Zaon MRX – Approx 1.9 miles – 500ft above descending.
The Zaon PCAS portable collision avoidance system is a very simple box which is considered as carry on – therefore requires no certification. Aircraft carry transponders which are interrogated by ground based radar and respond with a signal which may include height information. This is used to plot the aircraft on an ATC screen.
The Zaon device listens to the responses from nearby aircraft and estimates their distance from you based on signal strength. If the aircraft is transmitting height it will also show you that and if it is climbing or descending.
It prioritises the received aircraft and gives you a warning. It does not tell you in which direction it is – but its enough to add a level of safety.
Simple Design and Layout
The main PCB is split into 3 distinct parts. On the left there is the power supply, its a buck-boost circuit which provides 3.3 from batteries that may below 3v. An external connector uses the same power supply and can operate up to 28V.
On the right there is a radio front end which operates on 1090Mhz, with basic decoding of mode C signals.
In the middle there is a microcontroller that does all of the processing and drives the display. My guess from the label P16F877 is that its probably a Microchip Pic 16F177 controller. I’m familiar with these from my past, they are a 40 pin controller with USB connectivity – you can see it does have 40 pins. These are still in production in 2022.
LED display
The only other point worthy of note is the display which is insanely bright. A broadcom device found here.
Sadly, Zaon no longer seems to exist but Garmin make an XRX version which gives quadrant directional information. But to be honest you can achieve the same today with a Raspberry PI and a software defined radio – which also allows you to decode ADSB.
I recently bought a toy car for my Son which has a 2S lithium battery and comes with a USB charger FTX Tracer Truggy. I couldn’t get the toy off him, so I bought my own monster truck version – but my charger had a flashing red light whereas his has green when charged with red when charging. So I thought mine was broken. (it wasn’t it was just a slight variation)
When I removed the case I was puzzled for a while how this works. The battery is a 2s battery which is nominally 7.4V. Normally if you want more than 5V you’re going to need an inductor or switched capacitor boost circuit. But there are none on here. Only a couple of tiny supply decoupling capacitors; so how does it work?
Why is it interesting?
Cheap Chinese non-balanced charger
Above is what happened in my living room a few years ago when charging a toy helicopter. This was a 7.4v/2s battery of 800mAh capacity. Even for such a small battery the fireball was huge and very hot. This is why I never leave them unattended. You can see that the battery charger has only 2 wires and so there cannot be any balancing. I now charge lithium polymer batteries in the kitchen – on the stainless steel stove top. That way, fire cannot spread, there’s a cooker hood to extract any fumes and in the worst case, the stove can be easily replaced.
Unlike older rechargeable batteries, lithium cells do not tolerate over-charging with the risk of spectacular explosions. So battery packs with a series (2s, 3s….) of cells can be charged in series but if one cell has a lower voltage than the others then serious problems result. Its not as easy as limiting the charge current with a resister like in the good old days of NiCads.
Charging Voltage vs Capacity
You need to switch from constant current to constant voltage at around 60% state of charge and each cell may have a different state of charge. Cheap chargers monitor each cell and terminate charging when any cell reaches 4.2V, so a cheap drill battery may seem to get less and less capacity until its useless, all you actually need to do is charge them all independently to restore its capacity.
A proper balance charger monitors each cell independently and prevents that an individual cell charging whilst continuing to charge the rest. This is not trivial because they are all still electrically connected together.
A proper balancer will provide a voltage that is at least 4.2V x <cell count> and whist charging will shunt each cell as it becomes full. This means that current bypasses the cell through the transistor Q1 – Q3 and continues to charge the remainers.
Typical Balancing Circuit
Shown above is a discrete (made from bits not a chip) self-balancing system. It uses a simple zener and transistor that starts to turn on as the cell approaches 4.2V, its adjustable per cell because components are never exact. Its great because it can’t really go wrong.
No Shunt, No voltage boosting
Microprocessor controlled charging
Taking a closer look at the charger, we see USB plug on the right with 5V available. On the left we see the 3 wires going to the cells, the centre wire being a tap between the cells. To the right of those wires we see 5 transistors. And the whole thing is controlled using an anonymous (no label) micro-controller. This left me puzzled for a bit but an hour later I worked out what it must be doing. Its charging one cell, then the other and the transistors are used to connect each cell to the charging “bus”.
