July Issue Offers Data-Gathering Designs and More

The concept of the wireless body-area network (WBAN), a network of wireless wearable computing devices, holds great promise in health-care applications.

Such a network could integrate implanted or wearable sensors that provide continuous mobile health (mHealth) monitoring of a person’s most important “vitals”—from calorie intake to step count, insulin to oxygen levels, and heart rate to blood pressure. It could also provide real-time updates to medical records through the Internet and alert rescue or health-care workers to emergencies such as heart failures or seizures.

Data Gathering DesignsConceivably, the WBAN would need some sort of controller, a wearable computational “hub” that would track the data being collected by all the sensors, limit and authorize access to that information, and securely transmit it to other devices or medical providers.

Circuit Cellar’s July issue (now available online for membership download or single-issue purchase)  features an essay by Clemson University researcher Vivian Genaro Motti, who discusses her participation in the federally funded Amulet project.

Amulet’s Clemson and Dartmouth College research team is prototyping pieces of “computational jewelry” that can serve as a body-area network’s mHealth hub while being discreetly worn as a bracelet or pendant. Motti’s essay elaborates on Amulet’s hardware and software architecture.

Motti isn’t the only one aware of the keen interest in WBANs and mHealth. In an interview in the July issue, Shiyan Hu, a professor whose expertise includes very-large-scale integration (VLSI), says that many of his students are exploring “portable or wearable electronics targeting health-care applications.”

This bracelet-style Amulet developer prototype has an easily accessible board.

This bracelet-style Amulet developer prototype has an easily accessible board.

Today’s mHealth market is evident in the variety of health and fitness apps available for your smartphone. But the most sophisticated mHealth technologies are not yet accessible to embedded electronics enthusiasts. (However, Amulet has created a developer prototype with an easily accessible board for tests.)

But market demand tends to increase access to new technologies. A BCC Research report predicts the mHealth market, which hit $1.5 billion in 2012, will increase to $21.5 billion by 2018. Evolving smartphones, better wireless coverage, and demands for remote patient monitoring are fueling the growth. So you can anticipate more designers and developers will be exploring this area of wearable electronics.

AND THAT’S NOT ALL…
In addition to giving you a glimpse of technology on the horizon, the July issue provides our staple of interesting projects and DIY tips you can adapt at your own workbench. For example, this issue includes articles about microcontroller-based strobe photography; a thermal monitoring system using ANT+ wireless technology; a home solar-power setup; and reconfiguring and serial backpacking to enhance LCD user interfaces.

We’re also improving on an “old” idea. Some readers may recall contributor Tom Struzik’s 2010 article about his design for a Bluetooth audio adapter for his car (“Wireless Data Exchange: Build a 2,700-lb. Bluetooth Headset,” Circuit Cellar 240).

In the July issue, Struzik writes about how he solved one problem with his design: how to implement a power supply to keep the phone and the Bluetooth adapter charged.

“To run both, I needed a clean, quiet, 5-V USB-compatible power supply,” Struzik says. “It needed to be capable of providing almost 2 A of peak current, most of which would be used for the smartphone. In addition, having an in-car, high-current USB power supply would be good for charging other devices (e.g., an iPhone or iPad).”

Struzik’s July article describes how he built a 5-V/2-A automotive isolated switching power supply. The first step was using a SPICE program to model the power supply before constructing and testing an actual circuit. Struzik provides something extra with his article: a video tutorial explaining how to use Linear Technology’s LTspice simulator program for switching design. It may help you design your own circuit.

This is Tom Struzik's initial test circuit board, post hacking. A Zener diode is shown in the upper right, a multi-turn trimmer for feedback resistor is in the center, a snubber capacitor and “stacked” surface-mount design (SMD) resistors are on the center left, USB D+/D– voltage adjust trimmers are on top center, and a “test point” is shown in the far lower left. If you’re looking for the 5-V low dropout (LDO) regulator, it’s on the underside of the board in this design.

