Talk:2651: Air Gap

Worth noting that this is a large and inefficient version of an opto-isolator 108.162.221.79 05:37, 28 July 2022 (UTC)

Incandescent light bulb (assuming it the lamp does not use LED in the shape of light bulb) is not only less efficient than diode, but also much slower to warm up and cool down - it usually is much more sensitive to rapid switching, and has shorter life counted in the number of on/off cycles. --JakubNarebski (talk) 07:45, 28 July 2022 (UTC)

There’s not even any indication that the bulb is shaped like an incandescent bulb. Only that the front of the light (either fixture or bulb) is a convex curve. For all we know that could be a lens or diffuser in front of a flat LED. Whoever wrote that needs to go back and walk, because the claim that an incandescent bulb is depicted is quite simply false.172.71.142.89 10:35, 28 July 2022 (UTC)

The warmup and cooldown delays mentioned in the title text must imply an incandescent bulb, mustn't they? 172.70.210.145 03:29, 29 July 2022 (UTC)

I don't think it's less or more efficient than an opto-isolator, it essentially is just an opto-isolator. But an opto-isolator isn't supposed to be energy efficient to begin with; it's only designed to transmit data between circuits, not power. So the output side only needs to generate enough voltage/current to change the state of a transistor, and the input side only needs to generate enough light for the output side to do that. The voltages and currents involved aren't comparable to power circuits. --NeatNit (talk) 08:14, 28 July 2022 (UTC)

By the way, wikipedia links can be written like this: [[wikipedia:opto-isolator|]] result: opto-isolator (the final | automatically gets expanded to the article title without the wikipedia: prefix). --NeatNit (talk) 08:26, 28 July 2022 (UTC)

Or more often here on ExplainXkcd, {{w|article}} or {{w|article|anchor text}}. 172.70.206.213 08:35, 28 July 2022 (UTC)

Yes, thanks :) although there is a tiiiiiny advantage to the direct link without the template (the way I said), [[wikipedia:Pipe (computing)|]] becomes Pipe whereas {{w|Pipe (computing)}} becomes Pipe (computing). The pipe trick strips out the disambiguation parts of the title according to some rules. --NeatNit (talk) 12:30, 28 July 2022 (UTC)

Why not {{w|Pipe (computing)|pipe}}? 172.70.214.95 15:18, 28 July 2022 (UTC)

Yes we always use the {{w|xkcd|xkcd comic}} format here on explain to make xkcd comic these links. --Kynde (talk) 09:16, 29 July 2022 (UTC)

It is missing that air-gapping the power supply would protect your home from voltage surges in the power network caused by lightning strikes. Depending where the lightning hits the power network, there may be no fuses protecting your home or single fuses may fail to protect you. --172.70.246.115 07:57, 28 July 2022 (UTC)

That is true. But the suggestion that this might have anything to do with general energy security (as is currently very prominent in the explanation) is entirely unconvincing to me.

I also originally thought this was the main joke, until seeing the title text about bit rate. Certainly it's worth mentioning, even if this isn't the main joke, since it would actually work, with a wide enough gap, ideally with a vacuum in between.

Incandescent light bulbs convert most of their energy to infrared light. There are solar cells that work in this infrared spectrum, so this might not be all that inefficient as stated. This should in fact be a lot more efficient than any LED+visible spectrum based panel, as incandescent bulbs are very efficient in converting electricity into infrared light, much more than LEDs most likely will ever be. The (mostly) omnidirectionality of the light source might be an even bigger loss, as most of the light (however efficient) does not even reach the panel. And regarding sending data over this construct: As soon as there's a 0V state (which will be the case as soon as the transmission starts, due to some form of manchester coding, regardless of it being a 0 or 1 bit) the PC behind the solar panel would not only have a data transmission problem :) (With incandescent bulb, that is. A LED 0V might be short enough for capacitors in the PC's power supply to buffer it, if it is only at 50%(+PSU conversion loss) load max, as manchester coded signals per definition have a duty cycle of 50% to keep the DC bias at 0V) 172.68.51.204 08:26, 28 July 2022 (UTC)

What is the highest wattage commercial opto-isolator, and how can I get one mounted from the ceiling in my bedroom? 172.70.207.8 14:10, 28 July 2022 (UTC)

Ordinary florescent bulbs or neon tubes are close, the electrical circuit being isolated to flow through a gas which only becomes ionized and thus conductive in a way which causes it to emit light. But the signal stays as electrical without any intervening photons. You probably don't want a laser BBQ on your bedroom ceiling, but then again, the military gear referred to below isn't commercial, as it would never pass laser safety certifications. 172.70.210.145 01:01, 30 July 2022 (UTC)

