Sunday, May 1, 2016

Simple crystal tester

This is simple Colpitts oscillator to test commonly available passive crystals which range between 2MHz to 27MHz. This unit must connect to an oscilloscope and/or frequency counter to get the frequency of the crystal.

This circuit is design to work around 9V to 12V DC power source. Both 2SC930 transistors can replace with any high speed NPN transistor such as 2SC829, 2SC933, etc.

Testing 5030kHz FT-243 crystal using prototype version of crystal tester.

In our lab we use Manhattan style layout to prototype this crystal tester and it produces highly accurate results with above specified frequency range. According to our observations, after 27MHz, amplitude of the waveform starts to decrease steeply.

Schematic and Stripboard wiring layout for this crystal tester is available to download in here.

Monday, April 18, 2016

USB curve tracer for NPN transistors

Curve tracer is an electronic test instrument to analyze the characteristics of transistors and other discrete semiconductors. In this post we construct USB base curve tracer to analyze properties of NPN transistors. This curve tracer is build around Microchip's PIC18F4550 MCU and it use simple Windows based GUI application to plot captured data of a transistor.

In this design PIC18F4550 MCU is used to establish USB connectivity, perform voltage readings and control current/voltage flow into the test subject. To minimize the cost and to make it simple, we use R2R ladder circuit to generate discrete collector-emitter voltage levels for the transistor on test. In each scan session collector-emitter voltage level get increase from 0V to 7.5V in 256 steps. In this design, "tracer" scans the transistor for 7 base current levels which are in between 7µA to 60µA. In viewer application collector-emitter voltage levels are plotted on x-axis and collector current is plotted on y-axis.

Results generated for 2SC945 NPN transistor.

In current firmware, PIC18F4550 communicates with the PC over USB-HID class and because of that this may not need any special device drivers on host terminal. In most of Microsoft Windows operating systems the viewer application may be able to communicate with the device directly. Viewer application bundled in this project package are developed using Microsoft .net framework and it can run immediately on any new PC with minimum amount of prerequisites. Based on our experiences the most recommended operating system for this application is either Windows 7 or Windows 10.

Results generated for 2SD400 NPN transistor.

To get the perfect results we highly recommended to construct this project on PCB. According to our experiences breadboards and veroboards may generate lot of noises and may finally leads to wrong output(s).

This is an open source hardware project and all it's design files, source codes and compiled binaries are available to download at curvetracer.sourceforge.net. All the content of this project are released under the terms of GNU GPL version 3.0.

Saturday, April 2, 2016

Automatic monitor brightness controller

This is an automatic "monitor brightness controller" based on environmental light conditions. This system use USB port base sensor unit to measure the light level and control monitor brightness accordingly. We design this system to reduce the eye stress by matching the monitor brightness with environmental lighting.

The sensor unit of this system is build around PIC18F2550 8-bit microcontroller. To measure the light level we use LDR with MCU's inbuilt ADC. The control software of this unit is design to work with Microsoft Windows operating systems and it use Windows API's DDC/CI related functions to control the monitors/display devices.

Firmware of USB sensor use 50-point boxcar filter to get the average readout from the ADC and that value is passed to PC over USB HID interface. Thanks to this HID interface this system may not need any special device driver, and it can use in any compatible PC with minimum amount of configuration changes.

Prototype version of Automatic monitor brightness controller sensor unit.

We successfully test this setup on several PC configurations. At the designing stages we mainly use Viewsonic VA1901wm LCD monitor to test this setup.

This is an open source hardware project and all it's source codes, design files and compiled binaries are available to download at https://github.com/dilshan/brightness-control. All the content of this project are distribute under the terms of MIT Licenses.

Monday, March 21, 2016

Low voltage power supply unit

In this post we introduce simple and flexible, regulated low voltage power supply unit. This power supply has provision for 4 outputs such as 1.5V, 1.8V, 2.5V and 3.3V. We mainly build this low voltage power supply unit to test (and power-up) low voltage MCUs, CPLDs and radio receivers. For this power supply we choose 1.8V, 2.5V and 3.3V to get it compatible with most of the LVTTL/LVCMOS devices. Other than that, we include 1.5V because there are several analog ICs are available for that voltage level.

This power supply unit is based on LM1117/AMS1117 voltage regulator series and for this design we use AMS1117-1.5, AMS111-1.8, AMS1117-2.5 and AMS1117-3.3 fixed voltage regulators. Except to above regulators this board can be use with AMS1117-2.85 and AMS1117-5.0 regulators.

Internal view of low voltage power supply unit.

In our prototype we use center-tapped 6V (1A) step down transformer with this power supply unit. All the wiring diagrams to transformer and rotary switch are provided with the schematic.

The given PCB design for this power supply unit is based on SOT-223 (SMD) version of voltage regulators. Even though it use SMD components this PCB can build with single side PCB. Dimension of the PCB is set to 89mm × 69mm. All PCB designs and schematics of this power supply unit are available to download at elect.wikispaces.com.