Nuts and Volts - June 2017

I measured the voltage increase of batteries as they got colder, and created a voltage vs. temperature plot. Using a linear voltage ramp, I was able to plot the I-V curve of different diodes and match their response to Shockley’s diode equation to better than 1% accuracy.

I paid the huge sum of $15,000 for it. Now, I can build an even more powerful instrument with an Arduino, a shield, and freeware software for less than $100. It measures four input voltage channels at 16-bit resolution, and up to 40,000 samples per second directly into Excel. Once there, I can plot the data in any format I want.

This is a high performance general-purpose instrument that has become the Swiss army knife of my home lab. It is my go-to instrument to measure, record, and display data in just about every experiment I do.

I use it as a data logger, a strip chart recorder, and as an audio signal analyzer. I built a vibration analyzer to measure the impact of damping materials for a fan (see Figure 1). With it, I built a micro ohmmeter to measure the typically 20 µΩ contact resistance of a solder joint. I built a noise spectrometer to measure the excess 1/f noise in carbon resistors. I turned it into an impedance analyzer, and fit simple LRC models to the measured impedance profile of any component. I measured the voltage increase of batteries as they got colder, and created a voltage vs. temperature plot. Using a linear voltage ramp, I was able to plot the I-V curve of different diodes and match their response to Shockley’s diode equation to better than 1% accuracy.

These are just some of the measurements this instrument can do. Here’s how you can build this versatile precision data acquisition system.

The Arduino Due Microcontroller

The Arduino family of microcontrollers and boards come in a wide range of price, performance, and form factors. They each have their plusses and minuses. Most of us start out using the Arduino Uno because it is simple, cheap, and readily available. However, when it comes to performance and board size form factor, my favorite Arduino is the Due (Figure 2). I like the Due for three reasons: performance, memory, and form factor.

The Due uses a 32-bit Atmel ARM Cortex M3 SAM3X8E CPU with an 84 MHz clock. This is compared to the eight-bit and 16 MHz clock of the Uno. The combination of higher clock frequency and 32-bit operation means some functions can be more than 20x faster with a Due.

The Due has much more memory, which means larger programs can be written and larger data arrays can be used. A sketch is stored in the Flash memory, while variables are stored in the SRAM memory. The Due has 512K of Flash as compared to the 32K in the Uno, and it has 96K of SRAM compared with 2K in the Uno. I have never written a sketch that runs out of memory on a Due.

When collecting data, the SRAM is important as this is

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■ FIGURE 1. Example of a crystal microphone with its foil removed, mounted to a fan. The voltage from the PZT wafer picks up vibrations. The graph has 1,000 measurements during the 90 milliseconds.