I wonder if it's worth noting the significant understatement within the title text, where it says "the bitrate does drop a little" in contrast to the severe and drastic drop in bitrate that would actually occur, especially in light of today's typical Internet speeds. It might not be worth mentioning, but it struck me as a humorous understatement of the true impact. Ianrbibtitlht (talk) 14:25, 28 July 2022 (UTC)

We don't know if the light bulb is incandescent or LED, so we can't describe the bandwidth drop other than in very general terms. 172.70.211.88 15:34, 28 July 2022 (UTC)

It's safe to say the warmup and cooldown delays mentioned in the title text imply an incandescent bulb. Is there a standard or average response time for household bulb incandescent filaments? 172.70.214.43 03:33, 29 July 2022 (UTC)

Maybe this helps? https://ieeexplore.ieee.org/abstract/document/5625426 172.70.142.21 06:51, 29 July 2022 (UTC)

Yes! Figure 1.10 seems to suggest that frequencies above 10 Hz are filtered, and that seems consistent with Figure 3.14, in that recovery time is a tenth of a second (though drop-off "cooldown" time is much shorter.) So if I remember my modem math, even with the most sophisticated coding, anything more than 640 bits per second should be impossible. 172.69.33.225 07:39, 29 July 2022 (UTC)

640 bits per second should be enough for anyone. But seriously, Fig. 1.10 shows a fall-off, not a sharp peak. How do you know there isn't an 8,192-dimensional analog to the Leech lattice which would allow kbps? 172.69.33.199 22:56, 29 July 2022 (UTC)

I'm almost tempted to suggest that this should be an (honourary) addition to the Cursed Connectors comic-collection. 172.71.94.187 14:56, 28 July 2022 (UTC)

I agree that it reminds me of some of those. But it could never be added to the category, as it misses the cursed connector name and number. But I have added a mention of the similarity here above, and also mentioned it on the Category:Cursed Connectors page. --Kynde (talk) 09:22, 29 July 2022 (UTC)

It's a cursed disconnector. Barmar (talk) 15:24, 29 July 2022 (UTC)

Dis-connectors are always cursed! ;) 162.158.159.95 17:11, 29 July 2022 (UTC)

Yes, solar panels can transform electrical into electromagnetic signals. It is more on the side of the incandescent bulb that the capability to receive and forward these signals is missing. My source: https://www.youtube.com/watch?v=TGUteH93xNo Torge (talk) 15:11, 28 July 2022 (UTC)

Interesting! 172.70.142.21 06:51, 29 July 2022 (UTC)

Good link for free electrons in the recent 2649: Physics Cost-Saving Tips. For this comic, Part 1 might be better.,172.69.33.245 00:46, 30 July 2022 (UTC)

Nobody caught on that powerline networking is about sending data through powerlines? or that a crude opto-isolation setup could effectively scrub it? Where my networking geeks at? I am disappoint 172.69.71.51 15:19, 28 July 2022 (UTC)

An electrical engineer here: perhaps we should mention that the box pictured after the solar panel must be an inverter? The lightbulb/solar panel pair will be acting as a rectifier, putting out purely positive voltage, and to get back to AC to run appliances on there would need to be an inverter. 172.69.62.49 01:10, 29 July 2022 (UTC)

Can you please help out with the argument about rectifier efficiency at 2642? 172.70.214.95 03:24, 29 July 2022 (UTC)

Most of the bulbs in my house (before compact flourescent and LED) were 75 Watt or 100 Watt. 50 Watt would be a very dim bulb.

Mice wouldn't agree. 172.70.214.43 05:05, 29 July 2022 (UTC)

In Denmark 60 Watt was standard, with 100 and 150 possible in some lamps. --Kynde (talk) 09:22, 29 July 2022 (UTC)

There are actually usecases for optical power links. For example electric field probes use this: https://www.raditeq.com/products/electric-field-probes/ --Casandro (talk) 08:10, 29 July 2022 (UTC)

Some drones can be recharged in the air by lasers, but that's a really obscure application only the military needs, and then only as a contingency backup in most cases, but there were flying demos by companies interested in the space over the past decade. 172.70.211.88 22:09, 29 July 2022 (UTC)

I think it might be possible to construct a scenario where this could have some security benefit, albeit not one that justifies such poor efficiency. Suppose you have an embedded system running off USB power to perform some cryptographic task, but the USB port the cable's plugged into is under the control of an attacker who has it instrumented to sample the power consumption at a high frequency. He can perform power_analysis if the load changes based on the operations performed by your device, but the setup in the comic should prevent information from leaking upstream in this manner. D5xtgr (talk) 19:44, 29 July 2022 (UTC)