Hijack - iPhone Sensors Thru Audio Jack | Hacker News Reader

Hijack - iPhone Sensors Thru Audio Jack

seeedstudio.com

160 pointsJ3L240436 comments

The HiJack energy harvester can supply 7.4 mW to a load with 47% power conversion efficiency when driven by a 22 kHz tone from the output of a single audio channel through the iPhone/ipad/itouch headset port, no extra energy source is needed. The name "Hijack" refers to this energy harvestign method.

Can someone here explain the functional significance of the 22kHz tone? It would be more intuitive if they'd said "an audio signal driven at the headphone port's maximum volume." Why would there be more harvestable energy in a max-volume 22kHz tone than a max-volume 11kHz tone? Or might this be to avoid accidentally blasting the user with a super loud output tone (i.e. in a more audible range) from the device speaker?

The tone AC signal has to be converted to DC power supply. A higher frequency means smaller components needed to do the conversion. Size roughly inverse proportional to frequency. So they probably chose close to the highest frequency possible on the audio output.

If the samples are put out at CD rates (44.1KHz) then 22.05KHz is the highest AC frequency available.

It's probably no coincidence it's right out of human hearing range. Most headphone drivers response curve are likely designed to be less efficient at that freq, and thus need more power to create the same loudness. the iPhone's audio driver probably takes this into consideration and drives those frequencies with more power to compensate.

Just a guess, I'm sure they arrived at it purely through trial and error, and were happy to find that it's also a frequency that will be minimally annoying if a headphone happens to be plugged in when the tone is played.

Minimally annoying my ass. If you are still young enough to hear it it's downright painful!

How young is young enough to hear 22 kHz? I am 25 and can barely hear 14kHz. http://www.noiseaddicts.com/2009/03/can-you-hear-this-hearin... In the comments section there people claim to be able to hear 20 and 21, but it seems like not many can hear 22.

In practice I'm pretty sure it's more about exposure to loud noises than actual age, and these increase as you age. I know I was able to hear 20kHz at 16, but probably down to 17kHz or so now at 25.

In other words, if you've been to a couple of rock concerts with no earplugs then all bets are off.

Also worth noting that most speakers play 0.5dB below the actual signal at 20000Hz, where anything above that usually isn't specified. So you need specialist audio gear or some measuring equipment to actually try to listen to really high frequencies.

Don't forget with tests like the one that you've linked it depends on your speaker's response to get an accurate reading.

I don't know the theory behind this stuff, but here it goes:

Headphone jacks tend to have a capacitor in-line with the output terminals to get rid of any DC bias -- this can cause problems like battery drain, lots of noise when you plug headphones in, and damage to the driver coil of headphones -- not good!

I like to think of a capacitor like a shock absorber, with the length of the shock absorber being like the voltage it's currently storing. If you push on one end of a shock absorber, the larger the force (current), the larger the rate of change of voltage (length), so this analogy might be useful.

Imagine holding on to one end of the shock absorber while someone else tries to move it back an forth at a given frequency. If they try to move it back and forth slowly, the maximum force (current) is going to be low, and so the total energy will be low and you won't get shaken much -- the shock absorber will just change length (voltage).

If they try to move it back and forth quickly, the force (current) will be much higher, and so you can extract a lot more energy from it.

I believe it's the same reason portable devices have poor bass response.

Wouldn't it be a simple relationship: higher frequency -> higher energy?

I think this is a novelty at best.

The electrical interface standards may change between devices (voltage/current sink ability) and even between software releases (power saving etc) resulting in a sudden "it stops working" moment.

The basis of it is also flawed on the fact that it's a workaround for a closed platform at which point it's probably better to just use an open one.

Hack yes. Novelty yes. Don't build anything important with it.

Harvesting power is a bit questionable, but in terms of an input mechanism, Square built their reader on the idea.

