Blog – Page 12 – Pocket Science Lab

Expandable ListView In PSLab Android App

Post author:viveksb007
Post published:July 23, 2017
Post category:Android FOSSASIA GSoC Pocket Science Lab

In the PSLab Android App, we show a list of experiments for the user to perform or refer to while performing an experiment, using PSLab hardware device. A long list of experiments need to be subdivided into topics like Electronics, Electrical, School Level, Physics, etc. In turn, each category like Electronics, Electrical, etc can have a sub-list of experiments like:

Electronics
- Diode I-V characteristics
- Zener I-V characteristics
- Transistor related experiments
Electrical
- Transients RLC
- Bode Plots
- Ohm’s Law

This list can continue in similar fashion for other categories as well. We had to display this experiment list to the users with a good UX, and ExpandableListView seemed the most appropriate option.

ExpandableListView is a two-level listView. In the Group view an individual item can be expanded to show it’s children. The Items associated with ExpandableListView come from ExpandableListAdapter.

Implementation of Experiments List Using ExpandableListView

First, the ExpandableListView was declared in the xml layout file inside some container like LinearLayout/RelativeLayout.

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:orientation="vertical">
   <ExpandableListView
       android:id="@+id/saved_experiments_elv"
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
       android:divider="@color/colorPrimaryDark"
       android:dividerHeight="2dp" />
</LinearLayout>

Then we populated the data onto the ExpandableListView, by making an adapter for ExpandableListView by extending BaseExpandableListAdapter and implementing its methods. We then passed a Context, List<String> and Map<String,List<String>> to the Adapter constructor.

Context: for inflating the layout

List<String>: contains titles of unexpanded list

Map<String,List<String>>: contains sub-list mapped with title string

public SavedExperimentAdapter(Context context,
                                 List<String> experimentGroupHeader,
                                 HashMap<String, List<String>> experimentList) {
       this.context = context;
       this.experimentHeader = experimentGroupHeader;
       this.experimentList = experimentList;
   }

In getGroupView() method, we inflate, set title and return group view i.e the main list that we see on clicking and the sub-list is expanded. You can define your own layout in xml and inflate it. For PSLab Android, we used the default one provided by Android

 android.R.layout.simple_expandable_list_item_2

@Override
public View getGroupView(int groupPosition, boolean isExpanded, View convertView, ViewGroup parent) {
   String headerTitle = (String) getGroup(groupPosition);
   if (convertView == null) {
       LayoutInflater inflater = (LayoutInflater) this.context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
       convertView = inflater.inflate(android.R.layout.simple_expandable_list_item_2, null);
   }
   TextView tvExperimentListHeader = (TextView) convertView.findViewById(android.R.id.text1);
   tvExperimentListHeader.setTypeface(null, Typeface.BOLD);
   tvExperimentListHeader.setText(headerTitle);
   TextView tvTemp = (TextView) convertView.findViewById(android.R.id.text2);
   tvTemp.setText(experimentDescription.get(groupPosition));
   return convertView;
}

Similarly, in getChildView() method, we inflate, set data and return child view. We wanted simple TextView as sub-list item thus inflated the layout containing only TextView and setText by taking reference of textView from the inflated view.

@Override
public View getChildView(int groupPosition, int childPosition, boolean isLastChild, View convertView, ViewGroup parent) {
   String experimentName = (String) getChild(groupPosition, childPosition);
   if (convertView == null) {
       LayoutInflater inflater = (LayoutInflater) this.context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
       convertView = inflater.inflate(R.layout.experiment_list_item, null);
   }
   TextView tvExperimentTitle = (TextView) convertView.findViewById(R.id.exp_list_item);
   tvExperimentTitle.setText(experimentName);
   return convertView;
}

The complete code for the Adapter can be seen here.

After creating the adapter we proceeded similarly to the normal ListView. Take the reference for ExpandableListView by findViewById() or BindView if you are using ButterKnife and set the adapter as an instance of adapter created above.

@BindView(R.id.saved_experiments_elv)
ExpandableListView experimentExpandableList;
experimentAdapter = new SavedExperimentAdapter(context, headerList, map);
experimentExpandableList.setAdapter(experimentAdapter);

Roadmap

We are planning to divide the experiment sub-list into categories like

Electronics
- Diode
  - Diode I-V
  - Zener I-V
  - Diode Clamping
  - Diode Clipping
- BJT and FET
  - Transistor CB (Common Base)
  - Transistor CE (Common Emitter)
  - Transistor Amplifier
  - N-FET output characteristic
- Op-Amps
  - …
Electrical
- …

This is a bit more complex than it looks, I tried using an ExpandableListView as a child for a group item but ran into some errors. I will write a post as soon as this view hierarchy has been achieved.

Resources

The Pocket Science Lab: Who Needs it, and Why

Post author:Jithin B.P.
Post published:July 23, 2017
Post category:FOSSASIA GSoC Open Event Pocket Science Lab

Science and technology share a symbiotic relationship. The degree of success of experimentation is largely dependent on the accuracy and flexibility of instrumentation tools at the disposal of the scientist, and the subsequent findings in fundamental sciences drive innovation in technology itself. In addition to this, knowledge must be free as in freedom. That is, all information towards constructing such tools and using them must be freely accessible for the next generation of citizen scientists. A common platform towards sharing results can also be considered in the path to building a better open knowledge network.

