Sunday, April 26, 2015

Low cost DC millivolt source

DC millivolt sources are often used for thermocouple simulations, DMM, ADC and analog circuit calibrations and fault findings, etc. Most of the DC millivolt circuits are based on expensive (and sometimes hard to find) operation amplifier like OP7, LT1077, etc. In this article we introduce low cost DC millivolt source based on commonly available LM308 precision operational amplifier, ICL8069 low voltage reference and 78L05 voltage regulator ICs. This millivolt source is design to obtain 200mV - 700mV DC output with 100Ω impedance. At the time of writing we spend Rs.800.00 (LKR) to complete this project.

Finished view of DC millivolt source.

The most expensive component in this project is Bourns 10K linear multi-turn potentiometer. We use Bourns potentiometer because of its reliability and durability, but any 10K linear multi-turn potentiometers can be used with circuit.

The output impedance of this millivolt source is set to 100Ω to work with most of the high impedance test instruments.

DC millivolt source with 500mV output.

This unit need external DC power source and we recommend 9V to 25V (100mA) single rail power supply or 9V battery to drive this circuit. This DC millivolt source is easy to build and PCB is the most recommended way to construct this project. All the PCB design files and schematics of this project are available to download at

Monday, April 13, 2015

4 way Stereo Unbalanced Distribution Amplifier

This is 4 way stereo unbalanced distribution amplifier (DA), which can be used to distribute (or split) input audio signal to 4 isolated audio devices.

Distribution amplifier in this article is based on commonly available TL072 and TL074 low noise operational amplifier ICs and 7812/7912 voltage regulator ICs. To drive this system, 12V-0-12V or 15V-0-15V 1000mA center tapped step-down transformer is required. Also note that the operational amplifiers used this circuit can be replaced with identical high quality versions like OPA4134, OPA2134, etc. In our prototype we use TL07x series because of its availability and lower price.

PCB design of stereo DA project

To avoid any interference it is highly advisable to construct this circuit on PCB and also use double core screened wires for all external audio connections.

Schematic, PCB and "EAGLE project files" of this project are available to download at

Wednesday, April 8, 2015

AVR-HV: High voltage programmer for AVR microcontrollers

When it comes to AVR microcontrollers the most common programming option is In-System Programming (ISP). ISP interface is easy to use but in some scenarios it is totally unusable. The most common scenario is with some wrong fuse bit values. For example if we program SPIEN or RSTDISBL fuse bits, AVR microcontrollers may not respond to any in-system programmer(s). To overcome these problems the next available option is high voltage programming mode. In High voltage programming mode, 12V programming voltage is applied to RESET pin of target AVR microcontroller and user can change configuration fuses of AVR MCU with minimum amount of risk. The only drawback in this mode is that target microcontroller must need to be removed from the board to reprogram.

Most of the AVR high voltage programmers are expensive and difficult to find in ordinary electronic shops. As a solution we implement USB base high voltage parallel programmer for AVR microcontrollers and it allows programming, reading, verifying and configuring AVR microcontrollers through HVPP interface.

Prototyping version of AVR-HV programmer

This project is hosted at and all the project files, schematics, PCB design files, source codes and compiled binaries are available to download at there. Complete documentation is also available at project wiki section.

Current version of AVR-HV support ATmega series of AVR microcontrollers, but it can also be used with ATTiny microcontrollers which having high voltage parallel programming (HVPP) interface. To introduce new devices, only the configuration file need to be change and no source code changes are necessary.

AVR-HV Windows GUI

This project is an open source hardware (OSHW) project. AVR-HV source codes and compiled applications (including firmware) are released under the terms of MIT Licenses. AVR-HV USB programmer design files and AVR-HV documentation are released under the terms of Creative Commons Attribution 4.0 International License.

Sunday, March 8, 2015

BFO for SSB Reception

This post is about beat frequency oscillator (BFO) module which is used to receive SSB and CW signals from ordinary shortwave receiver. The original circuit which is related to this post is obtained through the internet and we modify it to get more precise output with commonly available electronic components. (Unfortunately at the time of writing this original link is no longer accessible due to dead link.)

This module consists with two general purpose NPN transistors and we select 2SC828 because of its availability in local marketplace. For the crystal we use commonly available 455 kHz ceramic resonator.

Assembled 455 kHz BFO module.

This BFO unit is easy to construct and may not need any test instrument for calibration. To receive the SSB signals, power this unit with regulated 5V or 6V DC power source and place BFO module near to the shortwave radio receiver. To get good results place output wire of BFO near to the IF amplifier section of the target receiver. Now switch receive into the shortwave band and tune it until you hear a whistling sound from receiver speaker(s). If whistling sound exists, tune receiver into amateur radio bands to get some SSB/CW radio broadcasts. To get clearer SSB reception try to change the distance between receiver and BFO unit, and change the output wire location also.

We try this BFO unit with several shortwave receivers which including National Panasonic RX-5090T, National R-218, National R-314, Panasonic RX-CS780, Kaito KA-11 and some unbranded Chinese receivers, etc. In most of the receivers this unit produces good results and in some of the receivers it becomes useless due to differences in intermediate frequency.

With proper tuning of trimmer capacitor this unit may be able to produce exact 455.000 kHz output and its waveform is closer to 2Vpp square wave (on 5V DC power source).

In our testing we got quiet good results with this BFO on National Panasonic RX-5090T and some of the recorded samples are available at this soundcloud profile. Our modified schematic and PCB design is available to download at