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Metajo Cum Dumpster posted:Thanks for the info on the circuit. You'll be able to use it if you can figure out how it works. It's probably proprietary. Good luck finding datasheets for it.
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Jan 1, 2010 03:07
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I just designed and built a simple gps receiver module which I am happy to report worked on the first try! This is a first on many fronts for me so I'm a little surprised it worked that easily.  Now that this proof of concept is done, I would like to optimize it. The problem here is I am new to rf design considerations and just plain electronics as a whole. Can any of you suggest good reading material on rf stuff? Ground planes, pcb trace widths, rf vs digital grounds, etc are all a mystery to me. I can follow datasheet design rules, but I would be much happier understanding why I am doing these things. Two of the burning questions in my mind are: - I used a calculator to figure out the size of the 50 ohm trace for my particular pcb board type and it did not seem to take into account the length of the strip.. so length doesn't matter? Only width and the strip's distance to grounding planes? What is the 50 ohms referring to? Why is 50 ohms such a popular value too? I have searched around and can't find a solid explanation for this. -Is it normal to have the ground plane for the rf signal be the same ground used by all other digital signals or should this have been separated. From reading around it seems it is fine to join them.. but in the past it was preferred to use more than one plane. Also, down the road I would like someone who knows what they are doing look over this design. If you have experience working in the 1-2Ghz rf range let me know. I can't pay much, but I will try to make your time worthwhile. Happy new year! macpod fucked around with this message at 06:23 on Jan 1, 2010 |
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Jan 1, 2010 06:20
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for microstrip antennas, width determines the characteristic impedance (along with the dielectric material and thickness). length should be matched to the carrier frequency (generally quarter wavelength). using the same plane for the antenna and the circuit ground is generally fine (though you should try to separate noisy circuitry from receivers). Keep in mind that your trace antenna is just one pole of the actual antenna, and the ground plane forms the other pole. most of the antenna theory for these setups assume a very large ground plane, so make it as big as allowable. it's hard to separate RF material from its underlying mathematics... and that can be kind of intensive. one possibility might be old HAM radio manuals. generally they give a good balance between qualitative and quantitative analysis. ANIME AKBAR fucked around with this message at 09:03 on Jan 1, 2010 |
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Jan 1, 2010 09:00
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Metajo Cum Dumpster posted:Thanks for the info on the circuit. actually touch screens are usually pretty simple. first figure out if it's a four or five wire type (most are 4). after that you'll need some hardware to interface with it, but I'm pretty sure the hardware won't care about the specific properties of the screen. here are a couple articles on resistive touch screens: http://www.circuitcellar.com/library/print/0100/dahlin-114/2.htm http://focus.ti.com/lit/an/slyt209a/slyt209a.pdf
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Jan 1, 2010 17:01
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macpod posted:- I used a calculator to figure out the size of the 50 ohm trace for my particular pcb board type and it did not seem to take into account the length of the strip.. so length doesn't matter? Only width and the strip's distance to grounding planes? What is the 50 ohms referring to? Why is 50 ohms such a popular value too? I have searched around and can't find a solid explanation for this. What you are building is a type of transmission line, which is composed of (or modelled with) repeating units of inductors, capacitors and resistors. The values for each component give rise to what is called the characteristic impedance of the line. You only care about the per-unit values, as the physical length of the line is modelled by a long chain of the single unit circuit. If you look up the circuit model for a transmission line, this will make much more sense. The 50-ohms is referring to the characterisic impedance of the strip. That is, the impedance seen by the source or load connected to the strip. 50-ohms is special because its the characteristic impedance that is achieved in coax cable when using 'reasonable' sized dielectic and conductor sizes in consideration of power dissipation. There is nothing intrinsically special about the value - it just works well in terms of the materials used in coax cable. macpod posted:-Is it normal to have the ground plane for the rf signal be the same ground used by all other digital signals or should this have been separated. From reading around it seems it is fine to join them.. but in the past it was preferred to use more than one plane. I'm an RF engineer, and general practice is to keep them separate. It is a much-debated topic though, as sooner or later, the grounds will be tied together. I think the more important point is to make sure your digital supply lines are all filtered (series chokes, caps to ground).