But before we get to scientists, we need to consider the talent pool in the student community that gave rise to successful scientists, and the potential talent pool that lost out on the opportunity to better contribute to society because of an inadequate support system. And this brings us to the Pocket Science Lab

How can PSLab help electronics engineers & students?

This device packs a variety of fundamental instruments into one handy package, with a Bill-of-materials that’s several orders of magnitude less than a distributed set of traditional instruments.

It does not claim to be as good as a Giga Samples Per second oscilloscope, or a 22-bit multimeter, but has the potential to offer a greater learning experience. Here’s how:

A fresh perspective to characterize the real world. The visualization tools that can be coded on an Android device/Desktop (3D surface plots, waterfall charts, thermal distributions etc ), are far more advanced than what one can expect from a reasonably priced oscilloscope. If the same needs to be achieved with an ordinary scope, a certain level of technical expertise is expected from the user who must interface the oscilloscope with a computer, and write their own acquisition & visualization app.
Reduce the entry barrier for advanced experiments.: All the tools are tightly integrated in a cost-effective package, and even the average undergrad student that has been instructed to walk on eggshells around a conventional scope, can now perform elaborate data acquisition tasks such as plotting the resonant frequency of a tuning fork as a function of the relative humidity/temperature. The companion app is being designed to offer varying levels of flexibility as demanded by the target audience.
Is there a doctor in the house? With the feature set available in the PSlab , most common electronic components can be easily studied , and will save hours while prototyping new designs. Components such as resistors, capacitors, diodes, transistors, Op-amps, LEDs, buffers etc can be tested.

How can PSLab help science enthusiasts ?

Physicists, Chemists and biologists in the applied fields are mostly dependent on instrument vendors for their measurement gear. Lack of an electronic/technical background hinders their ability to improve the gear at their disposal, and this is why a gauss meter which is basically a magnetometer coupled with a crude display in an oversized box with an unnecessarily huge transformer can easily cost upwards of $150 . The PSLab does not ask the user to be an electronics/robotics expert , but helps them to get straight to the acquisition part. It takes care of the communication protocols, calibration requirements, and also handles visualization via attractive plots.

A physicist might not know what I2C is , but is more than qualified to interpret the data acquired from a physical sensor, and characterize its accuracy.

The magnetometer (HMC5883L) can be used to demonstrate the dependence of the axial magnetic field on distance from the center of a solenoid
The pressure,temperature sensor (BMP280) can be used to verify the gas laws, and verify thermodynamic phenomena against prevalent theories.

Similarly, a chemist can use an RGB sensor (TCS3200) to put the colour of a solution into numbers, and develop a colorimeter in the process. Colorimeters are quite handy for determining molality of coloured solutions., and commercial ones are rather expensive. What it also needs is a set of LEDs with known wavelengths, and most manufacturers offer proper characterisation information.

What does it mean for the hobbyist?

It is capable of greatly speeding up the troubleshooting process . It can also instantly characterize the expected data from various sensors so that the hobbyist can code accordingly. For example, ‘beyond what tilt threshold & velocity should my humanoid robot swing its arms forward in order to prevent a broken nose?’ . That’s not a question that can be easily answered by said hobbyist who is currently in the process of developing his/her own acquisition system.

How can we involve the community?

The PSLab features an experiment designer that speeds acquisition by providing spreadsheets, analytical tools, and visualisation options all in one place. An option for users to upload their new experiments/utilities to the cloud, and subject those to a peer-review process has been planned. Following which , these new experiments can be pumped back into the ecosystem which will find more uses for it, improve it, and so on.

For example , a user can combine the waveform generator with an analog multiplier IC, and develop a spectrum analyzer.

The case for self-reliance

The average undergraduate laboratory currently employs dedicated instruments for each experiment as prescribed by the curriculum. These instruments often only include the measurement tools essential to the experiment, and students merely repeat the procedure verbatim. That’s not experimentation, it’s rather just verification. PSLab offers a wide array of additional instruments that can be employed by the student to enhance the experiment with their own inputs.

For example, a commonly used diode IV curve-tracer kit usually has a couple of power supplies, a voltmeter, and an ammeter. But, if a student wishes to study the impact of temperature on the band gap, he will hard pressed for the additional tools, and software to combine the acquisition process. With the PSLab, however , he/she can pick from a variety of temperature sensors (LM35, BMP180, Si7021 .. ) depending on the requirement, and explore beyond the book. They are thus better prepared to enter research labs .

And in conclusion , this project has immense potential to help create the next generation of scientists, engineers and creators.

Resources

PSLab-firmware : Source code and documentation for the firmware
PSLab-hardware : Layouts, Schematics, and other hardware design files
PSLab-python : Python Communication library
PSLab-desktop-apps : Experiment related applications built with the Qt Toolkit
PSLab-android : Android app for the Pocket Science Lab

Exporting Functions in PSLab Firmware

Post author:CloudyPadmal
Post published:July 23, 2017
Post category:FOSSASIA GSoC Pocket Science Lab

Computer programs consist of several hundreds line of code. Smaller programs contain around few hundreds while larger programs like PSLab firmware contains lines of code expanding over 2000 lines. The major drawback with a code with lots of lines is the difficulty to debug and hardship for a new developer to understand the code. As a solution modularization techniques can be used to have the long code broken down to small set of codes. In C programming this can be achieved using different .c files and relevant .h(header) files.