You could do this one cell until full then the other – but if you were to then interrupt the charge, say in a RC car, then the flat cell would end up reverse charging and that would ruin it. A quick connection to a multimeter shows that the charger charges each cell for 1 second then switches to the other and back again continually until charged. This swapping batteries coincides with the flashing charge light. I think the charge current is limited by the large 100 Ohm resistor, and the micro controller will also be able to measure that. Since USB is 5V and a fully charged battery is 4.2V that leaves 0.8V to be dropped by the transistors and the current limiting resistor, which you can just about do with normal transistors which have a voltage drop Vce of about 0.3V. So that would be 0.3+0.3 leaving 0.2V drop on the resistor.
Overall, its a clever design and may have been obvious, might even be common with cheap toys that require more than a single cell – but I hadn’t seen it before.
My concern is referring to the charge vs. capacity graph – when charging and relaxing as opposed to continual charging, I think its much more difficult to detect when the battery is at 100%. So it probably doesn’t optimally get the last 10%. Having said that, pulsed charging and charging to 85% is usually good for the battery. In an EV or Hybrid Vehicle for example, the battery takes charge and discharge intermittently at any state of charge.
Another concern is that the micro-controller runs a software which can easily glitch or get stuck. You’d hope the software had a watchdog reset that resets it if it does but if it doesn’t then you could find that the circuit dumps 1.2A into one cell until it explodes. That cannot happen with the discrete design, for safety you probably want to see that the cell voltage can never exceed 4.2V but doesn’t look like here is any stuck protection, unless those big blobs that look like diodes on the output are 4.2V Zener diodes, such a thing does exist – but they appear to be marked ss34 which is basically a 3A shottkey diode to stop the charger being `driven` from the batteries.
Finally, if a lithium cell is over-discharged (below 2.5V per cell) you cannot whack a full charge current on to it to start charging, you need to charge at a low rate until the voltage recovers, then resume charging. Similarly, you must also take care charging hot batteries, its better to reduce the current or wait for it to cool before charging. A proper BMS (battery management system) will typically have temperature monitoring for that reason.
For all its cleverness – probably best to never leave it unattended.
An engineer at my local Tesco store obviously needed to get a camera working but didn’t have the PoE power injector he needed. Was that going to stop him- not by the looks of it!
Smart Fish for a Smart Home – 3D Printed lamp housing.
If Apple made aquariums the Biorb Life tank is what it would be. Yes its modern but its also astonishingly expensive, £300 for something that really is worth no more than £50. But there we go, I have this ridiculous tank and I love it, now that the pain of paying for it is a distant memory. Unfortunately the Smart light at the top started flashing and causing me issues – presumably afte failure of one or more of the LEDs.
With Alexa you can set routines – like 30 mins before sunset turn Orange.
I looked at BiOrbs replacements and for the price of a small mortgage they have the Biorb MCR which offers any colour at the press of a button and it comes with a remote.
DIY Smart Lamp
Having made a number of projects using the ESP8266 wifi connected boards I thought I could make my own lamp if I could find a suitable LED and perhaps code up an app to change the colours.
Turns out its easier than that, with a little research all the building blocks are out there – if a little tricky to find out what they are called.
PowerDot 10W RGB LED – this was probably the most expensive part especially as I trashed the first one.
One issue I did encounter was that the blue stopped working and I thought this would be failure of the LED but it turned out that the solder was fractured, so simply re-flowing it with proper leaded solder fixed that.
ESP8266 – Generic, whatever happened to be on Amazon that day.
40mm graphics fan
These are cheap and readily available.
3D printed custom enclosure – I designed this is Solidworks in two parts that can be press-fitted together. I printed it in white PLA.
Software Libraries
Espalexa – library that simulates Philips Hue devices
FastLED – library that provides addressable LED functionality in addition to many colour utility functions, such as fading from one colour to another.
ESPAsyncWiFiManager – Generic library I always use instead of hard-coding the wifi credentials. This starts up a hotspot and then you can set the details using your phone.
