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646  Topics / Device Hacking / Re: Harvesting components from devices on: January 19, 2012, 01:28:15 pm
I too love to take dead devices apart... first with the hope towards arepair, second with an interest in how it was made, third to harvest components if it's truely dead.

If you're looking for high-voltage transformers, BTW, microwave ovens are a great source for them unless it's an "inverter" model. Some folk have used multiple microwave transformers in series to do stick welding projects.smiley-eek-blue

I expect these analog devices to disappear from microwaves over time thanks to the commodity price increases combined with a steady decrease in transistor prices. Just like the classic transformer/bridge rectifier/linear VR/smoothing cap circuit we used to see in all appliances..
647  Using Arduino / General Electronics / Re: How to calculate what capacitors to use? on: January 19, 2012, 01:06:44 pm
Read the datasheets carefully. One linear voltage regulator I used was not happy when it only encountered a ceramic capacitor on its output. Not only did I blow a voltage regulator, but the downstream SD card too (4.5VDC vs. 3.5VDC max allowable will do that for you). Other voltage regulators I have used subsequently specifically stated that they are safe for use with ceramic output capacitors. But they tend to sell at a premium and you have to confirm (via datasheet or inquiries with the manufacturer) that the regulator will be stable using only a (or several) ceramic output capacitor(s).

More often than not, linear voltage regulators are specified to use a large Alumimum Electrolytic or Tantalum capacitor along with a 100nF ceramic one. Data sheets and application examples by the manufacturer are invaluable to beginners like myself, they take a lot of guesswork (and magic smoke) out of the equation.
648  Using Arduino / Sensors / Re: Current Sensing on: January 19, 2012, 12:47:58 pm
A couple of suggestions:
  • Determine the maximum voltage you may be measuring. Add a safety factor (I use 25%) and then see if you can either use the internal voltage reference (1.1VDC, 2.56VDC, etc. depending on the model of your Atmel microprocessor) or whether it makes sense to apply an external voltage reference to the AREF pin. The latter approach is preferable, IMO, especially if you use a voltage reference or a small voltage regulator because variability in the power supply voltage won't propagate into your measurements as easily. However, many people use a cheaper approach using voltage divider networks that also do the trick (but which are more prone to be influenced by other devices attached to your Arduino).
  • Keep in mind: Once you set a alternate AREF voltage (through software or hardware) do not try to sample a higher-voltage signal on a analog input channel or you will blow your ADC. Here, the software approach of setting the internal reference is more flexible, you can easily switch back and forth between reference voltages though you have to add some settling time between measurements.
  • Consider using decimation for your signal. Keep the analogread results in their original integer format, oversample, decimate, and enjoy results of up to 16 bits or so. Atmel has a nice paper on this topic, just google atmel and decimate.
649  Using Arduino / Sensors / Re: Connecting 24V inductive sensors to Arduino on: January 19, 2012, 07:45:44 am
I'd recommend a optocoupler with a drop-down resistor to make it safe. In case the signal is AC, use a AC optocoupler. That's what I have used here with my air conditioning system (the signals for most residential AC control systems in America are 24VAC). Attach the opto to a digital input and then count the pulse train. Given how slow the signal is, you could use a ISR to do the counting and then let your Arduino refer back to the count whenever you want.
650  Using Arduino / Project Guidance / Re: need more accuracy out of my 60Hz frequency meter on: January 19, 2012, 06:23:28 am
FWIW, I'd go a different route. If you need a really accurate dedicated pulse counter, I'd hook up the SQW output of an inexpensive RTC to one of the interrupt pins (using DS1307, for example). Then program a 1Hz signal output on the RTC and set up an Arduino interrupt to set a flag or increment a counter for every second that goes by. In the meantime, read the frequency as fast as possible.

Since you don't really need amplitude, you may even do this at a ADC lower resolution, i.e. you can adjust the ADC pre-scaler to a lower value like 16 (vs. the default 128) to get tens of thousands of readings a second. If you want to speed up things further, you can read analog values directly from the relevant port and thus get another reading about every 13 ADC clock cycles (so CPU speed / pre-scaler / 13 = max theoretical sampling rate).

