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Does anyone have a preferred cheapish instrumentation amp? I'd like to do some current sensing.
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Jan 7, 2017 05:20
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Aurium posted:Does anyone have a preferred cheapish instrumentation amp? They're not instrumentation amps but the INA28x (e.g. INA282) series from TI is pretty decent for DC/low frequency AC current sensing. You can get them on a breakout board off eBay for a few dollars a piece.
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Jan 7, 2017 13:11
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Agree on ina series as purpose built current sensing amps. You can use any Opamp in a differential topology too with decent resistors or, better, matched resistor divider packs. Note that the high impedance input aspect of an instrumentation amp isn't usually important for shunt sensing applications. You'll see that many purpose built shunt amplifiers are simply a regular amp with matched resistors included.
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Jan 7, 2017 15:00
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poeticoddity posted:It's because of a focus on safety over efficiency. I'm looking into building a tDCS (transcranial direct current stimulation) device, so I'd rather have an assurance of a maximum possible current past a given point in the circuit than save a few mW. Well setting aside the already stated concerns I take back the statement about the POT. You're asking the pot to drain a fixed amount of current away from the load but an R is only a stable current source under stable voltage conditions. But if you're talking about skin contact conditions you may have widely varying 'load' impedance which means the current sharing between it and the fixed pot would also be unstable. So I think you want direct current regulation. There are literally a million ways to do this but keying off your choice of that current diode the next step up would be a depletion mosfet current source which is actually the internal structure used in many of those diodes. Being a two part solution it's as simple as an adjustable current source can be: This is from the LND150 depletion mosfet datasheet which is tailored for current regulating applications. It's just the mosfet with that series resistor. Note that, as shown, a given device is quite stable but device to device variation will be significant, hence individual pot adjustment would be needed:  Note that there is no harm in leaving your diode in series which will act as a backup 2mA limit. asdf32 fucked around with this message at 15:57 on Jan 7, 2017 |
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Jan 7, 2017 15:52
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Keep in mind that you can't use any old amp if the application was a PWM motor driver; there's a "common-mode transient immunity" spec, using the INA282 on an unfiltered PWM output will just result in a garbage output. In that case build a differential amplifier from matched discretes or get something like the LT1999-xx.
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Jan 7, 2017 15:56
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For high transient immunity hall effect works well though with other downsides . Allegro makes IC sized hall efects though accuracy isn't their strong suite (like +/-20% or so) http://www.digikey.com/product-detail/en/allegro-microsystems-llc/ACS712ELCTR-05B-T/620-1189-2-ND/1284593
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Jan 7, 2017 16:23
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asdf32 posted:For high transient immunity hall effect works well though with other downsides . Allegro makes IC sized hall efects though accuracy isn't their strong suite (like +/-20% or so) LEM sells some hall effect current transducers that aren't quite IC sized, but still easy to work with. They're more accurate than the Allegro ICs, but have a lower frequency response (for the most part) and are a little dicier to find in distribution http://www.lem.com/hq/en/content/view/269/206/
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Jan 7, 2017 16:44
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asdf32 posted:Well setting aside the already stated concerns I take back the statement about the POT. You're asking the pot to drain a fixed amount of current away from the load but an R is only a stable current source under stable voltage conditions. But if you're talking about skin contact conditions you may have widely varying 'load' impedance which means the current sharing between it and the fixed pot would also be unstable. Awesome. Thank you. I'll probably toss that mosfet sub-circuit into KiCAD along with everything else I had planned and keep the CRD in series as backup like you suggested.
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Jan 7, 2017 23:41
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How can digikey justify charging $50 shipping for a local delivery of a few components which could be sent through the mail. it's so frustrating having the best component store be unusable because of that.
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Jan 7, 2017 23:48
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Cumslut1895 posted:How can digikey justify charging $50 shipping for a local delivery of a few components which could be sent through the mail. it's so frustrating having the best component store be unusable because of that. You mean they are hand delivering and refuse to ship it? I think your account rep (or just online support chat) will be able to sort that out.
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Jan 8, 2017 00:00
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asdf32 posted:Agree on ina series as purpose built current sensing amps. Thanks. One of the things I'm trying to move away from rolling my own resistor networks. Letting the IC vendor make them for me sounds wonderful. (A differential amp would work pretty well too) longview posted:Keep in mind that you can't use any old amp if the application was a PWM motor driver; there's a "common-mode transient immunity" spec, using the INA282 on an unfiltered PWM output will just result in a garbage output. This looks great.