I wanted to prolong the life of the fan as much as possible as its a moving part in addition to keeping the noise as low as possible (Although the aquarium is noisy anyway with the pumps etc). So using the FastLED library you can work out the power dissipation of the current colour and brightness, I use this to produce a power factor between 0 and 1 and then use that to control the fan speed. In the end the fan starts to turn at 25% brightness and the maximum duty ratio is 40% of full power – so even at full chat its almost silent. The only exception is that on start-up I run the fan at full power for 5 seconds before reducing the speed to blow off the dust and give the bearings a good thrashing.
At night the fan isn’t required and in the day (8hrs) it runs at about 40%. I used an N-Channel logic level FET to control the fan speed. The fan is brushless and doesn’t really like being powered by PWM signal, so I created a simple RC filter, so the fan see’s an average voltage. This also stops it singing at the PWM frequency.
ESP8266, Powerdot 10W RGB LED and 40mm Cooling Fan
Logic level MOSFET to control fan speed
Fitted into 3D printed enclosure
Testing with Alexa App
Prototype fitting. At full power passive cooling was not quite sufficient
Happy Smart Fish
Alexa
Once you have configured the library, you use the Alexa app to search for your device – here I called it ‘Fish’ and it identifies as a Hue Colour device. You could also have just white, dimmable, colour tuneable etc.
You can set routines with Alexa and have the aquarium come on at a specific time, or sunrise if you like. I found it to be a little hit and miss because I have 3 different echo dots on different VLANs (I think that is the issue) – so for the initial turn on and off, I add a 2 minute delay and then repeat the command and that tends to work. It may be that we get a fix in the library at some point.
FastLED
FastLED is a great library, but to fade from one colour to another you need to working the the HSV colour space rather than RGB. The library finds the approximate closest HSV colour to what you have or want and fades smoothly. But it has issues with secondary colours. So orange looked green, magenta looked red. This was a pain as you didn’t get the colour you asked Alexa for unless it was White, or RGB. So in the end as there is only a handful of colours Alexa recognises I hardcoded them in so that it would translate on the device.
ESP8266 Limitations
The ESP8266 struggled with PWM and FastLED libraries at the same time, because its doing the PWM in software. So you saw random flashes of solid colour from the LED with occasional glitching when fading colours. What I did was to disable the FAN PWM when fading. You probably wouldn’t ever notice.
As an aviator its important to know what the weather is doing and in my case whether there is an point tuning up to the airfield.
I had been toying with the idea of making a retro LED clock for some time, but as I now have to use an ESP8266 connected device for everything because its cheap and east to use, I thought I could add value by displaying the current METAR. A METAR is an aviation text encoded weather observation that really belongs 100 years in the past, but is still useful. It tells us what the wind is doing, the pressure, cloud and visibility. These are all available online in JSON requests or via many free API’s. So all we need to do is make a web request.
The ESP8266 does not have a real time clock, but it is able to sync with NTP (Network Time Protocol) and keep time that way. So all we really need is the ESP8266 and the LED display.
Two 4 panel LED displays is too large for my 3D printer so its made it two halves. Simple affair, there is no back but it includes screw hangars and a notch for USB entry.To join the two LED panels, I used 5 wire links between them.CDS light cell / resistor tacked on to the board allows sensing of ambient light levels. Other than that its all driven directly from the ESP8266, the USB powers everything.
ArduinoOTA – allows for it to be updatable over the air, essential when its screwed to the wall.
Adafruit GFX Library – required for the LED panel.
MegunoLink – used to filter the light sensor data to stop the brightness hunting.
ArduinoJson@5.13.4 – specific version required to parse the JSON web request for the METAR data.
WifiManager – to create a hotspot to allow the device to be configured.
Max72xxPanel – the shift register driver for the matrix panel.
Refinements
I found that the string class, although very powerful, can lead to heap fragmentation on the ESP8266. The forums are not terribly helpful basically saying that people should learn to use char arrays and pointers instead and stop being lazy. Whilst that may be true especially on a tiny process with a few kb of RAM its not helpful when leveraging 3rd party libraries, particularly the JSON class – you could do this manually but its unwieldy.
The solution I went for is to restart the ESP every 24 hours instead to clear up the memory. The LED display will just retain whatever it was showing whilst the restart fires, and it only takes a few seconds. I may revisit this and refine it a bit later, but for now its perfectly workable.
Clones
As it happens by word of mouth I have made quite a few of these. Its ironic that the most expensive part is a nice USB cable! There’s one at Flightpath flying school at Wolverhampton Halfpenny Green.
