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Ask MAKE is a weekly column where we answer reader questions, like yours. Write them in to mattm@makezine.comor drop us a line on Twitter. We can’t wait to tackle your conundrums!

Jacob asks:

I’m new to electronics, and am interested in LEDs. One thing I can’t figure out is why some of them are colored, while others are clear. What’s the deal with that?

Hey, good question! I’d never actually thought about it before, and now that you mention it, it does seem a bit confusing. My initial guess was that the coloring might be used as a filter to block out other colors, but that doesn’t make sense- in general, LEDs put out a very narrow spectrum of light, so they shouldn’t need filters (and it would probably be difficult to build a filter with that narrow of a cutoff range). One exception would be more complicated LEDs such as white ones, which normally start with blue light and then use a phosphor to convert it to white light. It seemed possible that at least for those, the color could be part of the phosphor- except that white LEDs are almost always clear! Besides, the phosphor part turns out to be located right on top of the dye.

So, the best I can tell is that the tinting is added to make it easier to tell them apart when they are off. The clear ones are a pain to sort out, because you have to plug them in to figure out what color they might be. Kind of funny, but I guess that’s how it goes!

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Ask MAKE is a weekly column where we answer reader questions, like yours. Write them in to mattm@makezine.comor drop us a line on Twitter. We can’t wait to tackle your conundrums!

Paul asks:

I’m a museum educator, and working on an exhibit where I want to have a knob that you can spin around to change the color of an exhibit. I thought about using a potentiometer, but I am worried that it might break if people try to turn it too far. Any idea what I could use? Do they make a potentiometer that you can keep turning?

Interesting question. I now know that museums are pretty tough environments, so I understand your concern about it breaking. I can think of three options that might work:

• Rotary encoder
• Continuous turn potentiometer
• Regular potentiometer, with a slip clutch

Using a rotary encoder is the first thing that comes to mind. These devices have a disc with markings in them that spins when their shaft is turned, and have a sensor (usually optical) that detects when the marking move by. Because the sensor part is non-contact, they are generally more robust and can last longer than, say, potentiometers. The downsides to using them is that they are more complicated to use, and have a lower resolution than a potentiomer solution. The Arduino playground has a nice example of using a rotary encoder with a microcontroller.

A continuous turn potentiometer is an interesting option; I actually didn’t know they existed until now. They certainly solve the issue of breaking by turning too far, because they don’t have a hard stop. The only issue that I can think of is that their value will will probably jump suddenly from full on to full off as they make a complete revolution, which may require special handling. Actually, there is a second issue- I don’t know where to buy them! Anyone have some pointers?

If you can’t find a continuous turn potentiometer, a final idea might be to use a regular potentiometer, but connected indirectly through a slip clutch. The idea is to use something that will limit the amount of torque that the user can put on the potentiometer, which will prevent them from turning it too far and breaking it. You can purchase a special mechanical device that will do this, or perhaps even use a loose belt that will slip when the knob is turned too far.

Good luck with your project, and be sure to let us know how it turns out!

Have a different way to do this that you think might work better? Sound off in the comments!

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Brett writes in:

Hi, I am building an automated brewing system for home brew beer. It will be controlled via an Arduino and network interface. I am using 15 gallon stainless steel kegs as brewing vessels. What is the best method or type of sensor to use to accurately detect liquid levels in the kegs? The liquid won’t be water alone, and all three vessels will be heated. I was thinking of using an external sight glass + a pressure sensor of some kind. Maybe someone in the Make community has a better idea or has done this before?

I have been a Make subscriber for the last two years(since I found out about you guys) keep up the great work!

Thanks Brett! So, you want to measure the level of a liquid in a tank. These are some of the ways that I can think of to do this:

• Capacitive: Measure difference in dielectric constant between air and water
• Differential pressure measurement: Measure pressure at bottom of liquid
• Optical: Use an array of light sensors to sense where liquid is/isn’t
• Float: Put a float on a stick, then measure the displacement of the stick
• Weight: Weigh the liquid, and calculate volume based on density
• Conductivity: Measure liquid conductivity at a few discrete points

Out of these options, what would be best? I think for your project, the best way to go is probably to use a differential pressure sensor, like you suggested. It seems like it would be the most reliable and least invasive method. Also, it just so happens that the folks who wrote the Practical Arduino book have a Water Tank Depth Sensor project that might be a good starting point.

If you only want an approximation of the amount of liquid in a tank, a simpler way to go would be to test for conductivity at a few points along the side of the tank. You wouldn’t be able to sense the exact amount of liquid present, but it would be a fine way to check if the tank is half-full or not. The folks at electronics labs built a single-point Water level detector using a PIC microcontroller, which could be expanded to measure multiple points

Capacitive sensors might work, but might be hard to set up in a metal tank. Optical sensors work the same way as the conductivity sensors, and would be more expensive, so they probably aren’t worth the trouble. A float could potentially work, but could get gummed up by hops or dead yeast, so it might be unreliable. The weight sensor could also work, but I think it might be tricky to keep it calibrated.

Good luck, and remember to let us know when you have it working!