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Jan 8, 2017 02:36
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If I have a stack of batteries, can I run them entirely in series to a load, and also hook them up entirely in parallel to a battery charger, at the same time?
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Jan 9, 2017 02:35
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Zero VGS posted:If I have a stack of batteries, can I run them entirely in series to a load, and also hook them up entirely in parallel to a battery charger, at the same time? No. Imagine how this would work physically/electrically. You have a direct connection between the positive terminal of one cell and the negative terminal of the next. Now connect the charger. You’ve shorted its positive and negative terminals together, and it is not amused. What’s worse is that you have also shorted out each of the batteries.
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Jan 9, 2017 02:58
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I'm kinda curious though so I think you should try it and film the results.
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Jan 9, 2017 03:34
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Platystemon posted:No. OK, that's what I thought. If instead of doing them both simultaneously, I just wanted to switch between one or the other, are there some off-the-shelf components like relay, or a huge switch, that would let me change up whether they full-series or full-parallel?
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Jan 9, 2017 03:47
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Cumslut1895 posted:How can digikey justify charging $50 shipping for a local delivery of a few components which could be sent through the mail. it's so frustrating having the best component store be unusable because of that. Digikey's shipping costs have always seemed pretty reasonable to me - if you want to have something heavy overnighted it'll cost you an arm and a leg but a handfull of tiny parts shipped via ground is like $7-$10 which isn't crazy since they have to have some worker walk around their warehouse packing your 5 resistors and 3 capacitors and put them into a box for you. That said, does anyone know of any documentary video on how Digikey/Mouser/Newark style places function internally? I've always been fascinated by how these places can stock 150k+ individual weird-rear end parts and still pick them out for shipment while making a profit at such low per-part volume. I've seen hobby type people order ridiculous lists of parts with a single 2.5k resistor, three 0.1uF capacitors, etc. and wonder how they can have a person pack that stuff for 1.5 cents per part without losing money. On the other end of the scale, does anyone have a war story about a parts shipment that MUST have cost one of those places more than they made to mail it to you? I once ordered some 1/4" shaft plastic potentiometer knobs that came shipped in individually sealed anti-static metalized bags complete with desiccant packs and humidity indicators. The vendor waived the base shipping cost due to us having .edu email addresses and I'm pretty sure they lost money on that sale.
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Jan 9, 2017 04:52
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^^^ A co-worker of mine visited their headquarters while he was in Minnesota for the holidays. I'll ask him what he saw on the tour.Zero VGS posted:OK, that's what I thought. Traditionally, you would charge with the full series voltage and balance the cells periodically with a circuit that taps out the nodes in between for measurement and shunting. There are ICs designed expressly for this purpose. Is there a reason you can't charge with the full voltage? KnifeWrench fucked around with this message at 04:56 on Jan 9, 2017 |
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Jan 9, 2017 04:54
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I've never ordered something huge and heavy from Digikey so I don't know how bad the shipping can get but for small-ish orders (dozens of ICs and hundreds of passives and such) under 10 Canadian dollars for next-day shipping from the US to Canada (if you order in the morning) is excellent.
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Jan 9, 2017 05:00
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KnifeWrench posted:Traditionally, you would charge with the full series voltage and balance the cells periodically with a circuit that taps out the nodes in between for measurement and shunting. There are ICs designed expressly for this purpose. Well, this would be a 300vdc battery that would be charged by solar panels, but there is no MPPT (maximum power point tracker) that outputs such a high voltage. I'm planning a solar car project.
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Jan 9, 2017 06:20
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Are these lithiums? If so, build it in series modules of 8S cells, and each module gets an 8S balancer. They're about $20 ready-to-run on eBay. Throw the series voltage across it on CC/CV @ 1C, and let it chooch. If they're any other topology just overcharge them and don't care. They'll all tolerate overcharge at C/15. NiMH/NiCD is trickle at C/15 for 24 hours to have a dumb charger, and lead acid is CC/CV @ C/5 and 2.6Vpc.