The task is tricky as the main code contains a lot of interrelated functions and variables. Lots of errors were encountered when the large code in pslab-firmware being modularized into application specific files. In a scenario like this, global variables or static variables were a much of a help.

Global Variables in C

A global variable in C is a variable whose scope spans wide across the entire program. Updation to the variable will be reflected everywhere it is used.

extern TYPE VARIABLE = VALUE;

Static Variables in C

This type of variables preserve their content regardless of the scope they are in.

static TYPE VARIABLE = VALUE;

Both the variables preserve their values but the memory usage is different depending on the implementation. This can be explained using a simple example.

Suppose a variable is required in different C files and it is defined in one of the header files as a local variable. The header file is then added to several other c files. When the program is compiled the compiler will create several copies of the same variable which will throw a compilation error as variable is already declared. In PSLab firmware, a variable or a method from one library has a higher probability of it being used in another one or many libraries.

This issue can be addressed in two different ways. They are by using static variables or global variables. Depending on the selection, the implementation is different.

The first implementation is using static variables. This type of variables at the time of compilation, create different copies of himself in different c files. Due to the fact that C language treats every C file as a separate program, if we include a header file with static variables in two c files, it will create two copies of the same variable. This will avoid the error messages with variables redeclared. Even though this fixes the issue in firmware, the memory allocation will be of a wastage. This is the first implementation used in PSLab firmware. The memory usage was very high due to duplicate variables taking much memory than they should take. This lead to the second implementation.

first_header.h

#ifndef FIRST_HEADER_H 
#define FIRST_HEADER_H 

static int var_1;

#endif 

/* FIRST_HEADER_H */

second_header.h

#ifndef SECOND_HEADER_H
#define SECOND_HEADER_H

static int var_1;

#endif    

/* SECOND_HEADER_H */

first_header.c

#include <stdio.h>
#include "first_header.h"
#include "second_header.h"

int main() {
    var_1 = 10;
    printf("%d", var_1);
}

The next implementation uses global variables. This type of variables need to be declared only in one header file and can be reused by declaring the header file in other c files. The global variables must be declared in a header file with the keyword extern and defined in the relevant c file once. Then it will be available throughout the application and no errors of variable redeclaration will occur while compiling. This became the final implementation for the PSLab-firmware to fix the compilation issues modularizing application specific C and header files.

Resources:

Understanding PN Junctions with the Pocket Science Lab

Post author:Jithin B.P.
Post published:July 22, 2017
Post category:FOSSASIA GSoC Pocket Science Lab Tutorial

The boundary layer between two thin films of a semiconducting material with Positive type and Negative type doping is referred to as a P-N junction, and these are one of the fundamental building blocks of electronics. These junctions exhibit various properties that have given them a rather indispensable status in modern day electronics.

The PSLab’s various measurement tools enable us to understand these devices, and in this blog post we shall explain some uses of PN junctions, and visualize their behaviour with the PSLab.

One might easily be confused and assume that a positive doping implies that the layer has a net positive charge, but this is not the case. A positive doping involves replacing a minute quantity of the semiconductor molecules with atoms from the next column in the periodic table. These atoms such as phosphorus are also charge neutral, but the number of available mobile charge carriers effectively increases.

A diode as a half-wave rectifier

A diode is basically just a PN junction. An ideal diode conducts electricity in one direction offering a path of zero resistance, and it is a perfect insulator in the other direction. In practice, we may observe some additional properties.

Figure : The circuit used for making the half-wave rectifier and studying it. A bipolar sinusoidal signal is input to a diode, and the output voltage is monitored. The 1uF capacitor is used to filter the output signal and make it more or less constant, but it has not been used while obtaining the data shown in the following image

Figure : A diode used as a half-wave rectifier. The input waveform shown in green was passed through a forward biased diode, and monitored by CH2 (red trace ) .

We can observe that only the positive half of the signal passes through the diode. It can also be observed , that since this is not an ideal diode, the conducted portion has lost some amplitude. This loss is a consequence of the forward threshold voltage of the PN junction, and in case of this diode, it is around 0.6 Volts. This threshold voltage depends on the band structure of the diode , and in the next section we shall examine this voltage for various diodes.

Measurement of Current-Voltage Characteristics of diodes

In practice, diodes only start conducting in the forward direction after a certain threshold potential difference is present. This voltage, also known as the barrier potential, depends on the band gap of the diode, and we shall measure it to determine how the electrical properties affect the externally visible physical properties of the diode.

A programmable voltage output of the PSLab (PV1) will be increased in small steps starting from 0 Volts, and a voltmeter input (CH3) will be used to determine the point when the diode starts conducting. The presence of a known resistor between PV1 and CH3 acts as a current limiter, and also enables us to calculate the current flow using some elementary application of the Ohm’s law. I = (PV1-CH3)/1000 .

The following image shows I-V characteristics of various diodes ranging from Schottky to Light Emitting Diodes (LEDs).