I wouldn't count frequency with an ISR... every time the CPU has to go into ISR mode, you lose 51 CPU clock cycles as I recall. OK if your code has lots of idle time, maybe not so OK if you have time-sensitive stuff going on. Instead, I would dedicate a small period of time (let's say a second out of every minute) to just count the frequency and then dedicate the CPU to other tasks.

RTCs also feature other output frequencies. You could avoid the ISR altogether and do frequency measurements on demand by sampling the frequency signal with the ADC output while sampling the RTC output with a digital input. Since you can sample very quickly using a small pre-scaler, you can set up a for loop that increments every time a RTC signal drops low, then measure a known number of increments at a given frequency to determine your other frequency. Thus, you might be able to determine your frequency with a high degree of accuracy, as needed.
651  Using Arduino / General Electronics / Re: How important is a ground pour, or am I asking the right question? on: January 18, 2012, 11:25:44 pm
Looks awesome.

But if you want to consider going down this path of creating bigger and bigger pour areas, did you consider putting the driver chips on each of the vertical "legs"? That could save you space on the main PCB without increasing cost significantly. Another option is to find a vendor for flexible PCBs... the only downside being that flexible PCBs are even worse than FRP re: heat dissipation. Though given your application, you probably would be OK. You've probably thought of all this already, I was just thinking aloud.

Congrats again on a neat project.
652  Using Arduino / Sensors / Re: Measuring temperature with lm35 on: January 18, 2012, 10:48:20 pm
You may be able to calibrate the voltage levels by measuring the bandgap voltage between the Arduino internal AREF and the external voltage.

See http://arduino.cc/forum/index.php/topic,38119.0.html and http://arduino.cc/forum/index.php/topic,51708.0.html for further information.  Keep in mind, you have to calibrate your bandgap voltage first to be sure it's accurate.
653  Topics / Product Design / Re: Eagle - Double sided PCB - Vias on top but soldering on bottom on: January 18, 2012, 08:13:31 pm
Actually I think I was off by a factor or 10.
I design with 0.02" (20 mil) for power, and 0.01" (10mil) for signal traces.

Whoops, me too. 15-16mil and 24 mil respectively, it is.
654  Topics / Product Design / Re: PCB make+assemble service on: January 18, 2012, 08:05:04 pm
Thanks for making holland sound like a pile of weed and strippers btw smiley-wink
Well, the truth hurts! (just kidding!) The Swiss enjoy similar hospitality in German border towns in establishments that are happy to take Swiss as well as EU currency. That way, the Swiss can indulge in all sorts of naughty stuff that is banned in Switzerland without having to change currency. smiley-grin

Coming back to the subject at hand, thanks for the mention of the Seeedstudio assembly services! I will have to keep that in mind for my current project, Seedstudio costs about a quarter of what I would pay for similar services performed in the States. Makes sense, I guess, the PCB fabrication cost discrepancy is similar.
655  Topics / Product Design / Re: Eagle - Double sided PCB - Vias on top but soldering on bottom on: January 18, 2012, 07:56:27 pm
Go with 0.16" or 0.2" for power traces.

CrossRoads, just laughed out loud... I use 0.15-0.16" for signal traces, and 0.24" minimum for power...  One of my boards features a +5V ring around the perimeter that is about 0.5" wide... keeps losses low! Probably overkill.
656  Using Arduino / Microcontrollers / Re: ATmega1284P: End to End using 1.0 IDE on: January 18, 2012, 07:17:13 pm
Adi, nice looking board. The big solid ground pours and the signal, etc. lines show great attention to detail to keep things looking that 'simple'. I have spent many long hours optimizing my layouts too in order to minimize signal-line vias, etc. I am also very happy with the quality and cost of iteadstudio, now if only their PCB factories weren't on vacation until February!