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Jan 9, 2017 06:27
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PDP-1 posted:Digikey's shipping costs have always seemed pretty reasonable to me - if you want to have something heavy overnighted it'll cost you an arm and a leg but a handfull of tiny parts shipped via ground is like $7-$10 which isn't crazy since they have to have some worker walk around their warehouse packing your 5 resistors and 3 capacitors and put them into a box for you. A single resistor is 50 NZD, shipped from their NZ warehouse edit: and sometimes they'll triple your shipping costs, after you have paid
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Jan 9, 2017 06:32
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insta posted:Are these lithiums? If so, build it in series modules of 8S cells, and each module gets an 8S balancer. They're about $20 ready-to-run on eBay. Throw the series voltage across it on CC/CV @ 1C, and let it chooch. I'm using 14 of these 24v Tesla battery modules, all in series: http://www.ebay.com/itm/262321881194 Each of them has a built-in slave BMS, but it is a mystery how they operate. Someone who examined them said that a thermal camera showed that the resistors on the slave BMS will heat up on their own, so apparently they do some level of load balancing even when not connected to the master BMS. All of the Tesla BMS are kind of a black box though, there's no real documentation on how they work besides some reverse-engineering attempts. That's why I wanted to just charge them with a 24v charger in parallel, I figure that's the safest way to go about it for now.
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Jan 9, 2017 06:47
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PDP-1 posted:
https://www.youtube.com/watch?v=G3V5Oi9fdr4
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Jan 9, 2017 06:58
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Cumslut1895 posted:A single resistor is 50 NZD, shipped from their NZ warehouse Yeah both digikey and element14/farnell/newark murder you with shipping costs in Australia too, even when it's from the Australian warehouse. It ends up being cheaper to buy from jaycar for small projects, which is ridiculous.
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Jan 9, 2017 07:43
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Cumslut1895 posted:A single resistor is 50 NZD, shipped from their NZ warehouse For that price I could afford to have it shipped to my house and then re-ship it to you and still charge you more than shipping costs and make a profit myself I wonder if anyone's doing that already...
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Jan 9, 2017 15:55
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I like Digi-Key to Norway because they give you free shipping from around $60, even with moderately large boxes. Other than eBay direct from Shenzen I don't know of any other electronics supplier with a lower minimum order for free shipping. Elfa Distrelec has reasonable shipping costs but I think they stopped offering letter mail years ago.
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Jan 9, 2017 18:48
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Playing around with an X-band (~10GHz) VCO+PLL: https://www.youtube.com/watch?v=cGNaxBKaWAk I'm characterizing the power consumption now so I can strap some batteries to it, and then I'll be connecting it to a spectrum analyzer on Wednesday to verify that it's actually outputting what it's supposed to be. The plan is to attach this antenna to it eventually.
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Jan 9, 2017 22:35
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longview posted:I like Digi-Key to Norway because they give you free shipping from around $60, even with moderately large boxes. Yeah, Elfa only has tracked packages to Norway and a similar threshold for free shipping. I needed a single capacitor some weeks ago and ended up massively padding my order. At least Kjell has begun selling a bare minimum of stuff from their physical stores.
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Jan 10, 2017 17:40
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I asked this place about addressable LEDs not too long ago and that's proved really interesting, so thanks for that and now I come to you again, as I need to pause on that project. I have finally come up with a useful house project that would teach me a lot about electronics and Arduino's control of them. After buying and using a few cheap temperature/hyrgometer devices from China about the house, I think I would like to develop this into a number of devices that take these readings and send them every 2-3 hours to a central computer which would record them and upload the figures onto Google Docs where I could have an ongoing graph of days/weeks/months. A key point, I don't want this to change the temperature in my house, only record it. I'd like to keep this relatively cheap as possible and build as much of it myself. there are 3-4 locations I would record, with maybe one being outside. My main thought was whether to go one Arduino for each site,or whether I could get away with the sensor/senders being dedicated sensors/senders wired together to send to one central Arduino. I would prefer the latter solution but do not know what to look for when ordering or how to connect them. It may be simply to just Arduino Nano each site with a thermistor, a moisture sensor and a way to send the signal out, but then I would have to calibrate them whereas I would prefer something built for that purpose. Whatever I use, I'll call them 'the devices' for now Next part is power, I could place these near plug sockets so getting 5V power to them wouldn't be bad, but I don't know how power hungry they might be in relation to batteries. If I could power them by battery with a few changes a year (my current devices are lasting a long time on simple small batteries), that would be good, but I can't judge it at this point. Next, how to speak to a central device. This I know a lot less about, and any suggestions would be good. I have the Make book "Making Things Talk", so I think with that and some online research I could find out how to get the Arduino to upload the figures it receives onto an online database, but not sure the best way to get the info from the devices to the central Arduino. I wasn't sure if the devices needed a 'clock' of sorts to let them know when to send out values, but anything requiring a sort of Wait(120minutes) command would be messed about by a battery operated program, so maybe it would have to be mains driven. Anyway those were just my basic ideas and I would appreciate any thoughts or signposting for them that anyone can help with. This feels quite ambitious for a beginner but might be simple to more developed electronics people. If I can cover sensor reading, device-to-device communication, device-to-internet communication, timing and power concerns in one project, it feels like it will be quite educational.
