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Slanderer posted:I don't know where you got 22 ohms from (I pulled my numbers from this) but I recommend you don't do that. I wish I could remember. I found the info when I was researching thermistors since it wasn't anything covered in my schooling yet. I found a replacement NTC 5D-15 in a surplus power supply I had laying around. I'll do a 1:1 with that piece and see how it goes powering a couple 12v lamps for a day before I think about sticking it back in my PC or using it as a bench supply. Inrush current limiting makes sense now that I think about it.
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Nov 27, 2012 03:17
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Wow, I haven't checked out this thread in ages. Anyway, RF time! Lets say you form a series resonant circuit from a coil of wire and an appropriate capacitor. The impedance seen from your source will be purely real. If you bring anything conductive near it - a piece of metal for instance - its impedance will shift towards capacitive (this is easy to observe on a network analyser), as the effective inductance is reduced by the opposing eddy currents in the conductive medium. My question is are there materials that would instead be seen from the source as increasing the inductance of the circuit? I.e., apearing as an inductive load instead of capacitive?
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Nov 27, 2012 07:39
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I never realized how sensitive ceramic caps are to temperature changes. I was measuring a cap on my multimeter and it started at 8.8nF and was slowly dropping, a few pF every second. So the test current from the multimeter was warming it as it measured. To test the effect of heat on capacitance, I rubbed the cap in my fingers and the capacitance dropped to 8.0 nF. Then I rested a frosty mug on it and it went up to 10nF. I hadn't really given much thought before to how much effect temperature plays in a circuit's performance. But 20% or 25% variance (depending which way you look at it) from freezing to body temperature seems pretty significant. Is this sort of variance typical or just a really cheap and lovely cap? Are there specific types of capacitors with much lower temperature dependence?
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Nov 27, 2012 08:02
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peepsalot posted:Is this sort of variance typical or just a really cheap and lovely cap? Are there specific types of capacitors with much lower temperature dependence? There are different ceramic capacitor types, with different temperature coefficients. I generally use X7R, since they're not too lovely, nor are they too expensive. There is a graph of temperature characteristics for different materials on here: http://www.johansondielectrics.com/technical-notes/product-training/basics-of-ceramic-chip-capacitors.html
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Nov 27, 2012 08:07
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Cyril Sneer posted:Wow, I haven't checked out this thread in ages. Anyway, RF time! I don't think so, if I'm understanding your unspoken assumptions correctly--something that is added parasitically to the circuit without breaking it. However, a ferrite bead/choke added over a section of wire in the existing circuit would slightly increase the inductance of the wire it was over...maybe. If it did, it would be extremely slight, due to the geometry. But I'm not sure that would be what you're looking for. On the other hand, if you were allowed to bring a ferromagnetic core material anywhere near that coil of wire, you'd definitely be able to increase the inductance. For an even less effective case, consider an inductor that is capactively coupled to the circuit at both ends.
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Nov 27, 2012 08:14
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peepsalot posted:I never realized how sensitive ceramic caps are to temperature changes. I was measuring a cap on my multimeter and it started at 8.8nF and was slowly dropping, a few pF every second. So the test current from the multimeter was warming it as it measured. To test the effect of heat on capacitance, I rubbed the cap in my fingers and the capacitance dropped to 8.0 nF. Then I rested a frosty mug on it and it went up to 10nF. I hadn't really given much thought before to how much effect temperature plays in a circuit's performance. But 20% or 25% variance (depending which way you look at it) from freezing to body temperature seems pretty significant. I only typically only use X7R or C0G if I'm using ceramics. X5R in a pinch for decoupling, but nothing more. Not only does temperature play a role, but DC bias has a significant effect as well. If you want to have some more fun, and you have a scope, probe across the cap with the gain cranked up and tap on the cap. Cyril Sneer posted:Wow, I haven't checked out this thread in ages. Anyway, RF time! I think you have this backwards - increasing the relative permeability of an inductor core should increase the effective inductance. That's why inductors wrapped around ferrite can be much higher value than air core. Vacuum has a permeability of 1, and air is very, very close. I suppose you could have a material that has a permeability of less than 1, but I imagine it would be something fairly exotic, with atypical material properties. Maybe a superconductor?
