you are interested to see my single-box solution with amp and power supply I have posted some photos here (with the lid off). You won’t be impressed, it’s only built on stripboard, but hey it worked first time!, raw 12V in, amplified audio out, antennas built in. This is what PiAware SHOULD be shipping.
http://www.ge.tt/1dkF6Lp2
