# Newbie question - building a locking circuit

Discussion in 'General Electronics Discussion' started by KarenRei, Feb 22, 2013.

1. ### KarenRei

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Feb 21, 2013
I'm a relative beginner to electronics. I have a general understanding of electronics, I can solder, can do Kirchoff's Law, etc. But I don't have much practical experience with making circuits.

I'm also in the middle of planning to build a house, and I want there to be a really unique lock on the front door. I'm envisioning the lock as a hole with several electrodes in it. The "key" would be a random, non-uniform, irregularly shaped natural semiconductor (for example, a rock or gemstone or something) - something with non-uniform, frequency dependent capacitance and such. The negative electrodes would pump into the "key" a haphazard but consistent pattern of AC at different voltages and frequencies. The positive electrode would be connected to a circuit to measure the current returning through the "key". When programming the lock, the measured values would be recorded in nonvolatile memory. When testing the lock, the measured values would be compared to the recorded values, and if a match (within a reasonable tolerance), the lock would be toggled - locked if unlocked, unlocked if locked.

It doesn't *sound* like something that complicated, but I've never worked on such a project before and honestly don't know where I'd even start. Could you point me to a starting point for what sort of hardware I'd be looking at?

Thanks.

2. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
It's good to see nubes getting their feet wet with basic circuits. Do you have an arsenal of lab instruments and a machine shop available to you?

Chris

3. ### CocaCola

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Apr 7, 2012
So many variables so many things that WILL go wrong and NOT work as expected...

To the OP can you make an LED blink at 200Hz for 3.3 seconds, 222Hz for 2.7 seconds and 51Hz for 6.5 seconds? If not you might want to learn to crawl before you try out for an Olympic running team...

Honestly though it's good to see dreams but sometimes it's best to dial back those dreams to reality until or when you have the money and/or knowledge to make them reality...

4. ### KarenRei

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Feb 21, 2013
I have the money.

So is the basic summary that this *is* complicated, even though it doesn't sound complicated? I should add that I am a computer programmer by trade so I have no issues with learning to program a microcontroller.

Coca-Cola: I could draw a circuit diagram of a circuit that would do that. But as mentioned I have little real-world building experience.

Last edited: Feb 22, 2013
5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
No, the issue is that it *IS* complicated.

Do you even have any idea about the random non-linear semiconductor (or whatever) properties and whether that will change randomly with placement or any of a million other things?

6. ### sirch

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Dec 6, 2012
Surely picking the front door lock is not top of a theif's list of ways into your house. Better to spend your time putting in a security system?

7. ### CocaCola

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Apr 7, 2012
The circuit *is* easy in theory it's the millions of variables you will face that you have to attempt to compensate for in software and hardware if you want a chance of getting it to work reliably... And by the time you make the logic so fuzzy that it compensates for all the possible variables, it's likely to be so loose that it won't be much of a lock at all... On the flip side if you miss one single potential variable you are locked out...

Not when you have easy access glass windows in the house...

8. ### KarenRei

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Feb 21, 2013
Isn't that what I just said?

Probably not much if there's only one way to fit. And more on that in just a second. As for the "whatever":

"Random" - not anything specifically generally intended to be a key
"Non-uniform" - not having a consistent uniform structure throughout - variabilities in material, density, structure, etc
"Irregularly shaped" - self-explanatory

Who said anything about thieves? Nowhere did I state that the objective was to make something that was harder to break into than regular locks; I stated "really unique lock", not "really secure lock". "Unique" is the objective. The lot I'm working on buying is in the countryside, and this is a pretty low crime country to begin with. We're also planning on putting in things like "secret passages", one-way mirrors, a door that leads nowhere, etc. The goal is to have a place that's unique and fun, especially for children.

What "millions of variables" are you talking about? And if the main challenge you're envisioning is ensuring a good and consistent connection, I already thought of that, and if it turns out to be an issue, the solution is to securely bond matching connecting electrodes to the "key". But honestly, if there's only one way for the "key" to fit, it fits securely, and neither the key nor the electrodes are something that wear or corrode, I doubt it would prove necessary.

Last edited: Feb 22, 2013
9. ### CocaCola

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Apr 7, 2012
The ones you will encounter once you actually build said device, everything from environmental changes from one extreme to the other to errant radio interference, or environmental contamination of the 'key' and the list could go on and on...

You seem to be one of those people that has it all figured out already and will blindly argue even though you only grasp the very basic concepts involved, and that is a very slippery slope to be on...

Have you done any proof of concept prototyping? What were your findings thus far?

