I’m extremely sorry about all the downtime and lag lately. Dreamhost was trying be on the cutting edge by upgrading to Ruby on Rails 1.1 last night. Unfortunately, Typo does not currently support Rails 1.1. A post in the forums brought me back online using slow CGI this morning.
The good news is that the site is back up and running. Dreamhost has also rolled back to Rails 1.0 allowing Superpositioned to speed along on FastCGI again.
Unfortunately between this fiasco and a borked Typo upgrade earlier this week, I have exhausted what little spare time I had. I promise updates soon, though. My TODO list is full of circuits and projects!
Update: As of April 4, 2006 everything should be working again. Wait.. I am getting the “Typo failed to start properly” message randomly again.
Update: A few minutes later and you should no longer be receiving that message.
I am back from spring break. As of today, I should have significantly more time to dedicate to this site, unless school/real work builds up on me again.
Anyway, I just stumbled upon the Open Automation project. This ‘open source’ project has the following mission statement:
To fill the gap between the powerful mobile robot platforms typically used by researchers, and the small rug-roving robots with limited processing power that are popular with hobbyists.
The specific project goals are:
Design a coherent set of modular components (hardware and software) that conform to standards (where possible), and implement the functionality of an intelligent mobile robot. Use pre-built components that are readily available where possible (and when such pre-built components are affordable).
Minimize cost. It should be possible to build a robot for around the price of a PC (target: US$1,500 to $2,000). Consumer grade hardware components are to be used in preference to professional grade products.
Focus on stereo vision as the primary spatial sensor to produce useful space occupancy data. Central to the success of this project is the implementation of a functioning low-cost real-time vision system. The prevalence of FireWire-enabled WebCams and mainboards makes this goal reachable from the standpoint of cost; the difficult part here is the software.
With two firewire webcams for eyes, a mini-itx brain, 4-12 sonar sensors, an I2C speech synthesizer, an LCD, the drive and chassis, and more, the bill of materials is not cheap. However, the prototype is really cute. If you want an overview of how it works, there is a block diagram of the system.
Unfortunately, the project seems to be hibernating right now, and there is not much detail on what to do when you are finished building the it. I would really love to build my own robot, but I would not know what to do with it.
Ladyada has an interesting rant on her new blog. In linear circuit analysis, we tend to ignore the difference between the positive and negative inputs. However, in practice positive feedback results in a bistable circuit. (Also read up on negative feedback.)
Usually everyone ends up remembering this detail the hard the way. While designing a tape head preamplifier she was utilizing this circuit from an application note:
After a few hours of staring at the circuit and debugging and wondering “man why the hell is this railing?” I finally look back at the datasheet and realize: oh its in positive feedback, of course its railing.
Just a reminder that the textbook (or pdf) isn’t always right!
Over the past week, I’ve been really busy with exams and projects. Trying to save time by finding the speed of light on Google, I stumbled upon an extremely interesting article on measuring the speed of light with a microwave. As any decent cook knows, microwaves do not heat evenly. In fact, this article explains their heating patterns are relative to the speed of light!
Understanding how a microwave heats
As we all know, microwaves heat using electromagnetic waves. These waves are at a frequency perfect for rotating water molecules (f = 2.5 GHz). The rotating water molecules create friction and thereby heat.
Two types of electromagnetic waves
Although there are two types of electromagnetic waves, we typically only consider traveling waves. The amplitude of the wave travels forward in position over times. The following animation demonstrates the amplitude of a wave over space and time.
The waves in a microwave are not traveling. If they were, it would be nearly impossible to distinguish any uneven heating patterns!
Standing waves in a microwave
The waves in a microwave oven are standing waves. These waves are stationary in space with an amplitude changing over time.
With this demonstration, it is obvious that particular sections of the chips are heated more than others. In fact, these locations are located half of the wave’s length apart.
The physics of waves
We now know the frequency of the microwave and can presumably measure the length of the wave, but how are they related to the speed of light? Simple. Electromagnetic waves propagate through free space (like that in a microwave) at the speed of light. Therefore, their length is related directly to the speed of light by λ = c / f where λ is the wavelength, c is the speed of light, and f is the frequency of the microwave. Solving for the speed of light, c = λ * f.