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Jan 2, 2010 07:21
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macpod posted:Can any of you suggest good reading material on rf stuff? Ground planes, pcb trace widths, rf vs digital grounds, etc are all a mystery to me. I can follow datasheet design rules, but I would be much happier understanding why I am doing these things. The Art of Electronics has a good chapter on RF stuff, it touches and goes on most topics but you get a good amount of info. Definitely a book to get if you don't have it already. Besides that, David Pozar's Microwave Engineering is the gold standard textbook, but it's pretty dense for self-learning. Some websites cover some information on RF. http://www.microwaves101.com/ and http://www.rfcafe.com/ are more meant for industry insiders, but they have some good tutorials and tips/tricks to help out. http://www.amanogawa.com/ has some really excellent Java animations and calculators to help out with RF stuff. HAM radio and old army manuals are good sources for (somewhat)-easy-to-digest information on this arcane magic. macpod posted:Two of the burning questions in my mind are: What Cyril Sneer said. Length does matter though, a lot. If it's not matched up you lose a lot of energy and get standing waves. Length however does not affect the characteristic impedance, which you can think of as similar to the resistivity of a material (more length = more resistance), but special for RF stuff. macpod posted:-Is it normal to have the ground plane for the rf signal be the same ground used by all other digital signals or should this have been separated. From reading around it seems it is fine to join them.. but in the past it was preferred to use more than one plane. Generally you keep them separated if you can. You can merge them, but you may get weird effects that usually stem from your digital circuits dumping lots of high frequency noise into your delicate RF circuits. It's more important that you keep your supplies separated, as Cyril pointed out. Use different regs for your RF and digital circuits if at all possible, or at least shove an inductor or ferrite bead between the supplies if you have to use the same reg. Decoupling caps on RF components is a must (usually a 1uF cap connected between the supply pin on the IC and ground, with as short a trace as possible), and you should do the same to digital components to keep them happy and not dumping toxic waste noise into your system (digital circuits are kind of jerks). clredwolf fucked around with this message at 17:03 on Jan 2, 2010 |
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Jan 2, 2010 17:00
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Don't forget, Jan 7 9am-11pm mst Sparkfun is giving everyone $100 credit towards anything on the store.
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Jan 2, 2010 19:17
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Metajo Cum Dumpster posted:Don't forget, Jan 7 9am-11pm mst Sparkfun is giving everyone $100 credit towards anything on the store. What does this mean exactly? It sounds neat though.
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Jan 2, 2010 21:04
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Metajo Cum Dumpster posted:Don't forget, Jan 7 9am-11pm mst Sparkfun is giving everyone $100 credit towards anything on the store. Not everyone, the first thousand people (first $100k of merch max $100/person whatever) The site has 94,000 members, of which an unknown percentage are active
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Jan 2, 2010 22:15
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ANIME AKBAR posted:that circuit is a rectfier: if you put in an AC voltage, the top diode will rectify the positive part of the AC and the lower one will rectify the negative portion of the AC. The voltage between the two outputs will a DC voltage of twice the amplitude of the AC input (or equal to its peak to peak voltage). This circuit won't really do anything with DC voltages. indeed it will not, but if you feed it a DC source of alternating polarity, it will charge both of those capacitors to Vin, and give you your voltage doubling. not a very good circuit for much, but it works.
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Jan 2, 2010 23:36
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Metajo Cum Dumpster posted:Don't forget, Jan 7 9am-11pm mst Sparkfun is giving everyone $100 credit towards anything on the store. Also don't forget they "guess the time we sell out by" contest. See this post for details: http://www.sparkfun.com/commerce/news.php?id=312 200 bux! == Thank you for the reading suggestions and those websites. I keep hearing about "The Art of Electronics" book.. so yea I'll probably pick that up when the next paycheck comes in. I wish the new version would come out soon! The HAM radio reading sound like a great idea too. So the ohms measurement is referring to impedance at each "step" or "unit" on the transmission line. After reviewing what you folks said and searching around it sounds like the reason a consistent impedinance is important is to prevent energy from being fed back to the source, then bouncing back, etc. causing distortion. I guess sharp angles can affect capacitance in lines too and this is why they are discouraged on pcbs (in addition to fabrication related issues). Why the value 50ohms was choosen also makes far more sense to me now. The simplified analogy I am thinking of involves water in a trough. If the trough has a consistent width, then no "feedback" waves will be generated. On other hand, if the trough suddenly has a "step" that is narrower than the previous step, some water will hit the barrier and generate a wave that is