10. ### Osmium

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Jan 28, 2013
It's extremely complicated. You need to do a LOT of research. This is related to materials signature analysis. The following link describes using signature analysis for known componentry in circuit boards. You are trying to do something similar with a lump of rock. The REALLY hard part is determining what characteristics of the lump of rock used in your key make it unique and different from any other lump of rock.

I would strongly suggest that you heed the advice of the other respondents and either:

a) Abandon the idea - it's just too hard for an inexperienced individual to learn and undertake the scientific research needed in a 'reasonable' length of time - say two years or more of full time research.

OR

b) Think of another way to achieve the end that you want - using a lump of rock as a key. My thoughts on this are that you simply wire up the rock with a set of contacts which are either linked in pairs or not so that the connections represent a number. Then sense the number to open the lock... (you still have a lot to learn to do this but it's feasible and achievable).

11. ### KarenRei

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Feb 21, 2013
CocaCola: If I "had it all figured out", I wouldn't be here asking for information.

Osmium: Thanks for the paper!

12. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
I always find it fun when introductory statements are prefaced like this.
Then, within the same post we see a statement like this.

FDC is not exactly a commonly discussed topic. I'd venture to say that you could (I haven't) search this forum for hours for FDC and not get a single hit because it's not something that you'll experience below the microwave band. It's hardly a nubie topic.

Moving on to your last post we have morphed from your intro to this....

Something doesn't add up here. College project??

We need a "Skeptical" in the smilies.

Chris

13. ### KarenRei

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Feb 21, 2013
Excuse me, but both resistance and capacitance can change with frequency, and it doesn't have to be in the GHz band. Whether or not it's widely discussed here doesn't change the fact. You're thinking in terms of capacitors designed to be as close to ideal capacitors as possible. The real world is complicated. Even frog skin has variable capacitance in the Hz/kHz range.

And yes, I wrote a Kirchoff's law calculator years back. Want the code? Believe it or not, people can have an interest in learning about technology before they actually physically mess with it much.

Fine. You've all made it clear that you find it more fun to troll newbies than to answer or explain your viewpoint. I have better things to do than be your punching bag. And anyway, someone on another forum kindly pointed me to PICAXE, which looks to be right about what I'm looking for. Imagine that, actually helping someone who asks questions instead of insulting them.

Next time, if you decide you'd rather actually be helpful, if you feel a project won't work, try starting off with a simple explanation of the problems rather than, basically, "won't work, you're an idiot", which seems to be what most posts here (excepting Osmium's, who was at least kind enough to include a paper on the topic while expressing his viewpoint and suggesting alternatives) sum up to. My god, I can't imagine responding like that if someone new to programming came to me with a programming project which I felt was unrealistic.

Last edited: Feb 22, 2013
14. ### CocaCola

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Apr 7, 2012
LOL, I suspect you will discover REAL fast that the PICAXE is a toothpick when you need a bulldozer... Just because someone gave you advice doesn't mean it was good advice or they understood the problem...

We did, you just didn't like those answers and knew/know better...

You wouldn't tell them it was unrealistic and extremely complicated, likely well beyond their abilities like you were told here? The fact that you didn't like the answers doesn't mean the answers lacked substance...

CD I believe you might be onto something way to much terminology being tossed around for a true newbie...

At the end of the day if security is not the end game 'spoofing' the effect by practical means can be accomplished in days or weeks on a small budget vs years on a huge budget... One could easily make a pocket in a solid piece of rock, drop in a RFID chip, and cover with color matching polymers... Using a crystalline rock like granite would make this dead simple to hide and for all practices invisible... We could also go to the dead simple extreme of embedding a neodymium magnet in the rock and a magnetic read switch in the lock hole... The effect is 'spoofed' for a few bucks and an elementary circuit...

15. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
If you just want something that looks unique, you don't actually need to measure all the characteristics of the key material. You could use some conventional system to provide the actual key validation to control the lock, in conjunction with any kind of unique-looking material for the wow factor.

It could just be an RFID tag embedded into a piece of cool-looking mineral made into an appropriate shape.

Edit: Bugger! That's just what CocaCola suggested 14 hours ago! I should have read more carefully.

16. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
I never said or implied that you're an idiot. In fact I implied precisely the opposite. Your writings tend to lean toward having a physics education?

RE "Frog skin": I didn't think we were discussing organic/biological matter. They're riddled with variables and wild cards. Last year I thought I'd take a chance on purchasing some of those new environmentally friendly (green) caps with Frog skin dielectric but I had to return them because they were exceedingly unstable. I would not recommend caps with Frog skin dielectric to my friends here.

RE "Other forum":

http://www.eeweb.com/electronics-forum/newbie-question-building-a-locking-circuit/

You're pleased because you were told what you wanted to hear. The reply you refer to implied that you're correct in your assumption that your concept is easily achievable.

Chris