Where do the chocolate chips come in?
Chocolate chips are perfect for measuring the distance between melted spots. The heat does not spread as quickly through them because they are not uniform. This means the melted spots will be smaller and you will have more time to measure before they all start to melt.
It is hard to tell from the photos, but there were distinct melting spots almost exactly 6cm apart. Remember, this is only half of the wavelength, so λ = 12cm. Plugging all the known variables into our equation, we get c = 12×10-2 * 2.5×109 = 3×108. Not bad! The true speed of light is 2.9987×108.
Notes if you replicate this experiment
The chocolate chips only take 20-30 seconds to melt. The longer you have them in, the bigger the melted spots will be and the less time you will have to measure.
This will not work in a microwave with a spinning carousel. In fact, the microwave spins to counteract these effects. Usually, you can just flip the carousel upside down to stop it from spinning. (Thanks Ryan)
If you plan on putting the chips back in the bag, simply refrigerate them. Freezing causes them to stick to the plate.
You can microwave anything that melts. (Cheese or a chocolate bar) However, chips work particularly well.
USA Today was recently asked, “If both electric and magnetic fields can pass through paper why not light?” If you think about it, this is a really good question. The simple answer: paper absorbs/scatters light, but not low or high energy electromagnetics. Read the article if you want to learn more.
BigDog is a quadraped robot being developed by Boston Dynamics for possible military usage. It is by far the most advanced quadraped around. The site has more mechanical details and a video! About 30 seconds into the video, he kicks BigDog and it stays balanced!
So far, BigDog has trotted at 3.3 mph, climbed a 35 degree slope and carried a 120 lb load. BigDog is being developed by Boston Dynamics with help from Foster Miller, the Jet Propulsion Laboratory, and the Harvard University Concord Field Station.Development is funded by the DARPA Defense Sciences Office.
Scouring the Internet for information on LEDs, I accidentally stumbled upon a PDF detailing a flashlight made from PVC. For the torch, he biases ultra-bright LEDs with ballast resistors as described in my LED lighting guide. This is a simple solution, but a with a slightly more complicated circuit we can extend battery life by over ten times!
In this flashlight, each LED/resistor combination consumes 4.5volts at 30mA or about 135mWatts. The ballast resistor alone consumes 1.1volts at 30mA or about 33mWatts. Therefore, 25% of the power being drained from the batteries is lost in the resistor of each LED chain!!
Battery life
The ‘C’ size batteries in the PVC flashlight have a capacity of around 4 500mAh. If you build your flashlight with seven LEDs, there is a constant current drain of 7 * 30mA or 210mA. Dividing this into our battery capacity, it becomes obvious this flashlight will only stay lit for about a day. This is a far cry from the 50-100 hours claimed by commercial flashlights running on smaller ‘AA’ batteries.
Cutting your losses
As mentioned above, the ballast resistors are wasting 25% of our battery power. Electronic Design proposed a simple circuit to resolve this in a recent article. The front end of their circuit draws less than a milliamp of extra current.
The circuit is best described in two parts: one, the boost circuit function of Q1 and Q2, and two, the control circuit of Q3 and JFET1. Assume Q1 is off. With the battery voltage slightly above Q2’s VVB, a positive Q2 base current [iB = (battery voltage VBE)/RJET1] would flow. Q2 turns on, which switches inductor L1 to ground.
The end result is a 23volt pulse (as shown in Figure 2) across the series of ultra-bright LEDs. At 278kHz, the human eye cannot distinguish the difference between these pulses and a constantly lit LED. This saves even more battery power.
As the battery voltage decreases, the pulses become further apart. The brightness remains indistinguishable until the voltage falls near 2volts. (The circuit does not function well below 2volts) I doubt the PVC flashlight has this efficiency near the end of its life.
The extended battery life
According to Electronic Design, this circuit consumes an current equivalent to about 17mA. Powered by our ‘C’ cell in the PVC, this circuit could run for 265 hours! This is ten times the original PVC design.