bounced back to the source. Is that an ok way to think of it? Out of curiosity now, lets say you have a funnel-like transmission line where the source end is narrow and the destination side is wide. What would the effects be? == Before I continue, let me show you the design as-is too:   (ignore the goofy screenprinting towards the bottom, that was dumb) The trace I made goes between the reciever chip and an active antenna. Since it is a transmission line vs a microstrip antenna, I should be more concerned with keeping the line short vs. matching to wavelengths, right? Or do I need to go by some golden ratio? From the little bit of reading I am doing now regarding transmission lines, it seems I shouldn't have much of a problem as long as the length is under 1/4th a wavelength (a little under 2" for gps). I beleive they call these "electrically short" transmission lines. Thanks for the comments on filtering too. The analog gnd and gnd pins in the reciever are connected internally... so as far as my circuit is concerned I don't think I can do much there. The caps are a something I will add in the next design. I don't know why I didn't think about them.. I have used them before to remove ripple and the datasheet for the device even suggest them, doh! You know the more I think about this, perhaps it would be better to learn from someone suggesting improvements and pointing out why they should be done. If you folks would be interested drop me your email and we can discuss beer money. (nelsonjm@macpod.net) Thanks for the help! macpod fucked around with this message at 01:06 on Jan 4, 2010 |
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Jan 4, 2010 00:56
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So I got this electric scooter that uses (apparently) a Hall Effect throttle. It's all kinds of messed up from a previous owner. After lightly disassembling it into the shattered bits inside the shell that the last guy left it at, I see what appears to be a metal ring with a transistor mounted next to it. The rotating part of the throttle moves some magnets around the ring. I can get the scooter to move and the throttle to actuate by waving the transistor thing around in free space. Loosely attempting to re-mount the throttle in its correct configuration makes the scooter regulate its speed somewhat, but not predictably. How do I make a new throttle, or how do I fix this one? What kind of signal is a "hall effect throttle" sending? Should I be doing a voltage- or current-controlled signal?
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Jan 4, 2010 01:08
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babyeatingpsychopath posted:What kind of signal is a "hall effect throttle" sending? If it uses a hall effect sensor like the one sparkfun has in stock this may be helpful http://www.sparkfun.com/datasheets/Components/General/Hall-US1881EUA.pdf
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Jan 4, 2010 01:28
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Otto Skorzeny posted:If it uses a hall effect sensor like the one sparkfun has in stock this may be helpful Nope; as far as I can tell it's an analog device. A digital-latch throttle WOULD be fun, though.
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Jan 4, 2010 01:42
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macpod posted:The simplified analogy I am thinking of involves water in a trough. If the trough has a consistent width, then no "feedback" waves will be generated. On other hand, if the trough suddenly has a "step" that is narrower than the previous step, some water will hit the barrier and generate a wave that is bounced back to the source. Is that an ok way to think of it? Yes, this is a good way to think of it. macpod posted:Out of curiosity now, lets say you have a funnel-like transmission line where the source end is narrow and the destination side is wide. What would the effects be? This would be a matching circuit! macpod posted:The trace I made goes between the reciever chip and an active antenna. Since it is a transmission line vs a microstrip antenna, I should be more concerned with keeping the line short vs. matching to wavelengths, right? Or do I need to go by some golden ratio? So long as the chip input is 50-ohms and your antenna output is 50-ohms (is it really?) then the length of your trace, assuming its 50-ohms, doesn't matter (okay, not quite true, since you'll introduct resistive losses). But yes, try to keep it short.
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Jan 4, 2010 04:02
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babyeatingpsychopath posted:So I got this electric scooter that uses (apparently) a Hall Effect throttle. It's all kinds of messed up from a previous owner. hall effect sensors can have many types of outputs. generally it will be an analog voltage (either single ended or differential) which is proportional to the magnetic field parallel to one axis of the sensor's die. In simple sensors, the signal is very small and needs significant amplification and conditioning to be useful. More sophisticated sensors might give preconditioned outputs. Could you maybe look at the sensor and get a part number? The fact that it reacts noticeably to being moved in free space is.... pretty strange. You certainly wouldn't want that to be an issue when operating the thing.
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Jan 4, 2010 14:57
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Cyril Sneer posted:So long as the chip input is 50-ohms and your antenna output is 50-ohms (is it really?) then the length of your trace, assuming its 50-ohms, doesn't matter (okay, not quite true, since you'll introduct resistive losses). But yes, try to keep it short.
