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211  Using Arduino / Project Guidance / Re: Taking a shot at specific gravity measurements on: February 01, 2013, 06:30:32 am
I have been thinking and maybe the surface level change problem can also be solved by using a differential method.

Let's say you use a second similar floating device with complete voyancy (floating at the surface of the liquid). If this second device is not affected by changes in the liquid voyancy then you can use it to measure just the surface level. Then by substracting the two signals the difference will be the difference in the voyancy of the liquid. To do that, both sensors outputs (after demodulation) can be fed to a differential amplifier ( easy to build, check the LM324 datasheet) and that will be your liquid voyancy signal to be send to Arduino analog input. That way you will not need to calibrate the system that much; but just at the beginning. I think you Project its completly doable that way.
212  Using Arduino / Project Guidance / Re: Taking a shot at specific gravity measurements on: January 31, 2013, 08:52:08 am
You don't need a Pr Sensor to find the position.
Maybe...
Let's say you place the bouy support (tube) on the container cap. Then the bouy can slide vertically through that tube depending on the liquid voyancy.Then you place a rod with a ferrite section in the bouy shaft and windings in the tube coincident with the center of the windings at normal voyancy. Study LVDT (Linear Variable Differential Transformer). You can convert any displacement of the bouy shaft to electrical signal at the output of the LVDT. You can make your own LVDT by winding a wire on the tube (2 windings). There are specifics on how to do that of course. You need to study that. The advantage is the bouy will be a passive element with no wires and the electrical system will be on the support attached to the container cap and all that can be external if you make the bouy shaft long enough.
To make the sensor make to identical windings on the tube. One for excitation where you apply a high frecuency low voltage signal (let's say 10 KHz). The other winding is the pick up coil where you receive the signal. When the rod (ferrite position changes) the amplitude of the picked up signal changes. Then you do AM demodulation to recover the envelope with a simple diode and a low pass filter (a capacitor). The envelope is your voltage changing with the rod position and therefore voyancy. If you want you can go FM by using the inductance as a component to change the frecuency of your oscillator (study Clapp Oscillator). The ferrite movement will affect the inductance of the coil and then you can demodulate the signal by using a phase detector (XOR gate) or a PLL unit like the CD4076. You need to study that too. It will give you a voltage output dpending on the frecuency changes. You can amplify (using LM324 if required) and feed that to Arduino. The FM version will be more complicated but you will get better inmunity to noise.

The main problem I see is you will have to be Zeroing the stuff regularly as the position of the bouy will change not only with voyancy; but with the liquid level inside as it evaporates, etc... maybe others have an idea of how to solve that problem. If they don't come up with a nice solution you may end up with an evaporation meter instead. smiley-mr-green
213  Using Arduino / General Electronics / Re: Small power source for one century on: January 30, 2013, 09:50:29 pm
It's a good project you build your clock; but...
Also build a freezing chamber and I'll volunteer to get frozen to see if your thing pops after 100 yeas or not smiley-grin . Don't use the device to tigger my ice melting in case it fails...
214  Using Arduino / General Electronics / Re: Small power source for one century on: January 30, 2013, 09:06:40 pm
I have been thinking in somekind of thermoelectric geneator for years that maybe can be used for this... There is a temp difference from the surface  down as you dig a hole. Maybe placing some bi-metal rods can havest some of the energy from the difference and it will be electrical energy already. If you go all CMOS and sleeping mode most of the time maybe you can use that energy to recharge something while sleeping or have two sets of batteies... Problem will be corrosion... but what if the rods are protected with vinyl or something like ceramic painting (like ovens)?. Maybe they produce enough power to supply the thing without a storage device. Remember spacecrafts use thermopiles (with a nuclear capsule for heating) to generate  electricity. You don't need that much energy so you don't need the reactor. Multiple rods in parallel and series (like batteies) may meet both voltage and current requierements even with small differences in temp. Maybe... Testing is the poblem.
I also think semiconductors should last that long specially because there will be negligible amounts of heat generated.
215  Using Arduino / Programming Questions / Re: Computer Serial Number on: January 27, 2013, 06:17:34 pm
That answer is for pure java.  Maybe someone here did something like that.
216  Using Arduino / Programming Questions / Re: Computer Serial Number on: January 27, 2013, 06:15:00 pm
Depends on how you look at it. I want it for my Arduino project. I want to read the computer sn from processing and match arduino to it. You can remove the post; but maybe others need the same solution.
217  Using Arduino / Programming Questions / Re: Computer Serial Number on: January 27, 2013, 06:09:10 pm
Anything that's unique to the computer and does not change.
Thanks.
218  Using Arduino / Programming Questions / Computer Serial Number on: January 27, 2013, 03:26:29 pm
Hi;