This year, we’ve all been working from home. I’m very lucky to have a man shed to work in. In very hot weather though it can become far too hot to work in. The roof is dark and in direct sunlight on a hot day we are looking at 40C.
The shed has a gutter and a water butt, so I decided to create a water cooling circuit with some PVC pipe arranged as a spray bar. I used a PWM motor speed control circuit and a 12V bilge pump from eBay.
On a hot day the roof would reach 66C. With the water cooling it would bring that down to 28C. Its not so much the water cooling the roof as the evaporation of that water, so I don’t need to spray a lot of water, just keep it damp. The specific latent heat of evaporation of water is staggering so the cooling effect is very real and quite dramatic. On the hottest days, the roof evaporated 25l of water which is many megawatt hours of energy, powered directly from the sun – that otherwise would have cooked me.
Refinements
The evaporation was so effective that I had to add a float switch to prevent the pump running dry and I had to top up the water every other day unless it rained.
I added an irrigation inline filter to stop debris clogging the jets. Again, where else, a fiver on eBay.
A battery management system needed to be added and this proved to be tricky. For convenience we want a rechargeable battery and, like any gadget, to be able to charge it from USB. The unit draws 170mA and I opted for a 1000 mAh lithium battery which includes battery protection from overcharge, short-circuit etc. This should see up to 5 or more hours use. I used a charge management chip and programmed the charge current to 300mA. This conservative charge rate means a very safe 3.5hr charge time and doesn’t demand too much from the battery.
Case Design
Prototyping The Case
I wanted to make a device that was as small and lightweight as possible. After modelling the display PCB, all the other parts were modelled as accurately as possible. Finally we know the minimum size the case can be. Originally I designed it with screws, but then I realised I was still thinking like it would be made from milled metal – now I can 3D print a case I can make it snap together. Note the two tabs on the top part which snap in to the bottom part. The thinnest shell I was comfortable with was 2mm and this makes the case a feather weight and yet still surprisingly rigid.
For the switch I tried a few different things and again I was thinking along the wrong lines. Plastic allows you to make flexible parts. The switch can be made out of the body in one piece, which is quick cheap and simple. Just cut a slot so it can move. I added a raised area to make it easy to find and since the print direction is left to right, its optimal for the 3D print. It worked really well and I’ll definitely use this technique in future. I wanted a black case, but have lots of white PLA that I needed to use up so the prototype cases were white.
Prototype Case
The software was updated to hook up the ADC input to monitor battery voltage. A housekeeping task runs once per second and this monitors the voltage and displays approx capacity rounded to the nearest 25% – for simplicity.
In the previous project, we saw a traffic display involved a far bit of hacking of the Pilotware unit. The downside of this is that it makes it difficult to keep the Pilotaware unit up to date.
The Pilotaware unit provides traffic information via a Wifi hotspot. The system provides an interface on port 2000 which provides FLARM and NMEA GPS information.
What we need is something lightweight, cheap with wifi and a display. Until recently this would have been an expensive development, but today we have the internet of things. Esspressif systems have made this extremely easy with a system of a chip, which includes WiFi, can easily be programmed in C++ using arduino compatible libraries …..and all for the price of a McDonalds! It should be possible to create a unit about the size a fag packet that you can stick the dashboard for very low cost.
A complete computer (esp8266) for £5 and a £10 TFT colour display.
All that is required is to connect the esp8266 serial peripheral interface to the display. In this case I used a wimo d1 mini board.
Next we need to connect to the PAW which is only a couple of lines of code, then connect to port 2000 and handle the data stream. Once that’s done we install some SPI display libraries and we can draw whatever we want on the screen.
Space invaders
I chose to depart from the TCAS display symbols because I’m not used to them and I found a ‘spaceship’ pointing towards me far easier to interpret than squares moving sideways. Other than that the large number indicates the level and I can display the reg or whatever with it.
Next Steps
3D printed case
USB Rechargeable lithium battery – gives about 7 hours of use from a 1 hour charge.
I had forgotten to post updates to the project. Progress was extremely slow because of family etc.
I really liked the Pilotaware system having known people personally to die in GA mid-air collisions. If you have a certified aircraft, you cannot change anything and it must be regarded as carry on. However, what you don’t want is a mass of cables and obstructed views.