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Nov 27, 2012 08:22
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SnoPuppy posted:I think you have this backwards - increasing the relative permeability of an inductor core should increase the effective inductance. That's why inductors wrapped around ferrite can be much higher value than air core. Whatever the effect is, you could use some metamaterials to gain the opposite effect for a small frequency range. VictualSquid fucked around with this message at 11:27 on Nov 27, 2012 |
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Nov 27, 2012 11:25
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SnoPuppy posted:I think you have this backwards - increasing the relative permeability of an inductor core should increase the effective inductance. That's why inductors wrapped around ferrite can be much higher value than air core. See, that all makes sense, but it's not what I'm seeing. I'd encourage anyone whose interested to fashion a coil and tune it to series resonance then monitor its response on a network analyzer. If you bring a metal plate near it, the whole response shifts capacitive. Maybe I'm misinterpreting this result? I know argument-by-symmetry isn't exactly scientific but it seems that if you can parasitically load a coil one way, it should be possible to go the other way as well.
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Nov 27, 2012 15:22
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Cyril Sneer posted:See, that all makes sense, but it's not what I'm seeing. I'd encourage anyone whose interested to fashion a coil and tune it to series resonance then monitor its response on a network analyzer. If you bring a metal plate near it, the whole response shifts capacitive. Maybe I'm misinterpreting this result? That's pretty much the expected result, however. It's easy to detune certain RF circuits by bringing a finger near certain spots. Put a ferrite core near the coil, and it'll add inductance to the circuit. That's all you can do, really.
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Nov 27, 2012 15:43
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Slanderer posted:That's pretty much the expected result, however. It's easy to detune certain RF circuits by bringing a finger near certain spots. Sure, but this isn't really a rigorous understanding of the phenomenon. The idea is that loading a coil with conductive material should increase its effective inductance, however, this is not the result I get when I monitor this on a Smith chart with a network analyzer. Any kind of loading pulls the circuit off-resonance and makes it appear capacitive. The only thing I can think of is that I'm introducing so much parasitic capacitance that it swamps the change in inductance. This is kind of hand-wavy though. Slanderer posted:Put a ferrite core near the coil, and it'll add inductance to the circuit. That's all you can do, really. I'll try. I need to find a plate of ferrite. That might do some interesting things.
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Nov 27, 2012 17:05
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Cyril Sneer posted:The only thing I can think of is that I'm introducing so much parasitic capacitance that it swamps the change in inductance. This is kind of hand-wavy though. quote:I'll try. I need to find a plate of ferrite. That might do some interesting things. This may be the issue. Bringing something conductive near the circuit isn't enough, you need something with a sufficiently high magnetic permeability. Ferrite, iron, steel (although the permeability varies wildly for different steels), etc... However, iron and steel will be quite conductive, which means big eddy current loses at high frequencies, which can minimize the effect of the added inductance (possible to the point that the added capacitance dominates). Ferrite, being nonconductive, won't have that problem, but it doesn't have quite the permeability. You should be able to get away with grabbing a nail and bringing it near your coil (pointing into/out of the coil). Other geometries can work, but not nearly as well, which means that you can again find the capacitance of a big piece of metal (plus your body, if you're holding it) dominating the impedance. Also relevant--what frequency are you working at?
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Nov 27, 2012 18:00
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peepsalot posted:Is this sort of variance typical or just a really cheap and lovely cap? Are there specific types of capacitors with much lower temperature dependence? This article about capacitors happened across my inbox today and I thought of your post.
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Nov 27, 2012 21:07
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Is there a post about fabrication places anywhere? I could have sworn it was in the OP but I don't see it. A friend of mine wants to have this built: http://ts.md-network.de/cocoon/theme2/yprpb2rgb.html#top
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Nov 28, 2012 00:43
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SnoPuppy posted:Ah, almost everything I do uses custom enclosures so I forget that you can buy them off the shelf. You still might be able to make it work if you hack up their standard enclosure. Do you have experience with custom diecast aluminum enclosures then? I have no idea if getting one custom is economical. It's for the 1K-10K volume range. On average it seems like off the shelf models are $30-100, which seems very pricey. quote:Also, I've found that using something like sketchup can be extremely valuable for communicating what I want to customers/MEs/vendors/etc. It's easy to learn if you haven't tried it yet, and the free version does almost everything I need.
