Priority Interrupt: Elektor & Circuit Cellar Join Forces

A Handshake and a Future
by Steve Ciarcia

I got an interesting letter from a young engineer a couple months ago. He was a long-time Circuit Cellar reader who wanted to know how I got my start and if it was still possible to emulate my career path in today’s business climate. Apparently, he was at his third job out of college and increasingly disappointed that no one shared his entrepreneurial fervor and design creativity. Basically, he wanted to start his own company and was asking for advice... -More-

A LETTER FROM THE MANAGING EDITOR:

Dear Valued Readers:

The Circuit Cellar editorial staff is excited about the new strategic partnership between Elektor International Media (publisher of Elektor magazine) and Circuit Cellar. By partnering with Elektor (www.elektor.com)—which has a long history of publishing excellence, an large attentive audience, an in-house laboratory, and a successful online storefront (featuring products such as PCBs, kits/modules, books, and CD/DVDs)—Circuit Cellar now has the opportunity to deliver fresh content about embedded applications to an even wider audience than ever!

Elektor and Circuit Cellar remain separate magazines. We’re partners, not competitors.

On behalf of the Circuit Cellar editorial department, I want you to know that we will continue to do what we do best: provide talented engineers and academics with informative articles and competitive design contests featuring the newest embedded technologies. Circuit Cellar is about effective hardware and software methods for embedded-control systems. This won’t change.

As we approach the new year, I encourage you to continue subscribing to this beloved magazine begun by Steve Ciarcia more than 20 years ago. And since neither Steve nor the editorial staff is going anywhere, 2010 is sure to be our best year ever!

As usual, if you want us to consider publishing your work, feel free to e-mail me an article proposal or finished article at any time! Write to cj@circuitcellar.com.

Happy New Year!

Regards,

C. J. Abate

Managing Editor

Circuit Cellar

EQ Interactive

Problem 1—The output stage of a generic '1488 RS-232 driver is shown below. The input stage has an output impedance of roughly 10 Kilohms and functions so that Q1 is basically either cut off or saturated. One of the functions of an RS-232 driver is to limit the drive current to approximately 10 mA. Does this circuit accomplish that? If so, show how it does it for both output states.

Problem 2—What is the function of D3 in the circuit in question 1?

Think You Have a Great EQ Challenge of Your Own?
E-mail your best EQ question and answer to eq@circuitcellar.com for a chance to be recognized by Circuit Cellar as an EQ guru.

Answer 1—When the output is low, Q1 is saturated and drives Q3 into conduction. The emitter resistor of Q3 functions as a current-sense resistor. When the voltage across it begins to exceed the VBE drop of Q4 (about 0.7 V), Q4 begins to conduct and reduces the voltage at the base of Q1, which in turn reduces the drive to Q3. The output current is limited to 0.7 V / 70 ohms = 10 mA. When the output is high, Q1 is cut off and only Q2 is conducting. This transistor is wired as a current source, with the voltage reference being supplied by the two diodes, D1 and D2, between its base and emitter. One of the diodes compensates for the VBE drop of the transistor, while the other limits the maximum voltage drop across the emitter resistor. If this drop becomes too high, Q2's base current is reduced. Again, a maximum drop of 0.7 V across 70 ohms limits the current to 10 mA.

Answer 2—The collector of Q3 is connected to the junction of D1 and D2 in order to make sure that the pull-up current source (Q2) is cut off when the output is low. Otherwise, all of the capacity of the pull-down circuit would be used by the current supplied by the pull-up source.

D3 provides the path for the output current to flow to Q3 when the output is low. Without it, the current would be blocked by D2.

D4 and D5 are protection diodes that clamp the output voltage if something back-drives the output pin. The 300-ohm resistor helps limit the current in that case.

WIZnet iMCU Design Contest Update

WIZnet wants to hear from you. There are many reasons why a company may choose to sponsor a design contest. Companies like WIZnet understand that one of the major benefits is access to people with invaluable insight—people like you. This is why it’s a great idea to register for WIZnet’s discussion forums. Not only will you be able to see what your peers are doing with WIZnet iMCU, your forum entries will also be read by some very important and responsive team members at WIZnet. Know that your comments, questions, and input has a tremendous impact on the decisions of tomorrow that affect all designers interested in embedded Ethernet. Register today!

(Note: Those anticipating a Q1 2010 Circuit Cellar contest schedule should familiarize themselves with the following. Stay tuned for more details.)

New ARM® Cortex™-M3 Microcontroller with Integrated SafeRTOS in ROM

New Stellaris LM3S9B96 Microcontroller Saves Developers Tens of Thousands of Dollars in License Fees

Texas Instruments and WITTENSTEIN High Integrity Systems, providers of robust embedded kernels for use in industrial, medical, and aerospace applications, announced a new Stellaris microcontroller featuring the SafeRTOS real-time kernel integrated into on-chip ROM. The LM3S9B96 microcontroller extends the industry-leading capabilities of the fourth generation of Stellaris microcontrollers with an IEC61508 compliant RTOS kernel at no additional cost to developers.

>> LM3S9B96 Microcontroller
>> LM3S9B96 Development Kit
>> More about SafeRTOS™

Color Changing Christmas Ornament

Oliver H. Bailey

I always enjoy holiday projects for the experimenter and hobbyist. With this in mind, I was browsing my local craft store recently and found a couple packages of plastic snap-together ornaments in 3.5 and 4 inch sizes. I grabbed one of each size and started thinking about this column as I walked around the store.

When I got back to the lab, I dug up some tri-color LED’s, a battery pack, photocell, 8 pin PSoC, and small circuit board to solder everything to. I added a few resistors and built a color changing ornament in about an hour. This is a very simple circuit and requires three pins for the LED, one

pin for the photocell, VCC, and VDD for the power supply.

Getting Started -Click Here to Download the PDF-

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