Tag Archives: arduino uno shield

Arduino-based Inductance Meter

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Incuctance meter in action. It displays the resonance frequency together with the inductance

I’ve just finished a little Arduino project. It’s a shield for the Arduino Uno that lets you measure inductance. This is a functionality that I found missing in just about any digital multi meter. Yes, there are specialized LCR meters that let you measure inductance but they typically won’t measure voltages or currents. So I had to build my inductance meter myself.

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Close-up of the circuit with the display removed

The basic design is really simple. It a colpitts oscillator (http://en.wikipedia.org/wiki/Colpitts_oscillator) with the coil missing. You use the test leads to connect it to a coil which will make it resonate. The Arduino then measures the frequency at which the oscillator is resonating and calculates the inductance. The capacitors are part of the shield so the capacity is known.

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With the test leads open, the oscillator can’t resonate. The current calibration/zero-offset is displayed in stead

There is 1uH of inductance included on the schield which is placed in series with the coil to be measured. This serves two purposes: The oscillator can resonate when you short-circuit the test leads. When you then press the push button on the shield, the software will use the current measurement as new calibration. It also puts an upper limit on the resonance frequency. This ensures that the software the rest of the circuit can keep up with the oscillator.

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Pressing this blue button zeroes the meter

As can be seen from the schematic, the oscillator uses two 1nF capacitors in series. Together with the 1uH inductance, this limits the frequency to about 7.1MHz. In practice, it oscillates at around 5.4MHz when the test leads are short-circuited.

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The Arduino shield from below

The oscillator output is followed by a comparator turning the sine wave of the oscillator into a square wave. I’ve used an inexpensive but fast Microchip MCP6561R. It has a maximum propagation delay of 80ns which allows it to keep up at the maximum frequency.

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Viewed from straight above

But of course, 5.4MHz is way too fast for the Arduino to keep up. The Arduino runs at 16MHz and will need at least a few dozend instructions to process each pulse from the shield. My solution was to add a 74HC590 8-bit binary counter dividing the frequency by 256. That gives a theoretical maximum frequency of 7.2MHz / 256 = 27.7kHz. That’s something the Arduino can deal with.

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The entire shield with the display removed

For obvious reasons, there is also a display included on the shield. And then there’s that pushbutton which is debounced in hardware by running it through an RC low-pass filter and a Schmitt-triggered buffer. The button is used to zero the meter, i.e. the current measurement is used as the new zero-offset.

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Even very small inductance values can be measured

All related files can be downloaded as a .zip file: LMeterShield. This includes the Arduino source code (aka sketch) as well as the Eagle files and PDFs of both the layout and the schematic.

Now there’s also a stand-alone version: http://soldernerd.com/2015/01/14/stand-alone-inductance-meter/

Arduino Ultrasonic Anemometer Part 10: Arduino Shield Ready

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A world’s first: Ultrasonic Anemometer Shield for Arduino Uno

I’m happy to announce that my new Arduino wind meter shield is ready. I had posted the design as well as a photo or two of the naked board in my last post but now I’ve placed and soldered all the numerous components and it’s ready to go.

Click here for an overview over this series of posts on the Arduino Ultrasonic Anemometer: http://soldernerd.com/arduino-ultrasonic-anemometer/

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Looking pretty. Any good? Don’t know yet.

So now all my custom hardware boils down to this easy-to-use Arduino UNO shield. Just stack it on top of your Arduino,  attach four ultrasonic transducers and you’re ready to go.

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Bottom side

I have high expectations for this shield and hope I won’t be disappointed. I’ll check on the weekend when I’ll first power it up and see if it’s any good. I’ll let you know.

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Straight from above

Click here to see how the new shield perfoms: http://soldernerd.com/2014/11/30/arduino-ultrasonic-anemometer-part-11-testing-the-new-hardware/