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Nov 28, 2012 03:13
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Cyril Sneer posted:Wow, I haven't checked out this thread in ages. Anyway, RF time! Example, put a nice conductive sheet (like copper clad) over the coil, you should see resonant frequency increase due to eddy currents. Try a sheet of copper mesh, and frequency may decrease instead, since the mesh suppresses eddy currents while allowing displacement currents (effectively increasing capacitance). Same with lossy materials, like body tissue (saline). I see this sort of stuff all the time when tuning MRI coils. The effects are often dependent on the shape, size, frequency, and Q of the coil. For reference, I usually work in the range of 50-150MHz, with coils between 1-15cm wide, Q between 30 and 300. For fun, try etching/cutting a pattern on copper clad that separates the copper into very thin, long, parallel strips which do not contact each other. Position it so that the strips are parallel to the E field from the coil. You should see the capacitance jump substantially, without any apparent loss in Q. ANIME AKBAR fucked around with this message at 03:28 on Nov 28, 2012 |
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Nov 28, 2012 03:25
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ANIME AKBAR posted:Yeah I use sketchup, but it sucks for showing 3D models, unless the other person actually has sketchup installed and just opens the files. And getting a client to do that is usually a stretch. Check out Autodesk FBX Converter. It's a free application that can convert Sketchup's COLLADA file output to an FBX file and from that to pretty much any other standard 3D CAD format. I use it to draw stuff up in Sketchup that I then ship off to my mechanical engineer for integration into the full system design, and while it introduces an extra file conversion step there are loads fewer errors than when I try to explain ideas in words and badly hand-drawn sketches and hope he draws the thing that's in my head.
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Nov 28, 2012 05:04
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I think it's awesome that this thread exists (and disappointing that most of the images in the OP were lost in the Wafflecaust). Electronics has been something I've always been interested in, and I played with it a fair bit since I was young, but every time I try to do something substantial I always get overwhelmed. Anyway, emboldened by my reading of the thread, I made this in Circuit Simulator: Ideally, what I want the circuit to do is run off of the "12v DC" source, but if it is disconnected (or the voltage drops) I want it to automatically switch over to the battery source. The 1k resistor is there to simulate my load (I'm not sure how much resistance the load will be but likely more than 1k). As for the capacitor, it may not be necessary, but I used it to help smooth out any voltage drop from the switch. My question is: is this kosher? Will it do what I think it will do? Or is there some gotcha I have missed, and it will destroy my 12v source and battery, start a fire, and kill literally everyone?
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Nov 29, 2012 15:27
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Are there any print magazines for EE stuff worth subscribing to? I'm about 90/10 software/hardware at work, but I'd like to get better at the hardware side of things both to make myself a better engineer and also so I can do more interesting hobby stuff on the side. Also there is a dearth of stuff to read in my bathroom, I wind up reading the label on the toothpaste tube sometimes 
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Nov 29, 2012 15:46
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Michael Arlowe posted:I think it's awesome that this thread exists (and disappointing that most of the images in the OP were lost in the Wafflecaust). Electronics has been something I've always been interested in, and I played with it a fair bit since I was young, but every time I try to do something substantial I always get overwhelmed. Anyway, emboldened by my reading of the thread, I made this in Circuit Simulator: A Diode OR is dead simple, but a dedicated IC could do the trick too. Assuming you can live with slightly less than 12V. Otto Skorzeny posted:Are there any print magazines for EE stuff worth subscribing to? I'm about 90/10 software/hardware at work, but I'd like to get better at the hardware side of things both to make myself a better engineer and also so I can do more interesting hobby stuff on the side. Well, there's always the shampoo bottle. Seriously though, a lot of the "trade" mags are really lovely, with a few gems hidden inside. The poo poo is all ads and sponsored garbage, the gems are the few columns/articles inside about interesting problems and how they were solved by some awesome engineers. Very low signal to noise ratio. That being said, easy to get for free, just throw around corporate creds and EDN will start showing up in your mailroom.