I want to retrieve my computer serial number using Processing; but haven't found a way to do it. I posted in Processing forum with lots of viewers; but zero answers... Maybe the Guru's there are sleeping...When Google I get a lot of VB and DOS solutions; but can't use them in Processing.
Any ideas will be appreciated.
Thanks.
219  Using Arduino / General Electronics / Re: Power supply building technical help required on: January 26, 2013, 11:43:16 am
dc42, thanks for your clarification. Now I understand it better.
Thanks.
220  Using Arduino / Project Guidance / Re: Reading CSV values from an SD Card on: January 26, 2013, 11:24:43 am
Thanks for letting me know. Honestly I haven't done it directly from Arduino to an sd card, so I don't know the details.
Another way is to keep track of the value in a variable and save it in the sd card just at the end (at exit) and not constantly as you are doing. Use a counter if you just need the ID and save it at exit. Remember keep it simple whenever possible. Maybe it helps.

valuecounter=valuecounter+1; //Incremnt the counter every time a value is stored.

Good Luck.
221  Using Arduino / General Electronics / Re: Power supply building technical help required on: January 26, 2013, 10:02:11 am
I won't estimate the motor current in the way explained above. Motor's currents are not the simple Ohm's law calculation from the windings resistance. The current varies depending on the mechanical load applied to the motor, that is, how much force they are doing to move the load. Thats' why they get burnt when mechanically overloaded. Better to place an ammeter (your multimeter measuring current) in series with the motor and measure the real current with the motor working with its normal mechanical load. That's the normal current it will take from the power supply. Remember if the mechanical load increases it will take more current. I don't think the relationship between force and current is even linear by the way (I have to check). Therefore, careful with the calculations and estimates...
222  Using Arduino / Project Guidance / Re: radar trap with mattel hotwheels speed gun on: January 26, 2013, 09:30:10 am
From what I read of the device it already "takes out the doppler frequency" in its internal mixer. You don't have to worry about doing it yourself as working with such high frequencies is the hardest part. You just need to use the output signal. That means the frecuency components you will be working with are low and not the microwave signal itself. The doppler frequency range will be in the audio range and below that, even for the fastest moving objects (cars I guess). Those low frequencies will be a piece of cake for Arduino. Measure the frequency every second or so and then calculate the speed from there. I don't see why you want to use an Arduino if the device does all that by itself. Well I guess you should have your reasons.
Good Luck.
223  Using Arduino / Project Guidance / Re: Time an analog comparison? on: January 26, 2013, 09:04:49 am
Yes you can. With Aduino you can do almost anything. The prob is pogramming the thing.
It all sounds like an RL circuit time constant calculation. Study that first.

224  Using Arduino / Project Guidance / Re: Reading CSV values from an SD Card on: January 26, 2013, 07:59:06 am
Hi:
Processing has a lenght() instruction that will help u find the lenght of the recovered array; but that will be too complicated...
If you just need the last value, then save the last value in another file, either txt or csv. Same way you save the values in the csv file you do it in another file; but you overwrite the previous value with the new one. In that way at the end you will have only the last value in that file and you just need to read one value. I don't know what you are using; but with processing use filename.print() instead of filename.println(). The lines will be overrwritten over and over and you will have your last value there. You need to create the file first of course with PrintWriter(); but if you did the csv file already I assume you know that.
Hope you can do it.
225  Using Arduino / Project Guidance / Re: multiple RF transmitters on: January 25, 2013, 08:30:44 pm
If you can be a bit more explicative on what you want maybe we can give you better ideas. Maybe you don't even need to go RF which is a really complicated subject by the way.
Anyway you can use only one transmitter and change the modulator signal. Let's say if you go FM then you can use multiple diffenet oscillator frecuencies and modulate the carrier with all of them at once by simply summing them at the mnodulator end. Then in the receiver end you just need to listen to one RF channel and demodulate the message. Then you filter the message and separate all individual modulating frecuencies. If you want to go digital then you need to use the Fast Fourier Transform. This is going to be tough very tough even if you are in an advance level. Do you still want to go RF?
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