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Jan 12, 2017 19:25
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Fat Turkey posted:My main thought was whether to go one Arduino for each site,or whether I could get away with the sensor/senders being dedicated sensors/senders wired together to send to one central Arduino. I would prefer the latter solution but do not know what to look for when ordering or how to connect them. It may be simply to just Arduino Nano each site with a thermistor, a moisture sensor and a way to send the signal out, but then I would have to calibrate them whereas I would prefer something built for that purpose. Whatever I use, I'll call them 'the devices' for now A bunch of cheaper hygrometers have built-in temperature sensors so if you get those you're set. The DHT11, for example: https://www.amazon.com/Gowoops-Temperature-Humidity-Digital-Raspberry/dp/B01H3J3H82. Personally I'd put a separate Arduino at each location because otherwise you'll be relying on sending whatever weird protocol or signal the hygrometer uses over a long length of cable before decoding it and that could be iffy, depending. You can get 5-packs of Arduino Nano clones for $20 off amazon, (cheaper if you don't mind waiting for them from china off ebay): https://www.amazon.com/KOOKYE-Arduino-ATMEGA328P-Module-Micro-controller/dp/B019SXND4O Fat Turkey posted:Next part is power, I could place these near plug sockets so getting 5V power to them wouldn't be bad, but I don't know how power hungry they might be in relation to batteries. If I could power them by battery with a few changes a year (my current devices are lasting a long time on simple small batteries), that would be good, but I can't judge it at this point. Depends a lot on how you write your code - you can make arduinos "sleep" (basically turn off their oscillator and go into a super low power mode) and only wake up to check and see if they need to take a measurement or not. This lowers their power consumption to the point where it would last quite a while on a few AA's, but it can be a little tricky to program if you're completely new to this. Fat Turkey posted:Next, how to speak to a central device. This I know a lot less about, and any suggestions would be good. I have the Make book "Making Things Talk", so I think with that and some online research I could find out how to get the Arduino to upload the figures it receives onto an online database, but not sure the best way to get the info from the devices to the central Arduino. Fat Turkey posted:I wasn't sure if the devices needed a 'clock' of sorts to let them know when to send out values, but anything requiring a sort of Wait(120minutes) command would be messed about by a battery operated program, so maybe it would have to be mains driven. There's several ways this could be accomplished. If the actual precise amount of time between samples isn't super critical, and you're using the "sleep" function I mentioned earlier, you can basically say "well it sleeps for about 4 seconds each time, so each time it wakes up increment a counter, and when the counter hits 100 (for example) then it's been ~400sec and you should take a measurement". Then when it sends back the data, the computer can just save a timestamp along with it it as having been received at [whatever] time for logging purposes. If specific sampling times are required ("take a sample at 5:00 PM" rather than "take a sample every 400 seconds"), you might need to do something like trigger the measurements from the central control point - so the computer says "ok it's exactly 5:00 PM, send out a signal to tell everyone to wake the gently caress up and do some samplin'."
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Jan 12, 2017 22:10
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Depending on the distance you want between units, Bluetooth and Wi-fi modules for arduinos are very cheap and easy to use these days. For Wifi, ESP01 boards using the ESP8226 chip are cheap, everywhere and very easy to use. HC06 Bluetooth modules are even easier to use and similarly cheap and common: they come pre-programmed to the point where you don't have to configure them at all. I'd have an arduino nano at each location, but the central device could be your computer fairly easily, running a python script or whatever. Depending on how often you sample you can probably get away with batteries on each temperature device, though I haven't looked into how much power the Bluetooth and Wi-Fi modules I've mucked around with actually use.