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Nov 29, 2012 15:51
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movax posted:A Diode OR is dead simple, but a dedicated IC could do the trick too. Assuming you can live with slightly less than 12V. Thanks. I tooled around with this in the simulator and wow, it really is THAT easy. My original design had a relay with a protection diode, a capacitor to deal with the switching delay, and a bunch of other useless crap. The circuit I posted was the "simplified" version.  I still am going to need one mosfet, because I have to send a signal to a gpio pin when the device is running on battery power, but this setup means a lot less soldering.
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Nov 29, 2012 17:14
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Michael Arlowe posted:Thanks. I tooled around with this in the simulator and wow, it really is THAT easy. My original design had a relay with a protection diode, a capacitor to deal with the switching delay, and a bunch of other useless crap. The circuit I posted was the "simplified" version. If you don't want to drop all that sweet, sweet voltage across the diode, however, you might consider a simple switching using a comparator, a mosfet, a couple resistors and possibly a cap (if you want to add a switching delay).
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Nov 29, 2012 17:19
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Otto Skorzeny posted:Are there any print magazines for EE stuff worth subscribing to? I'm about 90/10 software/hardware at work, but I'd like to get better at the hardware side of things both to make myself a better engineer and also so I can do more interesting hobby stuff on the side. Elektor and Circuit Cellar are pay magazines that are decent. Neither has a lot of "pure" learning articles, though they do have some of that. They are both mostly about projects. My dad was a long time Circuit Cellar subscriber, so I grew up reading it. There are plenty of free magazines. EDN, ECN and EE Times are the major ones I know of. Like movax said, they have the problem of being supported by ads. Most articles are going to shill for some company or other. But, there are some gems in there from time to time. You can find some good stuff in the eetimes.com archives. You might also like Design News and Machine Design. Not electronics related, but they have some neat stuff. The teardowns in Machine Design are fun.
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Nov 29, 2012 17:28
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Otto Skorzeny posted:Are there any print magazines for EE stuff worth subscribing to? I'm about 90/10 software/hardware at work, but I'd like to get better at the hardware side of things both to make myself a better engineer and also so I can do more interesting hobby stuff on the side. Circuit Cellar is pretty good IMO. Most of the articles are based around DIY projects, but they're usually much more advanced than typical hacker-level stuff. They like, actually build circuits themselves. And they have a couple good contributors who consistently put out good technical articles. I think I still have the "build your own spectrum analyzer" series of issues sitting around.
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Nov 30, 2012 03:05
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ANIME AKBAR posted:I think I still have the "build your own spectrum analyzer" series of issues sitting around. Do they have back issues easily available, because that spectrum analyzer bit is 
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Nov 30, 2012 05:05
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I have some PTFE spray, and some lithium based grease. Will the PTFE negatively affect electrical contacts? I know silicone based sprays can form a layer over electrical contacts... The lithium grease says on the tube that it's designed for keeping electrical contacts moisture free and protects against oxidization so I think that should be fine...
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Nov 30, 2012 12:25
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Otto Skorzeny posted:Do they have back issues easily available, because that spectrum analyzer bit is http://www.cc-webshop.com/CC-Gold-The-ENTIRE-Circuit-Cellar-Magazine-Collection-CCGOLD.htm http://www.cc-webshop.com/Download-Archive_c2.htm
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Nov 30, 2012 16:16
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ANIME AKBAR posted:Well it seems like the client will let us get away with using a metal enclosure, though there still doesn't seem to be anything that really suits our needs (need to be IP65 rated still, but has to have appropriate mounting hardware so that the electronics can be pressed against the enclosure wall (probably with an aluminum shim in between) to get good thermal performance. And it shouldn't cost >$100... 1K-10K should be enough for a custom cast aluminum enclosure. You might have to spend $50K in tooling/NRE, but the piece price might only be like $10-$15. That would put the total cost at around $20 per enclosure, assuming an overall volume of 10K. Those are ballparks based on my last design, which was fairly complicated. The other nice this is that almost all your mounting hardware and features can be part of the casting, so you only have to do minimal post processing. A lot of the cost really depends on how complex you need to make the enclosure. If you need multiple slides, really fine features/wall thicknesses, post machining work, etc., then the cost will go up. If the design is relatively simple then the NRE and piece price might be much less. This is the place that I, or at least the mechanical engineers I work with, have used: http://www.mgprecision.com
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Nov 30, 2012 17:27
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Is there such a thing as a single-chip (8-pin or less) or otherwise very compact very low frequency pure sine wave oscillator? I want to create a ~6-8V peak-to-peak 10 Hz sine wave for use in a cable tester, I think I need the voltage since I need to drive LEDs through long cables, through capacitive coupling and some diodes. One thing I thought of is doing DDS on a small 10F202 MCU, then I'd just need an opamp connected to a 9V battery for gain, alternately I'll just put a small gain stage and use my phone as a signal generator. Another alternative is to just use a tiny AC wall wart, and increase the series resistance... Used for driving this:  It's an XLR tester: Basically I use an AC signal and half wave rectify it to positive and negative flanks, positive bit on the hot wire, negative on the cold and return through ground. At the other end there's two diodes in parallel per wire, one will light up if the signal is flowing the right way (i.e. positive for hot, negative for cold), and red diodes that light if the wires are crossed, or there's leakage between the wires. The capacitive coupling is there to eliminate DC-bias since otherwise it could light up a diode permanently. I could use a 555 but I tested the circuit with a square wave and I didn't like it, plus the capacitive coupling really messes up the pulse shape.