CrossRoads, your solution to the much-debated question of whether to or not to include a USB interface is simply brilliant, especially since those of use who have an FTDI cable can use it too.
657  Using Arduino / General Electronics / Re: How important is a ground pour, or am I asking the right question? on: January 18, 2012, 07:03:35 pm
A couple of observations...

For one, you appear to be using the Eagle(?) autorouter. As others have suggested, the autorouter usually doesn't do very well (and the one in Eagle has not impressed me). For component layouts (to gauge how efficient a layout is) I have had great luck with the http://www.freerouting.net/ project. Frequently, designs with few tweaks can be adopted and integration is pretty simple for Eagle users (i.e. run a DSN, route, then run a script to lay the board out). Just remember to define your polygons up front (i.e. not only draw the polygon, but name it GND as well) and then to connect polygons that have fallen apart after the fact.

Ground planes (and positive rail grids) can simplify layouts significantly since one connection is always taken care of. The key is to review carefully what the optimum connection pattern is (and the auto-router may or may not discover that) and then to replicate it for assemblies like yours. Last but not least, another option for a power hungry board is to have a positive polygon on one side and a negative one on the other side. Then use Eagle to determine what polygons still need connections, etc.

As CrossRoads mentioned, using vias to connect polygons is the way to go. See how small you can go (your PCB factory should be able to tell you) in order to maximize the effectiveness of your vias. Laen's dorkbot PDX service allows 15 mil drills, while iteadstudio goes as low as 12 mils, for example.
658  Using Arduino / Sensors / Re: Help with DHT11 on: January 18, 2012, 05:32:21 pm
Try opening a new file, copy and paste your existing code into the new file, then try and compile.

I had the same issue with Easy Transfer on a file that had worked in the past and the copy and paste did the trick for me.
659  Using Arduino / Sensors / Re: Need IR sensor but can't find. on: January 18, 2012, 08:50:37 am
I know you'd prefer something non-contact, but do consider a K-Style thermocouple. Determine the critical surfaces to be measured and then attach the thermocouples there - some even offer screw-heads to make the connections really secure.

K-style thermocouples offer plenty of range (i.e. well over 400*C) and all you need is a small ADC meant for thermocouples to do the conversion. For example, the folk over at rocketscream offer a reflow oven controller for toasters that uses the Maxim MAX6675. There are other ADC's out there, but you might decide to look for a similar board and recycle the Arduino code relating to the MAX6675. K-style thermocouples are cheap on e-bay and with decimation/oversampling you may even get very good results. Just be quite observant re: PCB layout with anything that is measuring millivolts - you can easily add a lot of un-necessary noise.  

Another reason to go with a thermocouple vs. sensing with an IR sensor is the error that may creep in over time as function of emissivity, etc. At the very least you'd want to calibrate the IR gun regularly with a reference thermocouple to ensure that your readings are accurate.
660  Using Arduino / Project Guidance / Re: Temperature variance using TMP36 on: January 18, 2012, 07:34:19 am
The 1-Wire stuff is neat but the commitment of Dallas/Maxim to the platform is questionable. For example, they took out an existing product line (the very popular counter chip) and have yet to replace it with anything. That counter chip was extremely useful for hobbyists like myself (2 channels and counting bandwidth that could be measured in kHz) and was used in every 1-wire weather station to count the precipitation. To the best of my knowledge, they have yet to bring out a replacement counter.

As for the tmp36, as suggested above, your answer may be a better power supply (i.e. 0.1uF cap across GND and 5V) as well as decimation (Google the Atmel paper on that) to boost your sensor resolution from its quasi-10 bit beginnings to something approaching 16 bits. Decimation (or oversampling) is not the same thing as averaging, and for slow moving signals it's a perfect way to make inexpensive hardware perform much better.

All that said, the 1-wire digital temperature sensors from D/M are great units because they are accurate, can operate more easily in noisy environments, and are offered in all sorts of configurations that the TMP36 is not - like in a waterproof sheath, for example.
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