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Jan 12, 2017 22:55
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Thanks for the quick replies, sounds like I was working on the right lines at least. Happy to get cheap parts from China and use them, although some UK sellers have them at reasonable prices too on eBay, so for a few pennies more and instant delivery I may side with them. I already have 2-3 Arduino Nanos kicking about so I can make a start on designing one and then if that works, make the others. The DHT11s look like exactly what I'm after. I'll buy one quick for testing. Wireless is a Must-Have, but all devices should be within access of my house routers so sending info to the WiFi itself for the central computer to pick up should be fine. Will those NRF24L01 be good for that too, or the ESP8226? Or barely anything in between? Power consumption and ease of use are the two main criteria for me. I could use my home PC as the central hub, but does that mean it would have to be on 24/7 to take the readings? Surely if it's not on to receive the Arduino transmissions, then the readings get missed for that time? Readings don't have to be 'posted' in real time, I've no problem with delays, it's more for long term comparison. But if something has to be 'always on, I think I'd prefer another cheap dedicated Arduino. Although my PC is always connected to the router so no receiver to worry about there. Hmmm. Specific sampling time are kind of important. I don't need 5pm exactly, but every 2-3 hours so it measures the same time of day every day on every device would be good. Your time stamp approach probably fixes that though. But adding a receiver and instructions to trigger and constant watching out sounds like a battery drain. If the Arduino is accurate enough I think I could get away with sleeps but it would have to be doing a lot of sleeping between my two hour measurements. I don't mind learning about power efficient coding. Best news is it sounds like doable. If you could point me into WifFi or Bluetooth (I'm thinking WiFi) and whether the PC would have to alays be on, or have another Arduino be perma-on, then I'll get ordering some parts and checking some Youtube tutorials 
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Jan 13, 2017 01:08
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I'd use NodeMCU's, which is a particular form factor of ESP8266. The USB is onboard, they're cheap and easy to program with Arduino (just install the ESP8266 package and set the board type to "NodeMCU"). Just get a kind with a "CP2012" USB chip rather than the fly-by-night CH340G chip, but that advice holds for any cheap China Arduino clone.
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Jan 13, 2017 03:50
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Fat Turkey posted:
You could do either, it's entirely up to how you code it. The arduino itself has enough memory to store literally tens of thousands of data points, so the remote sensors could buffer them and only transmit when they're asked for. To make you know when the individual samples were gotten you'd have to timestamp them on the individual arduinos themselves though. The arduino's clock will drift by a few seconds each day, so you'd have to either get a real time clock on each of them, or have them connect say, once a day, or week or so and sync up the time to keep it reasonable.
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Jan 13, 2017 05:49
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The ESP8226 board I'm using is very easy to use, I'm using mine without even programming it, as the software it comes with communicates over UART with AT commands that do everything I want to do currently. I'm looking to do more with it later, but for your project the default stuff is adequate. It can send and recieve TCP packets and communicate them back to your nano.
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Jan 13, 2017 08:25
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wow and after all that RSComponents sent my $20 order out in four (4) shipping-free packages.
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Jan 13, 2017 08:37
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So I got around to using some parts we ordered from Digikey before the holidays. I needed some 1 pF capacitors and we ordered 10 which was the first price break. I mean may as well, it was still like a buck and spares are nice. I've seen prepacks of 10 pieces, but these caps came as a prepack of 9 pieces and a prepack of 1 piece. wtf
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Jan 13, 2017 08:44
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BattleMaster posted:So I got around to using some parts we ordered from Digikey before the holidays. I needed some 1 pF capacitors and we ordered 10 which was the first price break. I mean may as well, it was still like a buck and spares are nice. End of a reel, perhaps.
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Jan 13, 2017 08:47
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Cumslut1895 posted:wow and after all that RSComponents sent my $20 order out in four (4) shipping-free packages. So for $20 from RS you ordered 2 resistors and a capacitor?
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Jan 13, 2017 08:56
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You could set up something like a Raspberry Pi as an always-on receiver; they are easy enough to hide somewhere (e.g. behind your router) and more than powerful enough to receive your readings and present them as graphs on a webpage or something.
Computer viking fucked around with this message at 10:05 on Jan 13, 2017 |
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Jan 13, 2017 10:03
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