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Dec 1, 2012 23:43
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I'm having some trouble figuring out what to do with capacitors when looking at the frequency response of transistors. Take this circuit for example: The values of R1 and R2 are 10k, R3 is 40k and C is 10uF. The first part of this calls for finding the transfer function at low frequencies. What I did was I looked for the impedance contribution of the capacitor by I=1/(2*pi*f*C), taking f to be 100Hz. This gives about 159ohm, which to me should act as a short circuit when the other resistors are in the kOhm range, and short R2. However, the solution treats the capacitor as an open circuit and involves R3 in the transfer function. What's going on here?
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Dec 2, 2012 00:30
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It looks like the assumption is taking low frequencies to mean DC, which would make the capacitor look like an open (as frequency approaches zero, impedance approaches infinity).
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Dec 2, 2012 00:38
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longview posted:Is there such a thing as a single-chip (8-pin or less) or otherwise very compact very low frequency pure sine wave oscillator? I want to create a ~6-8V peak-to-peak 10 Hz sine wave for use in a cable tester, I think I need the voltage since I need to drive LEDs through long cables, through capacitive coupling and some diodes. As far as I know, this will be a problem with newer ICs---DDS ICs will be operating at way too low a voltage, I'd imagine. quote:The capacitive coupling is there to eliminate DC-bias since otherwise it could light up a diode permanently. I could use a 555 but I tested the circuit with a square wave and I didn't like it, plus the capacitive coupling really messes up the pulse shape. What's did't you like about the square wave? It should, in fact, work better, so long as you aren't overloading the output (ie, a buffer may be required). You're RC network probably isn't helping the situation, either--it looks like a high-pass filter, though I can't be bothered to calculate the response. Take a step back, for a moment, and think about what a commercial cable testing gadget would do, and how it would be designed. The behavior is so simple that it is probably just using a simple oscillator circuit, buffered to drive the cable. Really, the 555 is the easiest, and should work as long as you choose your capacitors correctly (don't want anything that will be drastically changing capacitance with temperature and/or voltage). Then you can filter it, if you like, and buffer the output. Alteratively, make a fixed-frequency wien bridge oscillator or something. Lastly, you could just just go full old-school analog IC. This had the added benefit of allowing you to operate from 9+ voltages directly. Options include the XR2206 and the ICL8038 (both, I think, I made by others as well, albeit with different prefixes). They are both 16 pin, I think, so bigger than you'd like, but they'll generate the sine waves you want....
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Dec 2, 2012 01:26
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Delta-Wye posted:It looks like the assumption is taking low frequencies to mean DC, which would make the capacitor look like an open (as frequency approaches zero, impedance approaches infinity). This. Solving for the transfer function, you'd find that the impedance contributions of the capacitor to the equivalent of R3 || C will be small, and will diminish further as you approach DC, until it's just an open circuit.
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Dec 2, 2012 01:29
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longview posted:I could use a 555 but I tested the circuit with a square wave and I didn't like it, plus the capacitive coupling really messes up the pulse shape. Buffering the voltage on the cap will give you an sort-of-triangle wave, especially if it's near 50% duty cycle. However, it's probably simpler to build a triangle oscillator with a dual opamp, like this: http://www-k.ext.ti.com/SRVS/Data/ti/KnowledgeBases/analog/document/faqs/sscco8.htm There's ways to shape a triangle to an approximate sine if you really want a sine wave, generally by soft-clipping the signal. A triangle isn't a terrible approximation to a sine depending on what you're using it for, though.
