My last project brought up the subject of capacitive touch sensing. You may not have realized this, but capacitive touch sensing is currently on the forefront of electronics. Mechanical touch sensors are known to wear and ‘push-less’ sensors are just cooler. In case you did not know, the scroll wheel on an Ipod uses an array of capacitive sensors along with the infamous touch lamp.

For the engineer in you, Planet Analog has a lengthy overview. Basically, you design a circuit that is highly dependent on the value of a small capacitor. The capacitance in a finger then causes a significant and detectable change in the circuits output.

The doorknob touch alarm functions in just this manner. It does its job, but is not as precise as more complex circuits. For more schematics, check out Discover Circuits. A recent article on DesignLine proposed that this interface be adapted to mobile phone keypads.

There are two major microcontrollers in the hobby domain, the popular PIC and Atmel’s AVR line. PICs are tried and true in both commercial and hobby implementations, but the Atmel AVR offers affordable programming solutions, a free development environment, a free assembler and a stable gcc toolkit that work across the entire AVR line.

Depending on the application, charging batteries can be complex process. Charging methods range from constant voltage to pulsed and random charging. Once power is being delivered back into the battery, you have to know when to stop charging!

Once a battery is fully charged, the charging current has to be dissipated somehow. The result is the generation of heat and gasses both of which are bad for batteries. The essence of good charging is to be able to detect when the reconstitution of the active chemicals is complete and to stop the charging process before any damage is done.

Typically, common household batteries are charged with a current that is kept constant and relieved when the batteries reach a predetermined potential. However, solar cells typically generate a constant voltage of 0.5V and a varying current that depends on the amount of collected light. As such, a consant voltage charging model is easier to implement. I found two respectable tutorials on building your own charger:

1. AA Battery Solar Charger
2. Clean Power’s Solar battery project charger

The first solution uses a diode to stop the batteries from discharging when there is no sunlight. I highly reccomend including this protection. Unfortunately, neither project implements a charge limit. You have to remove the batteries and test their charge with a multimeter. A shunt regulator is the simplest way to regulate the upper limit.

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!

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.

Amateur Logic has been podcasting interesting projects including the active cantenna in episode 3. Their latest episode is straight from the Capital City Hamfest. There is an ARRL review of the amateur radio effort during Katrina, a presentation about Skywarn, and an inspiring amateur satellite radio demonstration. There is not much technical information, but it is a great introduction to what being a ham is about.

To learn more about amateur radio visit arrl.org or amsat.org.

LED lighting is becoming increasingly popular in fish tanks, case mods, and even household lighting. This article intends to be a comprehensive guide to their advantages, powering them, and creating dimming solutions.

• Why use LED lighting?
• Powering your LEDs with a DC source
• Using an AC source to drive LEDs
• Dimming your LEDs (with PWM)
• Purchasing LEDs

Why use LED lighting?

LED lights are extremely efficient compared to standard incandescent lighting. No other lighting source outputs as many lumens per watt. They are particularly efficient at producing a single color of light. Other light sources have to produce the entire spectrum and optically filter out unwanted colors. However, LEDs can be manufactured to produce only one wavelength of light. This makes them particularly useful in stop lights.

Several factors determine how dangerous electricity can be. Voltages, currents, resistances, and frequencies are the major factors that determine whether you will be hospitalized or just feel a little buzz. So consider all this the next time you plan on sticking your finger in an electrical outlet or straddling an electric fence.

Voltage doesn’t kill. Current kills.

Contrary to popular belief, you cannot feel high voltage directly and it will not kill you. Voltage is simply the amount of stored energy at a point in space. Current is the motion of that charge and can have lethal effects. As we will see later, there is a relationship between voltage and current that makes high voltage significant. This concept is best explained through the following analogy.

Spark Fun has barely had the capitol to invest in a hot-air rework station… as business has increased, revenue has allowed us to purchase a few bottom-dollar machines to help… it was all a waste of money…

If you have ever tried soldering ICs and surface mount elements to a circuit board, then you know how hard it is. Unfortunately, the only commercial alternatives cost thousands of dollars. At least until Spark Fun turned a $30 hot skillet into a reflow oven! “I don’t think anything can replace the large scale, multi-zone, IR/convection reflow ovens. But I could by 100 hot plates at $30 a piece”

They also review the traditional toaster ‘reflow’ oven and commercial alternatives. $30 is still outside my price range. I’ll take the soldering iron.

The good people over at E-DSP have churned out a comprehensive tutorial on how to use an lcd with your electronic devices. The Amtel AVR is their microcontroller of choice because of its cheap programmer. From pinouts to microcontroller code, they make it look easy. In a way it is.