As far as it being a hack for a closed platform, I'm not sure why that matters if your goal is to interface with the primary mobile phone platforms. This solution works, and if you need something more professional, working with the dock connector itself isn't an burden any more insurmountable than dealing with injection molding or other product design and manufacturing concerns.

Agreed.

Using the dock connector with a legitimate product requires you to join Apple's "Made for iPhone" program.

For a device like Square that works on multiple platforms, the audio jack is a common interface and the Square reader has the benefit of requiring little (to zero) power to function as it consists of a magnetic head that works when a card slides across it.

Looks interesting...but what is the real application?

Prototyping mobile accessories! When you're working with hardware, building things to make things talk takes a lot of time. This makes it easier.

It's also useful for student projects. I got one of the original wiretaps when they were giving away their first batch for a prototype medical device (we didn't end up using it in the end because it was still too much work).

Well there is a small company that created a credit card reader using it to enable smartphones to act as Point of Sale machines. They call it "Square".

great idea / technology, lame page. Would love to see a decent article on the topic, with specifications about the minimal number of components to do something useful (enough to use the input jeck ADC in a decent way), and some example application that you can actual build easily with schematics. IMHO a good show case could be a termometer that you can plug to your iphone with minimal components, using a classic temperature sensor that changes the resistance with difference of temperature.

I suppose you could use this to make a generic ir remote. That was one of the coolest things that all my old palm PDAs could do, and has always been a big dream feature on the iPhone.

That has already been done: http://rtfms.com/episode-4-turn-your-iphoneandroid-mac-pc-pl... albeit without HiJack

This is really cool! I've been thinking about fiddling with Square readers to read card data, but something like Hijack makes this so much easier.

The O'Reilly book "iPhone Hacks" also has schematics and details on how to create a headphone jack interface.

...which requires a Cypress PSoC chip for which you need to buy a $100+ programmer and use their Windows-only IDE.

The concept is pretty well explained (FSK mod/demod). Someone enterprising enough can implement it in ways that don't require the Cypress equipment.

What's the advantage of using the headphone connector rather than the dock connector?

edit: thanks all, I forgot about the licensing requirement for the dock connector

License fee and dependence on Apple?

See this http://www.quora.com/Apple-Inc-2/What-is-the-royalty-Apple-c...

Using the Dock connector involved paying Apple a licensing fee. This circumvents that, and it's similar (although more involved) to how Square's card readers work.

There is already an iOS serial cable though, which I would think would be a better option for many projects:

http://redpark.com/c2db9.html

" Under current Apple policy this cable may not be used with apps sold on the App Store."

That's an odd condition of Apple's terms. Why is this so? If I go through the process of designing and approving a hardware device, I can't sell an app to go with it? Or do I have to provide the app in tandem with the device?

"may not be used with apps sold on the App Store" probably just means "Apple will not approve an application which uses this cable for sale on the App Store".

But Apple approved the cable. Get the paradox?

Where does it say that Apple approved the cable? It seems to me like they're deliberately avoiding any step that requires Apple approval.

It's right on the front page.

You can't use the "Made for iPod" label on your product unless it's Apple approved and has an Apple-supplied chip in the dock connector.

I wrote an app that uses this cable. You CAN distribute an app using serial communications, but ONLY with a custom cable variant created by Redpark. This custom cable will ONLY work with your app, and your app will only work with this custom cable. Apple will approve this app because Redpark has done the legwork of MFI compatibility. What the web page points to is only a "evaluation" system, not production ready.

Thanks for that information. It's hard to get any understanding of what the whole Apple developer program is regarding the dock connector. Obviously the keynote dream of "look at all the things we'll be hooking up to iPhone/iPad!" is never gonna happen.

While I think the other commentators have the main reasons, I'll add that this solution also let's you charge the iPad at the same time.

i'd think potentially being android/whatever compatible as well.

Neat, we've been doing that for a while for Android.

http://phinominal.com/sited/

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