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Dec 2, 2012 01:35
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Ok thanks, I was caught up on the original design (which worked but needed a proper signal generator to drive it), I will try to keep the diode matrix at the far end, but I will try to build a relaxation oscillator using an opamp. Hopefully I can make it drive itself and the LEDs without buffering. Then I can use the other half of a dual opamp to drive the ground-pin to 1/2 VCC and remove the need for the capacitive coupling to give an AC signal. If not I'll get a quad opamp. I'd like to keep the diode system, where hot only had positive pulses and cold only has negative since it makes it very reliable in terms of detecting crossed wires. E:  There we go, it works in simulations with 9V power. Ignore the fact that there's no series resistor with the LED. longview fucked around with this message at 13:07 on Dec 2, 2012 |
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Dec 2, 2012 12:42
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longview posted:I could use a 555 but I tested the circuit with a square wave and I didn't like it, plus the capacitive coupling really messes up the pulse shape.
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Dec 3, 2012 09:11
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SnoPuppy posted:1K-10K should be enough for a custom cast aluminum enclosure. You might have to spend $50K in tooling/NRE, but the piece price might only be like $10-$15. That would put the total cost at around $20 per enclosure, assuming an overall volume of 10K. Those are ballparks based on my last design, which was fairly complicated. While we're on the topic of manufacturing, does anyone have any real experience with designing PCBs for automated assembly? In particular, has anyone done a design with press fit components? I'm having to do one right now, but I'm unsure how incorporating press fit stuff will fit into the manufacturing process.
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Dec 4, 2012 13:57
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ANIME AKBAR posted:10-15$ each at 10K sounds like a bargain to me, especially if it means that it can have mountings and thermal spreaders built in. Just remember, it will depend on size and complexity. I'd really recommend contacting a vendor with some rough specs/rough 3d model to get there take on the design. ANIME AKBAR posted:While we're on the topic of manufacturing, does anyone have any real experience with designing PCBs for automated assembly? In particular, has anyone done a design with press fit components? I'm having to do one right now, but I'm unsure how incorporating press fit stuff will fit into the manufacturing process. Guilty again  I'm not sure what you want to know, but press fit isn't hard to deal with from a PCB design perspective. The only thing to keep in mind is that you should have clearance for the bottom support - I think 100 mils from the press fit pins should be enough, but your assembler should be able to give you DFM rules. As far as the actual process goes, it happens after you reflow the top and bottom side. The board is placed on a support jig in a hydraulic press that has been fitted with appropriate tooling to push on the connector. Then the connector is pressed into place. Typical forces are around 5-7 lbs/pin, so it's not something you can do without tools for anything vaguely dense. The connector vendor should also sell you the appropriate top tooling to press the connector, but it's generally up to the assembly house to provide the bottom support fixture.
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Dec 4, 2012 16:46
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Does anyone have a good link or tutorial that would explain how a capacitive touch sensor works, and how one would DIY one up? I'm interested in making one that I can read with an MCU and will report different values depending on where along its length I press it. So, I guess like a slider. It's either harder than I thought to find this info, or I'm just not searching for the right keywords though. some kinda jackal fucked around with this message at 04:13 on Dec 6, 2012 |
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Dec 6, 2012 04:09
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Martytoof posted:Does anyone have a good link or tutorial that would explain how a capacitive touch sensor works, and how one would DIY one up? Microchip has tons of only tangently PIC related application notes: http://ww1.microchip.com/downloads/en/AppNotes/01101a.pdf Their note on BLDC motors was fantastic as well, and one of the more helpful sources I had while trying to learn about them. It featured a PIC-based solution, of course, but would be fairly applicable to any controller. Their cap sensing stuff looks the same. Here is a cap dev kit with some sliders, you can see how they arranged the top layer from the product page. Some digging may get you some more information like a schematic or a walkthrough of the design. http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en541143
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Dec 6, 2012 04:45
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Awesome, thank you
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Dec 6, 2012 04:48
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