It may be interesting to note that the frequency of the light emitted by LEDs is directly proportional to the threshold voltage. In case of the white LED, it is almost similar to the blue LED because white LEDs are composed of blue LEDs, and a phosphor coating that partially converts blue light to yellow. The combination results in white light.

Zener diodes

Zener diodes are a special variant of diodes that also conduct electricity in the the reverse direction once a certain threshold has been crossed. This threshold can be determined during the manufacturing process, and zener diodes with breakdown voltages such as 3.3V , 5.6V , 6.8V etc are commercially available.

In the following image, the I-V characteristics of a 3.3V zener diode have been measured with the PSLab . As can be observed, the diode starts to conduct small amounts of current from around 2V itself, but significant current flow is usually present once the rated voltage is achieved.

In the forward direction, the zener appears to behave as a regular diode.

Resources

Introduction to P-N Junctions : http://www.electronics-tutorials.ws/diode/diode_3.html
P-N junction theory : http://www.electronics-tutorials.ws/diode/diode_2.html
Zener Diodes : https://en.wikipedia.org/wiki/Zener_diode
Light Emitting diodes : http://www.electronics-tutorials.ws/diode/diode_8.html

Real time Sensor Data Analysis on PSLab Android

Post author:asitava1998
Post published:July 20, 2017
Post category:Android FOSSASIA GSoC Pocket Science Lab

PSLab device has the capacity to connect plug and play sensors through the I2C bus. The sensors are capable of providing data in real time. So, the PSLab Android App and the Desktop app need to have the feature to fetch real time sensor values and display the same in the user interface along with plotting the values on a simple graph.

The UI was made following the guidelines of Google’s Material Design and incorporating some ideas from the Science Journal app. Cards are used for making each section of the UI. There are segregated sections for real time updates and plotting where the real time data can be visualised. A methods for fetching the data are run continuously in the background which receive the data from the sensor and then update the screen.

The following section denotes a small portion of the UI responsible for displaying the data on the screen continuously and are quite simple enough. There are a number of TextViews which are being constantly updated on the screen. Their number depends on the type and volume of data sent by the sensor.

<TextView
       android:layout_width="wrap_content"
       android:layout_height="30dp"
       android:layout_gravity="start"
       android:text="@string/ax"
       android:textAlignment="textStart"
       android:textColor="@color/black"
       android:textSize="@dimen/textsize_edittext"
       android:textStyle="bold" />

<TextView
       android:id="@+id/tv_sensor_mpu6050_ax"
       android:layout_width="wrap_content"
       android:layout_height="30dp"
       android:layout_gravity="start"
       android:textAlignment="textStart"
       android:textColor="@color/black"
       android:textSize="@dimen/textsize_edittext"
       android:textStyle="bold" />

The section here represents the portion of the UI responsible for displaying the graph. Like all other parts of the UI of PSLab Android, MPAndroidChart is being used here for plotting the graph.

<LinearLayout
       android:layout_width="match_parent"
       android:layout_height="160dp"
       android:layout_marginTop="40dp">

       <com.github.mikephil.charting.charts.LineChart
               android:id="@+id/chart_sensor_mpu6050"
               android:layout_width="match_parent"
               android:layout_height="match_parent"
               android:background="#000" />
</LinearLayout>

Since the updates needs to continuous, a process should be continuously run for updating the display of the data and the graph. There are a variety of options available in Android in this regard like using a Timer on the UI thread and keep updating the data continuously, using ASyncTask to run a process in the background etc.

The issue with the former is that since all the processes i.e. fetching the data and updating the textviews & graph will run on the UI thread, the UI will become laggy. So, the developer team chose to use ASyncTask and make all the processes run in the background so that the UI thread functions smoothly.

A new class SensorDataFetch which extends AsyncTask is defined and its object is created in a runnable and the use of runnable ensures that the thread is run continuously till the time the fragment is used by the user.

scienceLab = ScienceLabCommon.scienceLab;
i2c = scienceLab.i2c;
try {
    MPU6050 = new MPU6050(i2c);
} catch (IOException e) {
    e.printStackTrace();
}
Runnable runnable = new Runnable() {
    @Override
    public void run() {
        while (true) {
            if (scienceLab.isConnected()) {
                try {
                    sensorDataFetch = new SensorDataFetch();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                sensorDataFetch.execute();
            }
        }
    }
};
new Thread(runnable).start();

The following is the code for the ASyncTask created. There are two methods defined here doInBackground and onPostExecute which are responsible for fetching the data and updating the display respectively.

The raw data is fetched using the getRaw method of the MPU6050 object and stored in an ArrayList. The data type responsible for storing the data will depend on the return type of the getRaw method of each sensor class and might be different for other sensors. The data returned by getRaw is semi-processed and the data just needs to be split in sections before presenting it for display.

The PSLab Android app’s sensor files can be viewed here and they can give a better idea about how the sensors are calibrated, how the intrinsic nonlinearity is taken care of, how the communication actually works etc.

