Serial Interface Using Python Software

A. Robson Benjamin is associate professor of Physics at American College, Madurai, Tamil Nadu. His interests include design and development of USB based data acquisition and automation of electronic gadgets

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Presented here is a project for a simple transistor’s output characteristic curve-tracer program through the serial interface and PIC microcontroller using Python programming language. Output characteristic curve of a transistor is automated using Python software and serial interface. Application software for collecting the measured value and displaying it on the monitor screen is developed using Python.

Engineers and academicians have been using high-level languages like Visual C++, Visual Basic and Delphi for connecting their gadgets/embedded systems with the computer for measurement and control purposes for a long time. Recently, many shifted their preference to Python as their computer language for measurement and analysis. The main advantage of using Python is the support of a large open source community.

Python is an interpreted programming language like BASIC, which saves considerable time during program development because compilation and linking are not required. Programs written in Python are typically much shorter as compared to ones written in C or C++.

Python can be used for writing standalone programs and scripting applications. It is free, portable, powerful and remarkably easy to use.

To run a Python program, an interpreter is required. Python interpreter is a program that reads a Python program and then executes it. To write a Python program, it is advisable to use an editor (for typing a program). An editor is designed for writing Python programs. It provides highlighting and indentation that can help you write a program without mistake.
The interpreter can be used to experiment with different features of the language. There are two modes in which interpreters can be used, namely, interactive and script. In interactive mode, the interpreter prints the result as the program is typed. The chevron, >>>, is the prompt the interpreter uses to indicate that it is ready.

Alternatively, you can store the code in a file and use the interpreter to execute the contents of the file, which is called a script. By convention, Python scripts end with .py extension. (Python installation package is available at http://python.org)

Writing the first Python program

After the installation of Python IDE and PyWin32 software, you can write a program and run it. The traditional first program is Hello World. This program simply prints ‘Hello World’ on the screen when it is run. This can be done with one statement in Python, as shown in Fig. 1.

Printing ‘Hello World’ on PythonWin window
Fig. 1: Printing ‘Hello World’ on PythonWin window

Mathematical operations are performed in a single line without declaring the variables involved, as shown in Fig. 2.

Interactive Python Shell
Fig. 2: Interactive Python Shell

Circuit and working

Circuit diagram of the serial interface and transistor curve tracer is shown in Fig. 3. The author’s prototype is shown in Fig. 4. The circuit consists of a PIC18F4620 microcontroller (IC1), AD780 voltage regulator (IC2), MAX5154 12-bit digital-to-analogue converter, also called DAC (IC3), LM358 voltage amplifier (IC4), MAX232 dual driver/receiver (IC5), power transistor SL100/CL100 (T2), BC107 transistor-under-study (TUS) and other essential components needed for microcontroller operations.

Circuit diagram of the serial interface and transistor curve tracer
Fig. 3: Circuit diagram of the serial interface and transistor curve tracer

Clock frequency required for microcontroller operation is derived from its internal oscillator. Base current of TUS (T1) is varied from 0µA to 175µA in eight steps using port D pins (RD0, RD1 and RD2) of IC1. Collector voltage of T1 is varied from 0V to 2.5V in eight steps using IC1 and IC3 through IC4 and T2. Output voltage from the DAC is amplified about four times using LM358 and given to the base of T2.

Author’s serial interface using Python software prototype
Fig. 4: Author’s prototype

Emitter of T2 is connected to the collector of TUS, whose voltage is being varied. Collector voltage of TUS is given to channel-0 (AN0) of the built-in analogue-to-digital converter (ADC) of IC1 after dividing the voltage by half using a potential divider arrangement (R19 and R20) to meet the voltage limitation of the microcontroller.

Assuming that the collector current of TUS is equal to its emitter current, output is taken across the 1-ohm resistance, and the drop across 1-ohm resistance is amplified about 50 times and given to channel-1(AN1) of the built-in ADC of IC1.

Construction

An actual-size, single-side PCB layout for the USB interface is shown in Fig. 5 and its component layout in Fig. 6. After assembling the circuit on the PCB, enclose it in a suitable box.

PCB layout of the serial interface using Python software
Fig. 5: PCB layout of the serial interface using Python software
Fig. 6: Component layout of the PCB

Firmware

The firmware is written in C and compiled using CCS C compiler. It implements the following tasks:
1. Generates different base currents
2. Generates different collector voltages
3. Measures the voltage and current (current in terms of voltage)
4. Sends the data to the PC

Host software

The application software to collect the measured value and display it on the monitor screen has been developed using Python. It requires the following components to be installed on your system: Python 2.5 (python-2.5.2) or higher IDE, Win32 Python (pywin32-210.win32-py2.5), Serial Port (pyserial-2.2.win32), Matplot Library (matplotlib-0.91.2.win32-Py2.5) and Numeric Python (numpy-1.0.4.win32-py2.5). Output pattern is displayed using Tkinter software, and values are displayed in a text file. Tkinter software is a part of Python package, so there is no need to separately install it.

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4 COMMENTS

    • This project was designed for PIC18F4620 microcontroller. However, you can use an equivalent PIC microcontroller. You can also use other microcontrollers for the same application but for that, the circuit, the code, and libraries will be different.

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