Last year I bought an old wooden sailing boat. This summer I sailed her from Ipswich in Suffolk, along the English Channel, to Dartmouth in Devon. Whilst afloat there was much woodwork to do. Tramontana's wake was often one of sawdust and wood shavings. Amidst the chaos of sawn timber and brass screws below decks was an unopened Raspberry Pi. One of the summer's objectives was to get this wired up and running. Often sailing for a few days without landfall, occasionally becalmed, I wanted to run the Pi and any associated electrical equipment from the 12 volt DC ship's batteries.
DC to AC Power Inverter
Tramontana is a suitably sized vessel for long periods of life at sea and so, with comfort in mind, I wanted a proper monitor to use with the Raspberry Pi. In principal, it is possible to run any 240 volt AC mains appliance from a 12 volt DC supply using a DC to AC power inverter. However, an LED monitor's internal electronics are all low voltage DC. So it would be somewhat convoluted to convert the ship's DC power to AC, only for the monitor to then be converting it back to DC again. In particular, the DC to AC conversion is highly inefficient. So much waste heat is generated that the power inverter contains a fan to dissipate it. It also features high grade thermal protection should it, even with the fan, become too hot. This is at odds with life afloat which is all about power conservation. A ship's battery can be trickle charged via a solar panel or a wind generator. Alas, the batteries in Tramontana are drained quicker than they can be topped up by the demands of navigation equipment, lighting and a fridge. I was determined that adding a Raspberry Pi and a monitor to the electrical systems on board would not worsen the existing power management issues. A tempting solution, of course, is to recharge the batteries periodically using the diesel engine. But who wants that noisy thing running when the whole purpose of being afloat is to peacefully sail ?
Perfect Pi Monitor
Having berthed in Torquay Marina for a few days, a walk ashore to Currys PC World was enlightening. Some TV's had the transformer to convert mains AC to low voltage DC embedded in the mains power cord. That is, outside of the TV. On one such TV the label on the back read:
22" FHD LED TV with combo DVD player
Model No: L22FEDW12
Power supply: DC 12V 3.5A
Power consumption 37W
Although being sold as a domestic TV designed to be powered by mains electricity, the label suggested that this good sized TV could also be run from a 12 volt battery. Whilst I was primarily interested in using it with the Raspberry Pi, I also liked the idea of being able to play DVDs and music CDs whilst at sea. There was, of course, no guarantee that this TV would work with the Raspberry Pi. I decided to risk making an expensive mistake and bought the TV for £139.99.
I need not have worried. Back in the marina, using mains shore power, I found that it gave an excellent image when connected to the Raspberry Pi. Next I needed to see if it would work, as I hoped, from the ship's batteries. I unplugged the TV from the mains and cut the power lead on the 12 volts DC side of the embedded voltage transformer. I soldered in a male and a female 2.5 mm in-line power plug and socket. This was so I could reconnect the two halves of the cut power cable. First checking it still worked with mains electricity, I then made up a wire that ran from the ship's battery and terminated in a second 2.5 mm in-line power socket. Knowing that I was voiding the warranty on the TV, I used this plugs and socket arrangement to connect the monitor directly to the ship's 12 volt DC battery. The experimentation paid off: my Pi, still mains powered, had a lovely large battery powered monitor as its display unit. Currys PC World also sold me a £12 small fixed TV bracket which I used to fix the monitor securely to the bulkhead by Tramontana's chart table.
Left: Tramontana's chart table with the Raspberry Pi battery powered
monitor fixed to the bulkhead on the right hand side of the photo.
Right: Tramontana at anchor for four days in Elberry Cove, Devon.
Efficient power management is needed to avoid having to recharge
the ship's batteries using her diesel engine in such situations.
The World of 12 volt Monitors
Since purchasing the L22FEDW12 monitor, I've discovered that I am not the first to have realised this is a great TV to use on boats and in caravans. Although the TV's user guide clearly states that the warranty becomes void if the 12 volts is supplied directly from a source other than the supplied transformer there are several eBay sellers that supply it specifically for use on boats and caravans. In fact, eBay is a good place to investigate the various monitors that can be powered from a 12 volt DC supply. If you subcribe to the idea that bigger is better, take a look at this TV which has a 26" screen and can be run from a 12 volt DC supply. It's more expensive, of course; £329.
Pi Power from Ship's Batteries
In Maplins I spent £19.99 on a Universal 3 Amp power supply that could take 12 volts DC and convert it to the 5 volts DC needed by the Raspberry Pi. It could easily cope with the 1 Amp needed by the Raspberry Pi. With surprisingly little effort, I was all set up to enjoy using the Raspberry Pi afloat.
Back on Land
The summer's voyages over, I came across an excellent article in Issue 6 of the free MagPi magazine about building your own power supply to convert any voltage between 7 and 40 volts DC to the 5 volts DC needed by the Raspberry Pi. Curious, I ordered the necessary components from RS Components. As there was a minimum batch size for many of the parts, I spent a Sunday afternoon building ten of these DIY power supplies. Two I intend to keep. The other eight will be up for grabs at the RISC OS Portsmouth Show 2013. They work out at around £13 each to make. I'll be selling them at the show at less than they cost to make, £10 each, on the MathMagical stand. At the show I'll demonstrate the complete battery powered system that is described in this article.
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