Another op amp parameter to pay attention to...

I recently managed to build a non-inverting amplifier with an op amp that acted as an inverting amplifier. I found this more than a little puzzling, and asked around for possible causes. My friends were kind enough not to laugh too much when they pointed out my error. 

The problem was that the op amp I had selected, the TL974, has a rather tight common mode voltage spec. It expects the common mode input voltage to be more than a volt from each power rail. The common mode voltage is the average of the two input voltages. Since those voltages are equal (or nearly so) in the non-inverting configuration, we can assume that the common mode voltage is equal to the input voltage.   

The input voltages on my circuit ranged from 0-1.8V against power rails of 0V and 3.8V. Not surprisingly, this didn't work very well; it caused the inverting and non-inverting inputs to swap places. As a result, instead of amplifying the input voltage across the intended gain of two, it shifted the signal up 1.8V and inverted it. Wheeee!

Here's a long but useful discussion of this issue on Planet Analog.

Circuit pattern trading cards

Arachnid Labs has come out with circuit pattern trading cards. Most electrical circuits are composed from a relatively small number of common patterns that can be designed and analyzed as units. The engineer can then compose these individual sub-circuits into a completed device.  

Arachnid Labs took this and produced a set of 32 trading cards that illustrate some of the most common ones. Once you recognize these patterns, you will begin to see them in every schematic you see.


New website!

If you haven't visited our website in a while, you may notice that it looks much different. That's because we've re-designed it and switched from Google Checkout over to Stripe for handling credit cards and the like. It doesn't require your credit card to be attached to your Google account in order to use it, which should help some of our institutional customers. 

We've tried to duplicate as many links from the old website as made sense. Please poke around and see if we've missed anything important. 

Come see us at Maker Faire Bay Area 2013!

It's that time of year again -- time for Maker Faire Bay Area! We'll be sharing a booth with the Metrix Open Hardware Alliance, which is a coalition of several Seattle-area open source hardware companies, including OpenBeam USA, organized by our friends at Metrix Create:Space. We'll be in booth 514, in the southeast quadrant of the Expo Hall.

Academic Zigduino projects

A couple of interesting Zigduino projects in academic settings have recently been published. The first is from Frank Zhao, the author of the ZigduinoRadio package. He and his seminar group at the University of Waterloo created ARUCI: Augmented Reality Universal Controller and Identifier.

The second is a paper presented at IMECS 2013 about using Zigduinos andContiki to build loosely coupled wireless sensor networks. They are working with a middleware layer for building sensor networks called LooCI and evaluating how the Zigduino behaves with it.

New Zigduino r2 documentation and availability

First, the entire first spin of the Zigduino r2 sold out in two weeks. Since it's Chinese New Year, and our bare PCBs are made in China, it will be several weeks before we have more in stock. However, for those of you who do have them, we've got some amazing new documentation to show you. 

Arduino forum user pighixx has created gorgeous and detailed pinout diagrams for every official Arduino variant and a number of unofficial ones, including the Zigduino.

As you can see, it covers every pin and every function. I love these diagrams, and I am very glad pighixx was willing to create them for us! You can also download a PDF version.

Introducing the Zigduino r2

Along with the New Year comes a new version of the Zigduino! It's based on a number of comments and feature requests we've gotten from users. We've made the Zigduino significantly more useful for many of the applications they're talking to us about. We endeavoured to retain full backward compatibility for most applications. Here's the most significant changes:


  • FTDI FT231X USB to serial interface
  • D0/D1 connected to UART1; UART0 remains connected to the USB interface
  • All extra I/O brought out to a 2mm pitch auxiliary header
  • Arduino Uno R3 header pattern


  • Added LiPo battery connector
  • Added LiPo battery charging circuit
  • Added battery monitoring circuit
  • The USB interface powered by USB bus; unpowered when USB is not connected


  • Arduino Uno R3 board layout
  • Reset switch relocated to the side of the board, between the power jack and the power header


  • The external antenna replaced with an on-board antenna
  • Added a footprint for an MMCX connector, allowing installation of an external antenna for greater range.


  • Added footprint for clock crystal & loading capacitors

Taken together, these changes make the Zigduino a more capable platform for remote sensor nodes, remote controls, and similar applications. We look forward to seeing a host of new Zigduino applications that use these new features to their fullest.

All of the boards and connectors are now in hand -- right now, we're working on getting them all packed and programmed so we can offer them for sale by the end of January. In the meantime, we are now out of the r1 version and we do not anticipate making any more. There are still a few with our distributors, including SeeedStudio and Snootlabs.

New Products!

We have two exciting new products we'd like to tell you about. We introduced both of these for World MakerFaire New York

Current/Voltage Monitor

We've created a tiny current and voltage monitor for monitoring power flow. Despite its small size, it can measure currents up to 30A, both unipolar and bipolar. The broad traces, Hall effect current sensing element, and the ability to connect ring terminals as large as #10 minimizes the insertion loss from adding this monitor to your circuit. It's thin enough to fit inside a piece of 1/2 conduit, and still has two more mechanical tricks as well. The main contacts are 1-1/8" apart, which allows you (with standoffs) to bolt the monitor across the contacts of a standard 3/8 barrier terminal strip. There is also a cut line of holes on each side, allowing the board to be trimmed for easy installation into a standard breadboard.

Electrically, the board outputs two analog voltages. The first first, labeled Isense, is proportional to the current flowing across the board. The Vsense voltage is the voltage between the main conductor and the provided signal ground. The Vsense output is diode clamped to a value between signal ground and the 5V power rail for the current sensor.

We're currently stocking four different current ranges and two voltage ranges, all for $15/ea. If none of these meet your requirements, send us an email at and we'll be happy to assist you.

RGB LED Matrix Backpack

We're particularly proud of the second product we've just introduced, the RGB LED Matrix Backpack. It abstracts the multiplexing and driving operations required to drive one of our 60 mm 8x8 RGB LED Matrix displays and gives the user a simple-to-use I2C interface. An HT1632C does the heavy lifting of driving the matrix while an ATtiny2313 provides the I2C interface. Just transmit the desired pattern as a 24 byte string and the backpack displays it on the included LED matrix until you remove power or send a different pattern. 

We decided to duplicate our earlier blinkenlights demo (Part 1, Part 2) with the new backpack. We edge-taped four of them into an array, hooked up an Arduino, and displayed the same three games of life we did before. You can compare the size and complexity of the code for the original demo with that for the new demo we were showing of at World MakerFaire. We'll get a blog post up analyzing this demo in the next week or so.

The kit comes with everything in the picture above for $30.