This is Tom Struzik’s initial test circuit board, post hacking. A Zener diode is shown in the upper right, a multi-turn trimmer for feedback resistor is in the center, a snubber capacitor and “stacked” surface-mount design (SMD) resistors are on the center left, USB D+/D– voltage adjust trimmers are on top center, and a “test point” is shown in the far lower left. If you’re looking for the 5-V low dropout (LDO) regulator, it’s on the underside of the board in this design.

 

Arduino MOSFET-Based Power Switch

Circuit Cellar columnist Ed Nisley has used Arduino SBCs in many projects over the years. He has found them perfect for one-off designs and prototypes, since the board’s all-in-one layout includes a micrcontroller with USB connectivity, simple connectors, and a power regulator.

But the standard Arduino presents some design limitations.

“The on-board regulator can be either a blessing or a curse, depending on the application. Although the board will run from an unregulated supply and you can power additional circuitry from the regulator, the minute PCB heatsink drastically limits the available current,” Nisley says. “Worse, putting the microcontroller into one of its sleep modes doesn’t shut off the rest of the Arduino PCB or your added circuits, so a standard Arduino board isn’t suitable for battery-powered applications.”

In Circuit Cellar’s January issue, Nisley presents a MOSFET-based power switch that addresses such concerns. He also refers to one of his own projects where it would be helpful.

“The low-resistance Hall effect current sensor that I described in my November 2013 column should be useful in a bright bicycle taillight, but only if there’s a way to turn everything off after the ride without flipping a mechanical switch…,” Nisley says. “Of course, I could build a custom microcontroller circuit, but it’s much easier to drop an Arduino Pro Mini board atop the more interesting analog circuitry.”

Nisley’s January article describes “a simple MOSFET-based power switch that turns on with a push button and turns off under program control: the Arduino can shut itself off and reduce the battery drain to nearly zero.”

Readers should find the article’s information and circuitry design helpful in other applications requiring automatic shutoff, “even if they’re not running from battery power,” Nisley says.

Figure 1: This SPICE simulation models a power p-MOSFET with a logic-level gate controlling the current from the battery to C1 and R2, which simulate a 500-mA load that is far below Q2’s rating. S1, a voltage-controlled switch, mimics an ordinary push button. Q1 isolates the Arduino digital output pin from the raw battery voltage.

Figure 1: This SPICE simulation models a power p-MOSFET with a logic-level gate controlling the current from the battery to C1 and R2, which simulate a 500-mA load that is far below Q2’s rating. S1, a voltage-controlled switch, mimics an ordinary push button. Q1 isolates the Arduino digital output pin from the raw battery voltage.

The article takes readers from SPICE modeling of the circuitry (see Figure 1) through developing a schematic and building a hardware prototype.

“The PCB in Photo 1 combines the p-MOSFET power switch from Figure 2 with a Hall effect current sensor, a pair of PWM-controlled n-MOFSETs, and an Arduino Pro Mini into

The power switch components occupy the upper left corner of the PCB, with the Hall effect current sensor near the middle and the Arduino Pro Mini board to the upper right. The 3-D printed red frame stiffens the circuit board during construction.

Photo 1: The power switch components occupy the upper left corner of the PCB, with the Hall effect current sensor near the middle and the Arduino Pro Mini board to the upper right. The 3-D printed red frame stiffens the circuit board during construction.

a brassboard layout,” Nisley says. “It’s one step beyond the breadboard hairball I showed in my article “Low-Loss Hall Effect Current Sensing” (Circuit Cellar 280, 2013), and will help verify that all the components operate properly on a real circuit board with a good layout.”

For much more detail about the verification process, PCB design, Arduino interface, and more, download the January issue.

The actual circuit schematic includes the same parts as the SPICE schematic, plus the assortment of connectors and jumpers required to actually build the PCB shown in Photo 1.

Figure 2: The actual circuit schematic includes the same parts as the SPICE schematic, as well as the assortment of connectors and jumpers required to actually build the PCB shown in Photo 1.