After the data is stored, the control moves to the onPostExecute method, here the textviews on the display and the chart are updated. The updation is slowed down a bit so that the user can visualize the data received.

private class SensorDataFetch extends AsyncTask<Void, Void, Void> {
   MPU6050 MPU6050 = new MPU6050(i2c);
   ArrayList<Double> dataMPU6050 = new ArrayList<Double>();

   private SensorDataFetch(MPU6050 MPU6050) throws IOException {
   }

   @Override
   protected Void doInBackground(Void... params) {
       try {
           if (MPU6050 != null) {
               dataMPU6050 = MPU6050.getRaw();
           }
       } catch (IOException e) {
           e.printStackTrace();
       }
           return null;
   }

   protected void onPostExecute(Void aVoid) {
       super.onPostExecute(aVoid);
       tvSensorMPU6050ax.setText(String.valueOf(dataMPU6050.get(0)));
       tvSensorMPU6050ay.setText(String.valueOf(dataMPU6050.get(1)));
       tvSensorMPU6050az.setText(String.valueOf(dataMPU6050.get(2)));
       tvSensorMPU6050gx.setText(String.valueOf(dataMPU6050.get(3)));
       tvSensorMPU6050gy.setText(String.valueOf(dataMPU6050.get(4)));
       tvSensorMPU6050gz.setText(String.valueOf(dataMPU6050.get(5)));
       tvSensorMPU6050temp.setText(String.valueOf(dataMPU6050.get(6)));
   }
}

The detailed implementation of the same can be found here.

Additional Resources

Learn more about how real time sensor data analysis can be used in various fields like IOT http://ieeexplore.ieee.org/document/7248401/
Google Fit guide on how to use native built-in sensors on phones, smart watches etc. https://developers.google.com/fit/android/sensors
A simple starter guide to build an app capable of real time sensor data analysis http://developer.telerik.com/products/building-an-android-app-that-displays-live-accelerometer-data/
Learn more about using AsyncTask https://developer.android.com/reference/android/os/AsyncTask.html

Creating a Four Quadrant Graph for PSLab Android App

Post author:Akarshan Gandotra
Post published:July 20, 2017
Post category:Android FOSSASIA GSoC Pocket Science Lab

While working on Oscilloscope in PSLab Android, we had to implement XY mode. XY plotting is part of regular Oscilloscope and in XY plotting the 2 signals are plotted against each other. For XY plotting we require a graph with all 4 quadrants but none of the Graph-View libraries in Android support a 4 quadrants graph. We need to find a solution for this. So, we used canvas class to draw a 4 quadrants graph. The Canvas class defines methods for drawing text, lines, bitmaps, and many other graphics primitives. Let’s discuss how a 4 quadrants graph is implemented using Canvas.

Initially, a class Plot2D extending View is created along with a constructor in which context, values for X-Axis, Y-Axis.

public class Plot2D extends View {
public Plot2D(Context context, float[] xValues, float[] yValues, int axis) {
   super(context);
   this.xValues = xValues;
   this.yValues = yValues;
   this.axis = axis;
   vectorLength = xValues.length;
   paint = new Paint();
   getAxis(xValues, yValues);
}

So, now we need to convert a particular float value in a pixel. This is the most important part and for this, we create a function where we send the value of the pixels, the minimum and the maximum value of the axis and array of float values. We get an array of converted pixel values in return. p[i] = .1 * pixels + ((value[i] – min) / (max – min)) * .8 * pixels; is the way to transform an int value to a respective pixel value.

private int[] toPixel(float pixels, float min, float max, float[] value) {
   double[] p = new double[value.length];
   int[] pInt = new int[value.length];

   for (int i = 0; i < value.length; i++) {
       p[i] = .1 * pixels + ((value[i] - min) / (max - min)) * .8 * pixels;
       pInt[i] = (int) p[i];
   }
   return (pInt);
}

For constructing a graph we require to create the axis, add markings/labels and plot data in the graph. To achieve this we will override onDraw method. The parameter to onDraw() is a Canvas object that the view can use to draw itself. First, we need to get various parameters like data to plot, canvas height and width, the location of the x axis and y axis etc.

@Override
protected void onDraw(Canvas canvas) {

   float canvasHeight = getHeight();
   float canvasWidth = getWidth();
   int[] xValuesInPixels = toPixel(canvasWidth, minX, maxX, xValues);
   int[] yValuesInPixels = toPixel(canvasHeight, minY, maxY, yValues);
   int locxAxisInPixels = toPixelInt(canvasHeight, minY, maxY, locxAxis);
   int locyAxisInPixels = toPixelInt(canvasWidth, minX, maxX, locyAxis);

Drawing the axis

First, we will draw the axis and for this, we will use the white color. To draw the white color axis line we will the following code.

paint.setColor(Color.WHITE);
paint.setStrokeWidth(5f);
canvas.drawLine(0, canvasHeight - locxAxisInPixels, canvasWidth,
       canvasHeight - locxAxisInPixels, paint);
canvas.drawLine(locyAxisInPixels, 0, locyAxisInPixels, canvasHeight,
       paint);

Adding the labels

After drawing the axis lines, now we need to mark labels for both x and y axis. For this, we use the following code in onDraw method. By this, the axis labels are automatically marked after a fixed distance. The no. of labels depends on the value of n. The code ensures that the markings are apt for each of the quadrant, for example in the first quadrant the markings of the x axis is below the axis, whereas markings of the y axis are to the left.