The Pilotaware radar view is awesome as it is, but its a fiddle to connect to it on your phone and a distraction you don’t need. My idea was to create a purpose built display. I searched on eBay until I found a 1950’s ADF that would form a chassis that I could use. It was plenty big enough and had standard connectors already on it- someone already did all the hard work!
Then I realised that I’d be creating a system ideal to support the actual Pi. So I stuck the Pilotaware Pi in the back. One box, one set of connections.
System Internals
Internally it is based on the following
Anker 5V 3A charger – recommended by Pilotaware as its very low noise and seems to cause the least issues. This is happy with anything between about 7V and 28V and provides a rock-solid 5.1V output.
A Pilotaware radio module, attached to a Raspberry Pi 2 Model B (that’s how it came)
Pi Zero with Bluetooth and Wifi – amazing that this is only a tenner.
Adafruit 2455 Pi TFT 2.4 Inch touch display – I very carefully cut the touch overlay off because it was too reflective and not required. Getting the display to work on the Pi Zero was a bit of a pig.
Vero Strip board – some mil-spec stock I acquired ages ago.
Pi Zero
The Pilotaware unit
see all the bits
Early bench prototype – The LCD bezel is just printed on a laser printer
Software
Setting up the Pi Zero was a bit of a fiddle. I wrote a script to keep trying to connect to the Pilotaware system. Next, it starts the x windows environment and opens a windowless browser to the radar page. There is a script that checks that all is well and if it does loose connection it will sort itself out.
Hacking the Pilotaware
The radar app isn’t quite right. The lines are too thin and on a low resolution the screen its too difficult to see – so I needed to alter it. You cannot get access to the Pilotware system because they don’t give you the password. However that doesn’t offer much of a challenge. In the end I mounted the card under windows using some Paragon software.
I altered the CSS significantly and changed the javascript that draws on screen, to make everything using fully saturated colours and 3 pixel width line to make it really easy to see. I drew my own Compass Rose which is a PNG file. In the end it looks like a proper glass cockpit design. I wanted to keep mods to a minimum otherwise updating it is a pain. In the end I created a Radar2 folder such that updating the system leaves my hacked version intact. It sounds like I know what I’m doing with this; I didn’t it took ages – this project was on an off over 18 months. I created a debug environment and could locally test it using chrome with mobile device screen set up.
From the pilot’s seat
I wanted to see what it would look like on a real panel, from the pilots seat its lucky that this is a good angle for this screen.
Sunlight
Unfortunately, in sun light the screen reflections are a problem. I looked at anti-glare film and bonding the display to the screen like a smartphone but these are messy solutions.
3D printed bezel
What would be better is no glass. I 3D printed a prototype bezel. Shown here was an early prototype. The volume / power knob can now be added as there is no glass to worry about. I also added an LM386 audio amplifier connected to the Pi so you can hear the traffic alerts. Volume / Mute was a must. I scrapped the text and mention of TCAS because it isn’t TCAS, its a traffic aid.
Most connections are made remotely over a 2m cable with a Tesco USB hub. For some reason all sorts of expensive micro-hubs wouldn’t work but a 4 year old one from Tesco’s works perfectly. This allows easy placement of GPS and ADSB receiver at the back of the cockpit out of the way rather than trailing cables all over the place possibly jamming the controls!
I’ve worked hard to ensure that it produces as little radio frequency interference as possible. I have the old non 8khz navcom to play with on the bench and I also have a scope that shows the radio frequencies and strength being emitted so they can be addressed. The power inlet is RF filtered and the case forms a Faraday cage. Additional shielding was still necessary and grounding was a bit of a dark art. In the real aircraft, I realised I probably didn’t need to worry as the existing electrical noise was 3 orders of magnitude higher. There is a lot of electrical noise from the strobe and the alternator, its a wonder any radio navigation equipment works at all.
Final Thoughts
I think this project worked out quite nicely and I learned a lot doing it. It gives you an appreciation for the complexity and radio compatibility issues you can have with airborne systems. My end result with its black panel doesn’t look like its homemade and the performance is remarkable and I tested in the car at the airfield.
Unfortunately, you can’t fix something like this into a certified aircraft – though I’m sure many would. You can place it on the top of the dash although I hat anything obstructing the view – a non-moving dot hiding behind it is exactly what traffic on a collision course looks like.
