Week 9: Programming - Input Devices

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Week 9: Programming - Input Devices

Incorporate a sensor into a microcontroller board.
Read something with the sensor.

After experiencing difficulties inducing a magnetic field in Week 7, I opted to incorporate a Hall Effect sensor into my microcontroller board to further assist with final project design concepts. Previously, Anthony provided me a prefabricated Hall Effect sensor that used Texas Instruments' DRV5055 Ratiometric Linear Hall Effect sensor, which is a single axis magnetic field sensor that can be operated by 3.3 V or 5 V. The typical schematic and component diagram below are from the component’s datasheet.

Having a Hall Effect sensor that I was confident would work, I set out on designing and fabricating my own Hall Effect sensor with components available from the fab lab library. My design was built around the Allegro's A1302 Hall Effect sensor, which is a single axis magnetic field sensor that operates with 5 V. The functional block diagram below is from the component's datasheet.

Using Eagle through Fusion 360 I generated the schematic for my sensor board. I based the circuit on requirements from the component’s datasheet. My CAD design and .PNG milling files are hyperlinked to the below images.

While designing my Hall Effects sensor I was fully aware that the Allegro sensor required an input voltage of 5 V, but I still included a voltage regulator footprint in my design. I based this off the theory that I could use a 5 V regulator to ensure more consistent voltage being provided to the sensor. It turns out this was a silly consideration and there aren't even 5 V regulators in the fab lab inventory. So, I ended up soldering a jumper wire across the voltage regulator pads. When I redesign my sensor for the final project, I will remove this component and add vias to serve as mounting points for the board.

Post Milling

Final Product

I was running a little behind on work this week, because I missed the memo on fabricating your own sensor, which is obviously the moto the class, ‘Make it, don't buy it’. After using the code I wrote for this week, I recognized it was horribly inefficient due to the LED delay command. I incorporated the blinking LED function to provide visible reassurance that the microcontroller was programmed. In hindsight, I should have just removed the LED function or used a 'while' command for reading the Hall Effect sensor.

Prior to being able to receive an input from the Hall Effect sensors, I had to power to them. I failed to break out VCC legs on my board during the design phase, so to power the sensors I had to use microcontroller pins or add jumper wires. The DVR sensor only required 3.3 V, so I could have set one of the microcontroller pins I did break out to power it, but instead opted to add a jumper wire. The Allegro sensor required 5 V, so I have to solder on a jumper upstream from the board's 3.3 V voltage regulator.

The below serial plotter images are the result of changing proximity and polarity of a N42 permanent magnet. The DVR sensor appeared to perform better with little to no apparent hysteresis after changing polarity. In the future I would like to refine input of these sensor by applying a moving average filter, as demonstrated during this week's recitation.

DVR Hall Effect Sensor

Allegro Hall Effect Sensor (Own Fabrication)

Using a power supply to energize different electromagnets, I used my sensor to measure the relative strengthen of the induced magnetic field from each electromagnet. To see the results of this testing please visit my Final Project page here.

The group assignment this week involved probing an input device's analog/digital signal. Using an oscilloscope, I probed the input voltage from each sensor to my microcontroller while conducting the same permanent magnet test described above. Both the serial plot and scope window for each sensor are shown below. As expected, the input voltage follows the serial plotter position. The DVR potential spanned 2.95 V, and the Allegro potential spanned 3.7 V; both of these ranges are within the spans specified by their datasheets. In the future, I would like correlate magnetic field strength between the two sensors based on datasheet sensitivity information.

DVR Hall Effect Sensor

Allegro Hall Effect Sensor (Own Fabrication)

Sensors typically require three leads (VCC, Ground, Output). When building a generic board it is important to jump out supply voltage to power sensors.
In coding using 'delay' commands can be very inefficient, because no other functions occur while it is running. Alternately, a 'while' command can be used to run multiple functions at the same time.