Have you attempted a system like this before? Have some experience to share, or a better way to accomplish this? Sound off in the comments!

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Bjorn writes in:

I’m doing my 2nd Arduino project (and 2nd electronics project ever) and am trying to design some of the circuit stuff on computer instead of just wiring it up or scribbling some stuff on paper. I’m trying out Fritzing and am wondering if you have any other suggestions for software that can be used for breadboard, schematic and/or PCB design. I would like to maybe figure out/learn/teach myself enough about electronics to be able to design my own PCBs, so it’d be nice if I could do all the types of designing in one program. I downloaded Eagle but haven’t really tried that out yet and am not entirely sure what that’s used for, but I think it relates to PCBs.

You sound like you are off to a good start. The kind of programs you are looking for are referred to as Electronic Design Automation (EDA) tools, and traditionally allow you to draw out a symbolic representation of a circuit using a schematic capture interface, (sometimes) simulate it with a circuit simulator, and then finally lay out the circuit on a printed circuit board (PCB). In the commercial world, many companies use tools designed by Synopsys, Cadence or Mentor graphics, but these are prohibitively expensive and overly complex for most hobbyist use. You are on the right track with Fritzing and Eagle. They are both good choices for documenting your circuits and designing PCBs, although they have been designed for somewhat different purposes.

For people who don’t already have a strong background in electronics, Fritzing is a great place to start. Instead of forcing you to understand the circuit schematics for each piece, you can use it’s pictorial ‘breadboard’ mode to connect drawings that look like the actual pieces that you would place on your breadboard. It sounds kind of cutesy, but it is pretty powerful because it also generates a schematic view of your circuit. This way, you can learn what the symbols for all of your parts look like by just connecting them up and flipping between the two views. You can either leave it at that to document a project that you made on a breadboard, or try out the PCB mode to design a simple 1-layer board that you can then etch yourself. To get started, check out their tutorials.

While Fritzing is a great way to get started with electronics, if you start to do more complex things, you will eventually need to turn to a more powerful tool. The second program that you mentioned, Eagle (Easily Applicable Graphical Layout Editor), is a good choice for this. It isn’t open source software, however they offer a freeware version that is powerful enough to build many useful circuits. It doesn’t have a breadboard mode, so you have to start by making your circuit as a schematic. Once you have a schematic drawn up and have checked to make sure everything is connected properly, you create a new PCB and lay the components out. To get started, check out this tutorial by the folks at Sparkfun.

There are a multitude of other free PCB design programs out there. For instance, ExpressPCB is a proprietary schematic capture/PCB layout program that is closely integrated with the companies PCB fabrication service. gEDA aims to be a comprehensive open source circuit simulation/design environment, but appears to be quite complex. FreePCB looks good for designing PCBs, however it doesn’t appear to include a schematic capture program.

My advice would be to stick with Fritzing and Eagle for a while, use them to design and build a few PCBs, and if you aren’t satisfied with them, try out one of the other tools to see if it works better for you. Good luck!

[photo by Flickr user Zach Hoeken]

Related:

• Primer: Printed Circuit Boards
• How-To: Make your EAGLE schematic into a PCB

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Ian writes in:

I was looking at buying a digital camera, and read that there are two kinds of sensors that they can use to take a picture- CMOS and CCD. Can you tell me what the difference is, and if one is better to get?

Sure! It’s actually a pretty topical question, as the inventors of the CCD just won this year’s Nobel Prize! As you mentioned, there are two basic kinds of image sensor that are used in today’s digital cameras, CCD (charge-coupled device) and CMOS (complementary metal oxide semiconductor). They both work by converting light energy (photons) into electric charge (electrons), and the difference is in how this charge is read out.

To start, both kinds of sensor are made of a grid of ‘buckets’ placed evenly across a flat semiconductor surface. Each bucket acts as an individual sensor, which only sees a tiny portion of the image. By displaying a bunch of these tiny points in a grid (using a computer monitor or printer), we see the image.

Making color images is a bit more complicated. Because the buckets are sensitive to any wavelength of visible light, if we just looked at the results we would see a monochrome image. To get color information, we arrange the sensors into groups of four, and place tiny red, green, and blue color filters over them. Each group of four sensors is what we call a pixel, and it is interesting to note that modern cameras have millions of them.

Ok, so both CCD and CMOS sensors are basically just big arrays of individual sensors, so how are they different? The difference is in how the charges are collected and read out. In a CCD, the ‘bucket’ that collects charge is just a capacitor. To read the image data out of the CCD, the charge in each bucket is pumped individually over to an ADC (analog to digital converter), which actually measures charge. In a CMOS sensor, each bucket contains a photodiode and some amplifier circuitry. To read the image data out, the output of each amplifier is connected to an ADC through a multiplexer, which measures the voltage at each cell.

I don’t think that either technology is necessarily better, but each has its own quirks. There is an interesting site at dvxuser which talks about the different kinds of sensor artifacts associated with each kind of sensor. For most cases, though, I think that other specifications, such as ease of use and sensitivity to light, are probably more important to think about when choosing a digital camera. Good luck!

[photo by SarahCartwright]

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