Friday 21 October 2016

Four Pies

In my office, I have four Raspberry Pies. On runs the Geocaching Robot Arm, one monitors my front garden and road outside, one drives a seven inch screen (which wants a 12 volt power supply, isn't that handy?) showing me a continuous update of the usage of my 100 mbps line, and the fourth one drives a 17 inch screen that shows the details of line usage.

I've just reorganised the way they're powered. Before, it was a mish-mash of different power supplies, reflecting the fact that these systems have evolved over the last few years. Now I've rationalised things.

I have a computer - a rather small box, which I used to run several terminals on the same screen. A few years ago, the power supply in that failed, and because it's such a small box, it can't take a standard PSU (ATX power supply). So I put a standard ATX power supply on top of it, and led the wires inside. This is what is technically known as a kludge. What I realised just yesterday, was that the same ATX power supply could be used more widely.

I have another computer that I use as my main workstation. Several months ago, the power brick failed for the monitor (a lovely big 27 inch screen, 2560 by 1440 pixels). I looked on Ebay for a replacement power brick, couldn't find one, then realised that all it wanted was 12 volts. The answer is obvious. So I take 12 volts from the ATX supply, and it powers the monitor just fine, meaning I won't need to shell out a couple of hundred pounds for a replacement monitor.

The big change was the Pies. I'm using PoE, Power over Ethernet. In an ethernet cable, only two of the four pairs are used for data. The other two pairs just aren't used for 100 mbit ethernet, only for gigabit, which I'm not using in my office.

So I bought a bunch of PoE splitters, £1.24 per pair on Ebay. I'm using four of them, they're connected to the 12 volt line on the power supply, and to an ethernet switch.

At the other end of the ethernet cable, I put the other half of the splitter, so now the same cable is carrying the ethernet data, and the power. But hey, you're thinking, that's 12 volts, and the Pies want 5 volts. If I sent five volts down the ethernet cable, by the time it got to the Pies, the voltage would have dropped, and since the lengths of cable are different, the voltage drop would be different. So I put 12 volts down the line, and at the end where each Raspberry is, I put a voltage converter  with included voltmeter, to step the 12 volts down to 5.25.

The Pi wants less than 2 watts (under half an amp at 5 volts). So the 12 volt line will be transmitting under 0.2 amps, and the PoE spec says it can handle 1 amp. Still, I put a 5 amp fuse at the end where the PSU is, and that's carrying four Pies and the screen, which I reckon would be about 2 amps total - I had a car fuse that blows at 5 amps, left over from a bike project.

So now all my cabling is nice and neat, and I've dispensed with three power supplies that have gone back into my box of bits.

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