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Jan 4, 2010 16:12
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macpod posted:So the ohms measurement is referring to impedance at each "step" or "unit" on the transmission line. After reviewing what you folks said and searching around it sounds like the reason a consistent impedinance is important is to prevent energy from being fed back to the source, then bouncing back, etc. causing distortion. quote:Out of curiosity now, lets say you have a funnel-like transmission line where the source end is narrow and the destination side is wide. What would the effects be? If you really want to dabble in stuff (and it can be very beneficial to do so), you could look for a magnetic simulator (free ones do exist, I'm sure). It's been a while since I've really studied the stuff. This semester I'll be taking a grad level waves course so I'll probably have a much better grasp of it then.
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Jan 4, 2010 16:24
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I've heard that cheap multimeters are usually horribly inaccurate at measuring very low resistance values. I'm trying to measure something in the 0-10 ohm range, and am getting completely different results every time. If I measure a higher fixed value, say a 100 ohm resistor, and then put the part I'm measuring in series and measure the difference is this likely to be any more accurate, or am I going to have to figure out a better way to measure it?
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Jan 4, 2010 23:15
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Beg, borrow, or steal the use of some better quality equipment. Buying it is an option too, I guess.
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Jan 4, 2010 23:25
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Cyril Sneer posted:So long as the chip input is 50-ohms and your antenna output is 50-ohms (is it really?) then the length of your trace, assuming its 50-ohms, doesn't matter (okay, not quite true, since you'll introduct resistive losses). But yes, try to keep it short. The antenna says it needs a 50 ohm trace, as does the receiver so I assume so. This is the data sheet: http://www.sarantel.com/downloads/specifications/SL1206_SL1204%20Electrical_Guide_v10.pdf They suggest to keep it close to the IC to minimize loss.. in my case I guess the advantage would be reducing resistance since it's already an electrically short trace. So right now I'll try and shorten the trace by rotating the receiver chip 45 degrees and running the trace all the way to the rfin pin. I'm going to put in a 0.1uF and 1uF cap on the IC's vin line to gnd line as close to the chip as suggested too. Should I try to filter the rx/tx lines going to the chip too or will this matter? Any other comments to filter power better other than adding a regulator? With the new orientation, I'll also try and move the lines that are up by the trace away from that area. The reason I didn't put vias under the chip is because the chip itself has exposed vias. Thanks again all, my offer is still open for reviewing for some cash too.
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Jan 5, 2010 05:18
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ANIME AKBAR posted:I've never worked with active antennas before, but I would imagine that in this case the trace going from the active antenna to the gps chip isn't very critical at all. All the critical stuff should be built into the antenna chip. That's generally the point of fancy components like that... Whatever calculation he used for the strip will probably put it at something like 40 - 60 ohms, which is definitely good enough. macpod posted:The antenna says it needs a 50 ohm trace, as does the receiver so I assume so. This is the data sheet: No, filters on your tx/rx lines are not necessary. Why 0.1uF and 1uF? You might want to consider adding series chokes (several uH SMT chokes exist) to your power lines as well. Cyril Sneer fucked around with this message at 06:31 on Jan 5, 2010 |
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Jan 5, 2010 06:28
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I am researching chokes now.. as for the caps, no specific values are mentioned for the chip. 1uF was suggested earlier, and as I started searching around for other gps modules .1uF/.47uF were popular as a second cap 
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Jan 5, 2010 06:53
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Sweevo posted:I've heard that cheap multimeters are usually horribly inaccurate at measuring very low resistance values. I doubt it, but don't know for sure to be honest. The way I see it, the meter might be more accurate in a percentage-sense at 100 ohms, but it wouldn't follow that it is necessarily more accurate in an absolute-sense. That said, if you are getting a reasonable and steady value doing it that way, who am I to judge? You might have some luck (and fun!) with some sort of bridge circuit (http://en.wikipedia.org/wiki/Wheatstone_bridge for example) so you can measure voltages directly instead of resistances. Draw backs abound, from complexity to the requirement that you know some values accurately ahead of time. Delta-Wye fucked around with this message at 09:38 on Jan 5, 2010 |
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Jan 5, 2010 09:35
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Are 3 photodiodes arranged as a triangle sufficient for triangulating the position of a light source? I think I have a handle on the math involved but I'm still working out how to turn it from theory to a project.