float temp = 0.0f;
int n = 8;
for (int i = 1; i <= n; i++) {
    if (i <= n / 2) {
           temp = Math.round(10 * (minX + (i - 1) * (maxX - minX) / n)) / 10;
           canvas.drawText("" + temp,
                   (float) toPixelInt(canvasWidth, minX, maxX, temp),
                   canvasHeight - locxAxisInPixels - 10, paint);
           temp = Math.round(10 * (minY + (i - 1) * (maxY - minY) / n)) / 10;
           canvas.drawText("" + temp, locyAxisInPixels + 10, canvasHeight
                           - (float) toPixelInt(canvasHeight, minY, maxY, temp),
                   paint);
       } else {
           temp = Math.round(10 * (minX + (i - 1) * (maxX - minX) / n)) / 10;
           canvas.drawText("" + temp,
                   (float) toPixelInt(canvasWidth, minX, maxX, temp),
                   canvasHeight - locxAxisInPixels + 30, paint);
           temp = Math.round(10 * (minY + (i - 1) * (maxY - minY) / n)) / 10;
           canvas.drawText("" + temp, locyAxisInPixels - 65, canvasHeight
                           - (float) toPixelInt(canvasHeight, minY, maxY, temp),
                   paint);

By using this code we get the following results

Plotting the data

The last step is to plot the data, to achieve this we first convert float values of x axis and y axis data point to pixels using toPixel method and simply draw it on the graph. In addition to this, we set a red color to the line.

paint.setStrokeWidth(2);
canvas.drawARGB(255, 0, 0, 0);
for (int i = 0; i < vectorLength - 1; i++) {
   paint.setColor(Color.RED);
   canvas.drawLine(xValuesInPixels[i], canvasHeight
           - yValuesInPixels[i], xValuesInPixels[i + 1], canvasHeight
           - yValuesInPixels[i + 1], paint);
}

This implements a 4 quadrants graph in PSLab Android app for XY plotting in Oscilloscope Activity. The entire code for the same is available in here.

Resources

Designing A Virtual Laboratory With PSLab

Post author:Jithin B.P.
Post published:July 17, 2017
Post category:GSoC Pocket Science Lab

What is a virtual laboratory

A virtual lab interface gives students remote access to equipment in laboratories via the Internet without having to be physically present near the equipment. The idea is that lab experiments can be made accessible to a larger audience which may not have the resources to set up the experiment at their place. Another use-case scenario is that the experiment setup must be placed at a specific location which may not be habitable.

The PSLab’s capabilities can be increased significantly by setting up a framework that allows remote data acquisition and control. It can then be deployed in various test and measurement scenarios such as an interactive environment monitoring station.

What resources will be needed for such a setup

The proposed virtual lab will be platform independent, and should be able to run in web-browsers. This necessitates the presence of a lightweight web-server software running on the hardware to which the PSLab is connected. The web-server must have a framework that must handle multiple connections, and allow control access to only authenticated users.

Proposed design for the backend

The backend framework must be able to handle the following tasks:

Communicate with the PSLab hardware attached to the server
Host lightweight web-pages with various visual aids
Support an authentication framework via a database that contains user credentials
Reply with JSON data after executing single commands on the PSLab
Execute remotely received python scripts, and relay the HTML formatted output. This should include plots

Proposed design for the frontend

Responsive, aesthetic layouts and widget styles.
Essential utilities such as Sign-up and Sign-in pages.
Embedded plots with basic zooming and panning facilities.
Embedded code-editor with syntax highlighting
WIdgets to submit the code to the server for execution, and subsequent display of received response.

A selection of tools that can assist with this project, and the purpose they will serve:

Backend

The Python communication library for the PSLab
FLASK: ‘Flask is a BSD Licensed microframework for Python based on Werkzeug, Jinja 2 and good intentions.’ . It can handle concurrent requests, and will be well suited to serve as our web server
MySQL: This is a database management utility that can be used to store user credentials, user scripts, queues etc
WerkZeug: The utilities to create and check password hashes are essential for exchanging passwords via the database
Json: For relaying measurement results to the client
Gunicorn + Nginx: Will be used when more scalable deployment is needed, and the built-in webserver of Flask is unable to handle the load.

Frontend

Bootstrap-css: For neatly formatted, responsive UIs
Jqplot: A versatile and expandable js based plotting library
Ace code editor: A browser based code editor with syntax highlighting, automatic indentation and other user-friendly features. Written in JS
Display documentation: These can be generated server side from Markdown files using Jekyll. Several documentation files are already available from the pslab-desktop-apps, and can be reused after replacing the screenshot images only.

Flow Diagram

Handling graph plots using MPAndroid chart in PSLab Android App

Post author:viveksb007
Post published:July 16, 2017
Post category:Android FOSSASIA GSoC Pocket Science Lab

In PSLab Android App, we expose the Oscilloscope and Logic Analyzer functionality of PSLab hardware device. After reading data-points to plot, we need to show plot data on graphs for better understanding and visualisation. Sometimes we need to save graphs to show output/findings of the experiment. Hence we will be using MPAndroidChart library to plot and save graphs as it provides a nice and clean methods to do so.

