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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!

Andy writes:

Recently I acquired a vintage Leslie speaker cabinet. The speaker cabinet uses ac motors to turn baffles and horns to create a Doppler effect. Upon opening up the speaker, I found that the motors were working, but very dirty and coated with gunk. What is the best way to clean a motor with an excess of build up?

Congratulations on your acquisition! Since the motors seem to be working fine, my guess is that it might be best to clean them cosmetically, but not to try and take them apart and rebuild them. Even though they are electrical devices, you should be able to clean them like anything else, using some form of solvent and a brush. Just make sure to let them dry out completely before you power them up!

I would start a mild detergent (soap and water). If that doesn't do the job (which it probably won't), try mineral spirits or a specialized electric motor cleaner. The biggest things I can think to look out for when using a solvent to clean the motor are that it doesn't damage the varnish on the motor windings or get into any greased bearings. The varnish is used as a coating on the motor windings, to keep them from touching each other and shorting out, so removing it would not be a great thing to do. If the motor does have greased bearings, you might want to lubricate them as well.

I've taken apart a few motors, but admittedly don't have a lot of experience in this field. Does anyone have a favorite technique or solvent that they use to restore vintage machines like this?

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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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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!

Bjorn writes in:

I'm wondering if you could help me out with resistors. I'm a computer science guy and am very new to electronics. Most of my resistor usage in my experimentation has been because I read about situations when I should use them online and just used a specific resistor because it was recommended. Obviously this only gets me so far. How do you go about determining what level of resistance you need and when you should or need to use resistors? I didn't know I was supposed to be hooking up LEDs to my Arduinos with resistors until I started burning out LEDs and read somewhere what type of resistor someone recommended. I know it has to do with the Arduino running at 5V and most LEDs being lower voltage, but how do you know what voltage an LED is if it's not labeled? And more importantly (like my question above), even if you know what voltage and LED is how are you supposed to figure out what kind of resistor to use?

Sure thing! This question actually comes up quite a bit, and it's a good thing to understand.

Putting a resistor in series with an LED limits the amount of current that can flow through it, preventing it from blowing up. It's nice to be able to figure out an exact resistance to use, but if you aren't too worried about your light being a bit dim, you can always just stick in a largish resistor, and you will get some light. For instance, if I am making a prototype circuit and just want an indicator light that I can control from an Arduino, I'll grab a 220 ohm resistor and put it in series with the first LED I can find. It's probably fine for a quickie circuit, but if you are soldering up a project that you intend to keep around, it's probably a better idea to actually calculate the correct amount of resistance to use.

So, how do you do it? Well, for a single LED, you can use the LED circuit equation:

Unfortunately, this only helps if you know the forward voltage and current rating of your diode. So, how can one figure those out? You can make an estimate abut the forward voltage of the diode based on it's color, because the wavelength of the color and the forward voltage are roughly inversely proportional to one another. Some typical forward voltages are: Red:1.8V, Green: 2V, Blue: 3.4V. Once you have an estimate, you can measure the forward voltage of your LED by hooking it up with the suggested resistor value, and measuring the voltage across it. Then, if you were way off, you can re-calculate the resistance with the actual voltage to get a better result.

Once you know the forward voltage drop, you can choose a current value based on how bright you want the LED to be, as long as it is lower than the current rating of the device. Unfortunately, it's a little bit harder to figure out the current rating, because most LEDs will continue to operate at higher than the recommended amount. If they are, though, their lifetime will be much shorter, and they won't be as efficient at producing light. If you don't have the specifications for your LED, 20ma is probably a safe maximum.

If you don't feel like solving all of that by hand, or have a more complicated design involving multiple LEDs, you could also use an online LED calculator. This one looks nice, because it will also tell you how to hook up multilple LEDs.

Ok, enough calculations. This is what can happen if you don't connect the LED up properly:

Boom! Engineers sometimes call these 'unauthorized thermal events'

Related:

• Wallet-size LED resistance calculator!
• MAKE Presents: The LED
• LED throwie analysis

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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!

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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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!

Chris writes in:

I have been experimenting with my Arduino and piezo buzzers as simple speakers - noisy and fun. But one question has been bugging me, what on earth is the third blue lead for on some piezos? If I leave the blue lead disconnected, the piezo seems to behave identically to its two-lead cousins. Am I missing out on some noise making opportunities?

Aha, good question. The short answer is no- the third lead is most likely used for feedback in an oscillator circuit, so leaving it disconnected shouldn't affect your circuit. The long answer is, well, maybe, if want to make your piezo into a buzzer.

There are two kinds of piezoelectric devices that are commonly sold as piezos: buzzers and transducers. Though they both use the same kind of ceramic disc to make noise, the difference is in how they are controlled. A piezo buzzer already contains some circuitry to create a buzzing noise, so all you have to do to make it work is connect it to a power source. The buzz can range from a tolerable alert to signal that your clothes are dry, to the ear-splitting noise of a fire alarm. A piezo transducer works more like a speaker, where you have to feed it an audio signal to get it to make noise. This is what you are using if you are generating your own frequencies with your Arduino.

So what does this have to do with your question? Well, it turns out that a really simple way to make a piezo buzzer is to use the feedback electrode that you were talking about to make a Harley oscillator circuit. As an example, I found this schematic in Murata's (a piezo manufacturer) Piezo Electric Sound Components Applications Manual:

The circuit is a little advanced, however the basic idea is that a small amount of the energy fed into the piezo device is fed into the input of the transistor, which amplifies the signal and feeds it back into the piezo. If the component values are chosen correctly, the resonation can be very efficient and loud, perfect for that fire alarm!

[title photo by Flickr user Josh Kopel]

Related:

• HOW TO - make piezo crystals at home
• Hacking the Glade Wisp

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