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Jan 5, 2010 17:19
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BattleMaster posted:Are 3 photodiodes arranged as a triangle sufficient for triangulating the position of a light source? I think I have a handle on the math involved but I'm still working out how to turn it from theory to a project. photodiodes are generally quite directional, so that's going to be a big problem. try to find emitters and detectors with very large viewing angles/half power angles, and don't operate near those constraints. transconductance matching is an issue to, but that can be calibrated away if necessary. macpod posted:I am researching chokes now.. as for the caps, no specific values are mentioned for the chip. 1uF was suggested earlier, and as I started searching around for other gps modules .1uF/.47uF were popular as a second cap ANIME AKBAR fucked around with this message at 21:42 on Jan 5, 2010 |
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Jan 5, 2010 21:28
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I'm brand new to Arduinos and for that matter, programming. But as I'm in severe need of a new hobby outside of the hardware side of PCs... I thought I'd take up electronics and programming. I ordered an Arduino, it's great. I have already mashed together sketches to blink an LED, perform PWM "fading" on it. I've written to a 4x20 character LCD, though its backlight is fried so you can barely read it. All of this without any C, Wiring, Processing knowhow. But I'm learning. ANYWAY... In preparation for the Sparkfun Free Day, I have made a wishlist of parts that I must get. My first major Arduino project involves making a wireless thermometer probe with a twist. It'll use a solar panel and two of those "xbee" wireless modules, thermometer probe and 4 row 20 column/character version of this LCD with RGB LED backlighting, ordered directly from some guys ebay store in Poland: http://www.youtube.com/watch?v=8ZDXuR6EIi4 What does this mean? I plan on making a thermometer probe that outputs the temperature (and maybe humidity) to the LCD screen. But I also plan on having the LCD change colors based on the temperature. Below freezing? White. And then I'll figure out how to scale it up from there all the way to red for 95+ F.
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Jan 5, 2010 23:50
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ANIME AKBAR posted:photodiodes are generally quite directional, so that's going to be a big problem. try to find emitters and detectors with very large viewing angles/half power angles, and don't operate near those constraints. transconductance matching is an issue to, but that can be calibrated away if necessary. I figured that was going to be a problem. I was about to file down some of the LED package photodiodes that I have until I realized that accurately knowing the half-power angle was actually really important. I'm new to optoelectronics, though. When a datasheet lists a half-power angle, is this what it looks like? 
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Jan 6, 2010 00:19
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ANIME AKBAR posted:photodiodes are generally quite directional, so that's going to be a big problem. try to find emitters and detectors with very large viewing angles/half power angles, and don't operate near those constraints. transconductance matching is an issue to, but that can be calibrated away if necessary. Where did ferrite beads come from? By chokes I'm referring to inductors, which I thought was standard usage.
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Jan 6, 2010 01:14
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BattleMaster posted:I figured that was going to be a problem. I was about to file down some of the LED package photodiodes that I have until I realized that accurately knowing the half-power angle was actually really important. yeah essentially. a lot of datasheets just give a degree number, but what you should really look for is a chart like this:  this is for a photodiode with a half power angle of about 65 degrees (pretty good). but for good accuracy in your application you're probably restricted to 20 degrees. Cyril Sneer posted:Where did ferrite beads come from? By chokes I'm referring to inductors, which I thought was standard usage. same thing, pretty much. ferrite beads typically act as small value, low Q inductors suited for power supply filtering. ANIME AKBAR fucked around with this message at 03:06 on Jan 6, 2010 |
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Jan 6, 2010 02:59
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I can't even get sparkfun to load right now 
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Jan 7, 2010 16:38
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Haven't been able to get it to load all night. They must have gotten exposure somewhere
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Jan 7, 2010 17:03
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Sparkfun is totally down here. I think they DDoS'ed themselves.
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Jan 7, 2010 17:04
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Sparkfun is getting hammered... I don't think anyone will be able to checkout.
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Jan 7, 2010 17:05
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I got the payment screen to load, arggghgh
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Jan 7, 2010 17:09
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I suppose that's what happens when you announce you're giving away 100,000 in product :-P They must be pretty profitable to be able to give away that much poo poo.
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Jan 7, 2010 17:09
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I got the main screen to load! edit: They've only given away $1,600 so far.
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Jan 7, 2010 17:22
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Looks like it's up to about $6000. I keep getting stuck right after the "submit my order with the discount" button 
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Jan 7, 2010 17:43
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I got through to the payment page, $9630 given away so far. There's still hope.
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Jan 7, 2010 17:54
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Mine went through... has anyone else gotten through or gotten a confirmation email? It makes me a little nervous that my email doesn't mention the $100 discount at all.
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Jan 7, 2010 18:07
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