First add MPAndroid Chart as dependency in your app build.gradle to include the library

dependencies {
 compile 'com.github.PhilJay:MPAndroidChart:v3.0.2'
}

For chart view in your layout file, there are many available options like Line Chart, Bar Chart, Pie Chart, etc. For this post I am going to use Line Chart.

Add LineChart in your layout file

<com.github.mikephil.charting.charts.LineChart
        android:id="@+id/lineChart"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

Take a reference to LineChart of layout file in your Activity/Fragment

LineChart lineChart = (LineChart) findViewById(R.id.chart);// Activity
LineChart lineChart = (LineChart) view.findViewById(R.id.chart);// Fragment

Now we add dataset to LineChart of layout file, I am going to add data for two curves sine and cosine function to plot sine and cosine wave on LineChart. We create two different LineDataSet one for the sine curve entries and other for the cosine curve entries.

List <Entry> sinEntries = new ArrayList<>(); // List to store data-points of sine curve 
List <Entry> cosEntries = new ArrayList<>(); // List to store data-points of cosine curve

// Obtaining data points by using Math.sin and Math.cos functions
for( float i = 0; i < 7f; i += 0.02f ){
sinEntries.add(new Entry(i,(float)Math.sin(i)));
cosEntries.add(new Entry(i,(float)Math.cos(i)));
}

List<ILineDataSet> dataSets = new ArrayList<>(); // for adding multiple plots

LineDataSet sinSet = new LineDataSet(sinEntries,"sin curve");
LineDataSet cosSet = new LineDataSet(cosEntries,"cos curve");

// Adding colors to different plots 
cosSet.setColor(Color.GREEN);
cosSet.setCircleColor(Color.GREEN);
sinSet.setColor(Color.BLUE);
sinSet.setCircleColor(Color.BLUE);

// Adding each plot data to a List
dataSets.add(sinSet);
dataSets.add(cosSet);

// Setting datapoints and invalidating chart to update with data points
lineChart.setData(new LineData(dataSets));
lineChart.invalidate();

After adding datasets to chart and invalidating it, chart is refreshed with the data points which were added in dataset.

After plotting graph output would look like the image below:

You can change the dataset and invalidate chart to update it with latest dataset.

To save graph plot, make sure you have permission to write to external storage, if not add it into your manifest file

<manifest ...>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    ...
</manifest>

To save the photo of chart into Gallery:

lineChart.saveToGallery("title");

To save a some specific location:

lineChart.saveToPath("title", "Location on SD Card");

If you want to do some resizing in chart or save two three charts in a single image, you can do so by taking out the Bitmaps and processing them to meet your requirements:

lineChart.getChartBitmap();

Resources

MPAndroid Chart Wiki

Packing and Unpacking Data in PSLab Android App

Post author:viveksb007
Post published:July 16, 2017
Post category:Android FOSSASIA GSoC Pocket Science Lab

In PSLab we communicate with PSLab Hardware device, to exchange data, i.e we give a command or instruction and it responses accordingly. So this giving and receiving is in terms of packed byte string. Thus, we need some solid knowledge to pack and unpack data. In python communication library, there is struct module available. In JAVA we can use NIO’s ByteBuffer or implement our own functions. In this blog post I discuss both methods.

In Python we have struct module for packing data in byte strings. As different languages interpret data types differently like Java takes 4 bytes for int and C++ takes 2 bytes for int. To send and receive data properly, we pack data in a byte string and unpack on other side with it’s data type properties. In PSLab, we have to communicate with device for various applications like getting calibration data during power up time as raw data doesn’t make much sense until calibration is applied on it.

You also need to take care of order of sequence of bytes like there are generally two types of order in which a sequence of bytes are stored in memory location:

Big – Endian: In which MSB is stored first.

Source: Wikipedia
Little – Endian: In which LSB is stored first.

Source: Wikipedia

In Python

The standard sizes and format characters of particular data type can be seen in the image below.

Format	C Type	Python Type	Standard
x	Pad byte	No value
c	char	string of length 1	1
b	signed char	integer	1
B	unsigned char	integer	1
?	_Bool	bool	1
h	short	integer	2
H	unsigned short	integer	2
i	int	integer	4
I	unsigned int	integer	4
l	long	integer	4
L	unsigned long	integer	4
q	long long	integer	8
Q	unsigned long long	integer	8
f	float	float	4
d	double	float	8
s	char[]	string
p	char[]	string
P	void*	integer

Source: Python Docs

For Packing data

import struct
struct.Struct(“B”).pack(254) # Output -> b’\xfe’
a = struct.Struct(“I”).pack(2544) # Output -> b’\xf0\t\x00\x00′

Now a is the byte string that has packed value as 2544, this can be send to some device byte by byte and reconstructed on receiving side by knowing how many bytes does the data type received contains.

For Unpacking data

import struct
struct.unpack(“I”,a) # Output -> (2544,)

In JAVA

For Packing data

Suppose you have to pack an integer, in java int takes 32 bits (4 bytes)

Using JAVA’s NIO’s ByteBuffer

byte[] bytes = ByteBuffer.allocate(4).putInt(2544).array();

If you want hardcore method to see what exactly is happening, use

byte[] intToByteArray(int value){
 return new byte[]{
     (byte)value >>> 24,
     (byte)value >>> 16,
     (byte)value >>> 8,
     (byte)value
  };
}

“>>>” is used for unsigned shifting, you can use according to your requirements.

