Monday, September 7, 2015

DIY resistor box

Resistor box is a quiet useful instrument in analog electronic based design and prototyping tasks. It can easily build with few sets of resistors and with couple of rotary switches.

This quick post is about resistor box which we construct recently using 5W wire-wound power resistors. For this resistor box we use five, single pole 12 position rotary switches and 50 wire-wound resistors. It gives 5 dials to controls 10Ω, 100Ω, 1KΩ, 10KΩ and 100KΩ steps.

Final view of homebrew resistor box.

Internal view and high resolution photographs of this resistor box are available at my Flickr page. Schematic of the resistor box is available in here.

Sunday, August 30, 2015

Low cost TEA5767 based FM stereo radio receiver

TEA5767 is electronically tuned FM stereo radio receiver from NXP for low voltage applications. TEA5767 based radio modules are now common in the field and this project is based on Samsung TT-502 radio module. The main objective of this project is to design low cost FM tuner based on this TT-502 / TEA5767 module and commonly available electronic components.

In this receiver the main controller is ATmega8 MCU, except to that TDA2822 is used as AF power amplifier, PT2257 is used to control the volume. In this given design MCU, PT2257 and TEA5767 radio module is set to work with 3.3V power source. LD33V voltage regulator is used to obtain 3.3V from supply voltage.

To control the receiver, MCU provide five push buttons. Those can be used to control volume, perform automatic scan and manual tuning.

Final version of TEA5767 based FM radio receiver.

The firmware of this receiver is developed using MikroC for AVR and it is available to download with project package.

This receiver is design to drive using 9V - 12V DC power source and we recommended using regulated AC power supply to drive this tuner.

While at the testing we got very good results with this tuner and it perform similar to the commercial grade portable FM stereo radio receiver. All the schematic and firmware related to this project are available to download at

Tuesday, August 25, 2015

Calculator for audio output transformers

Audio output transformers are heavily used in vacuum tube and some (older) transistor base audio power amplifiers, but these days output transformer are quiet hard to find and expensive item. For homebrew projects the best option is to construct those transformers by ourselves and this script helps to calculate winding parameters for those transformers.

This "AF output transformer calculator" script is written using Python and it works with most of the commonly available Python interpreters. The script is available to download at under the terms of GNU General Public License version 3.0.

Homebrewed 25k : 4 output transformer

Once supplied the input parameters this script provide winding ratio, number of turns required for primary and secondary winding and required copper wire gauges for both primary and secondary windings, etc.

We construct several AF output transformers based on results of this script, which including transformers for Mullard 7W and 10W power amplifiers, several 6N6C/6V6GT and 6L6G based power amplifiers, etc.

Friday, July 31, 2015

USB to 3.3V - 5V serial TTL adapter

This post is about experimental USB to 3.3V/5V serial TTL adapter which we build recently to work with both 5V and 3.3V MCUs. This project is mainly based on CH340G seral-USB chip. CH340G is cheap and commonly available serial-USB convert IC and now it's commonly found on Chinese versions of Arduino development boards.

FTDI's FT232 is another possible alternative for this kind of project, but we drop it because of its famous device driver base chip locking issues. Also compare with FT232, CH340G is cheap, commonly available and simple to construct with minimum number of components.

In this module we incorporate 5V/3.3V voltage selector switch to avoid interface matching issues due to different voltage levels of main-board/MCU and serial TTL adapter.

Prototype version of CH340 USB to serial adapter.

In prototyping stages we use CH340G with breadboard by using SO-16 to DIP16 adapter and supplied PCB is also based on using the same adapter. Except CH340G all the other components in this project are standard through-hole type components and may not need any special soldering techniques for construction.

In prototyping we test this adapter successfully with several MCUs which including PIC18F452, PIC16F887, LPC2103, ATmega8, ATmega32 and STM32F103C8. The device driver for CH340G is available to download at Schematic and PCB design related to this project are available at