After you have your byte array, you can easily create a string out of it and transmit.

For Unpacking data

Using JAVA’s NIO’s ByteBuffer

int fromByteArray(byte[] bytes){
int a = ByteBuffer.wrap(bytes).getInt();
return a;
}

It assumes that byte array is stored as Big Endian, if bytes in byte array is stored as Little Endian, add order() after wrap()

int fromByteArray(byte[] bytes){
int a = ByteBuffer.wrap(bytes).order(ByteOrder.LITTLE_ENDIAN).getInt();
return a;
}

Note: Make sure the bytes array that you provide has same number of bytes as that of the data type that you are trying to unpack. For example: if you want int, bytes array should have 4 bytes as int type in JAVA has 4 bytes. If you want short, bytes array should have 2 bytes as short type in JAVA has 2 bytes.

To visualise underlying implementation, see

int from byteArray(byte[] bytes){
return bytes[0] << 24 | bytes[1] << 16 | bytes[2] << 8 | bytes[3];
}

In all above implementation big-endian order was assumed, you can modify function if you are using little-endian or some other sequence.

References

Reducing UI lags with AsyncTask in PSLab Android

Post author:Akarshan Gandotra
Post published:July 16, 2017
Post category:Android FOSSASIA GSoC Pocket Science Lab

In the Oscilloscope Activity, communication with the PSLab device goes in parallel with updations of the graph which result in inoperable UI if both these functions are performed in the main thread. This would severely degrade the user experience. In order to avoid this, we simply used AsyncTask. AsyncTasks are used to perform communications with the device in the background thread and update UI when the task in background thread completes. AsyncTask thus solves the problem of making UI super laggy while performing certain time-consuming functions. The UI remains responsive throughout.

More about AsyncTask

AsyncTask is an abstract Android class which helps the Android applications to handle the Main UI thread in a more efficient way. AsyncTask class allows to perform long lasting background operations and update the results in UI thread without affecting the main thread.

Implementing AsyncTask in Android applications

Create a new class inside Activity class and extend AsyncTask:

private class Task extends AsyncTask<Void, Void, Void> {
  	protected Long doInBackground(aVoid) {
     	}
 	protected void onProgressUpdate(aVoid) {
     	}
 	protected void onPostExecute(aVoid) {
     	}
}

Execute the task:

new Task().execute();

How they are used in Oscilloscope Activity?

The following diagram explains how AsyncTasks are used in Oscilloscope Activity.

AsyncTask in PSLab Android App

A public class extending AsyncTask is defined, this task is executed from another thread.

public class Task extends AsyncTask<String, Void, Void> {
   ArrayList<Entry> entries;
   String analogInput;

doInBackgroundMethod performs the part related to communication with the PSLab device.

Here we are capturing the data from the hardware using captureTraces and fetchTraces method.

 @Override
   protected Void doInBackground(String... params) {
       try {
           analogInput = params[0];
           //no. of samples and timegap still need to be determined
           scienceLab.captureTraces(1, 800, 10, analogInput, false, null);
           Log.v("Sleep Time", "" + (800 * 10 * 1e-3));
           Thread.sleep((long) (800 * 10 * 1e-3));
           HashMap<String, double[]> data = scienceLab.fetchTrace(1); //fetching data
           double[] xData = data.get("x");
           double[] yData = data.get("y");
           entries = new ArrayList<Entry>();
           for (int i = 0; i < xData.length; i++) {
               entries.add(new Entry((float) xData[i], (float) yData[i]));
           }
       }
       catch (NullPointerException e){
           cancel(true);
       } catch (InterruptedException e) {
           e.printStackTrace();
       }
       return null;
   }

After the thread, completely executed onPostExecute is called to update the UI/graph. This method runs on the main thread.

 @Override
   protected void onPostExecute(Void aVoid) {
       super.onPostExecute(aVoid);
       LineDataSet dataset = new LineDataSet(entries, analogInput);
       LineData lineData = new LineData(dataset);
       dataset.setDrawCircles(false);
       mChart.setData(lineData);
       mChart.invalidate();    //refresh the chart
       synchronized (lock){
           lock.notify();
       }
   }
}

This simply solves the problem of lags and the Oscilloscope works like a charm.

Implementation of Experiments List Using ExpandableListView

Roadmap

Resources

How can PSLab help electronics engineers & students?

How can PSLab help science enthusiasts ?

What does it mean for the hobbyist?

How can we involve the community?

The case for self-reliance

Resources

A diode as a half-wave rectifier

Measurement of Current-Voltage Characteristics of diodes

Zener diodes

Resources

Additional Resources

Resources

What is a virtual laboratory

What resources will be needed for such a setup

Proposed design for the backend

Proposed design for the frontend

A selection of tools that can assist with this project, and the purpose they will serve:

Backend

Frontend

Flow Diagram

Recommended Reading

Resources

References

More about AsyncTask

Implementing AsyncTask in Android applications

How they are used in Oscilloscope Activity?

AsyncTask in PSLab Android App

Resources