Monday, December 28, 2015

Light operated voice messenger

This is simple electronic project which we implemented for some local event to guide audience automatically with some prerecorded voice messages. The core component of this unit is ISD1820PY voice record/playback chip which is manufactured by Nuvoton Technologies. Except to ISD1820PY, this unit use commonly available low power LASER and LDR assembly to detect people. To drive the loudspeaker we use popular LM386 audio power amplifier.

Basic setup of electronic voice messenger.

Thanks to ISD1820PY and LM386 this system can implemented on breadboard or on strip-board with minimum number of external components. This circuit is designed to drive using 6V or 9V DC battery but it can also drive with 6V or 9V DC regulated power supply. To get 3.3V we use LD1117V33 voltage regulator, but this can also be replaced with some commonly available 3.3V regulators such as LD33V, MC33269-3.3, etc. For the speaker we use 4-inch 8Ω full-range speaker.

Prototype version of electronic voice messenger.

This unit can used with both dynamic and condenser microphones but according to our experiments, ISD1820PY produces excellent results with dynamic microphones. As per our observation with some condenser microphones results are not up to the satisfactory level.

Detailed schematic of this project is available to download at google drive.

Monday, December 21, 2015

74181 ALU design on Altera MAX II CPLD

74181 is 4-bit arithmetic and logic unit introduced by Texas Instruments in March 1970 and it is known for being the first ALU in single package. This chip is no longer in manufacturing but still it is quite popular in CA (computer architecture) courses in many colleges.

To test the behavior of this 4-bit ALU we decided to construct it on Altera MAX II CPLD. For this project we choose EPM240T100C5 CPLD and logic design is implement using Altera Quartus II. For logic design we refer Fairchild Semiconductor’s DM74LS181 datasheet. This setup is tested on basic EPM240 series development board with some mechanical switches and couple of LEDs.

74181 4-bit ALU test setup with EPM240T100C5 development board.

There are several test circuits available for 74181 ALU and our test setup is partially based on Electronics for Your (EFY) 1997 article. In another test setup we use HEF4040 to drive, A and B operands of this ALU.

The test setup attached into this article is the minimum configuration which anyone can build with small number of commonly available electronic components.

All the downloadable content related to this project are available at google drive.

Friday, December 18, 2015

Replacement LED driver for AN6877

AN6877 is linear AF level meter IC produced by Panasonic and it is commonly found on many audio equipment. This chip is no longer manufactured by Panasonic and finding replacement chip for AN6877 is also quiet difficult.

The circuit described in this article is design to replace AN6877 base LED drivers and it is based on commonly available components. This replacement LED driver is design using 10, MMBT3904/2N3904 transistors and it can easily modify to get necessary number of outputs.

SMD version of PCB design for 8-dot driver.

During the prototyping stages we test this driver with minimum of 7 LEDs (AN6877 configuration) and up to 12 LEDs. This LED driver is design to work with 9V to 12V DC power source.

With 7 LEDs and 9V power source, this module can directly replace AN6877 base LED driver modules.

Schematic and PCB design related to this LED driver is available at google drive.

Thursday, November 26, 2015

ZFM-20 fingerprint capture library

ZFM-20 is optical fingerprint sensor module series developed by Zhiantec. Resolution of this fingerprint modules are 256×288 pixels and depending on the module it can be accessible over UART or USB interfaces. Other than capturing fingerprints this module has inbuilt DSP to process and verify captured fingerprints.

This ZFM-20 fingerprint library is developed to capture and retrieve fingerprint images from the above sensor module. The main purpose of this library is to use ZFM-20 sensor with PC as low cost fingerprint capture unit and because of that, in-module fingerprint processing and verification functions are not implemented in this library.

ZFM-20 fingerprint sensor test setup with CH340G serial to USB converter.

ZFM20-LIB is x86 native shared library and its implemented using Lazarus/FPC. This library is developed to use with most of the programming languages which including C, C++, Delphi, FPC, Microsoft .net framework based languages, etc. Object oriented wrappers for this library are available for both .net framework support languages and for FPC.

This version of ZFM-20 Fingerprint capture library is tested with ZFM-20 series serial fingerprint module with CH340G USB to serial converter. Generally this library is design to work with any USB to serial converter which including FT232, CP2101, etc. 

ZFM-20 fingerprint capture library is an open source shared library project and it's source code and compiled binaries are available at This library is released under the terms of MIT License.

Monday, November 9, 2015

Dynamic microphone preamplifier

This is single chip dynamic microphone preamplifier which is design to drive using commonly available 12V single rail power supplies. Preamplifier in this article is build around popular NE5532 dual low noise operational amplifier IC and this preamplifier is specifically constructed to drive professional 600Ω dynamic microphones.

Internal view of dynamic microphone preamplifier prototype.

This preamplifier consists with two amplifier stages and initial amplifier is design to get 36dB gain and final preamplifier is used to get 16dB of gain. Preamplifier gain controller is placed in between those two amplifiers and this separation allows reducing clipping in output waveform.

In prototyping stages we test this preamplifier with few 600Ω dynamic microphones with TDA2050 power amplifier. According to our observations this unit produces excellent output with all of these microphones. To get professional results use this preamplifier with 12V battery or with regulated power supply unit and use screened wires for both input and output connections.

Schematic, wiring diagrams and PCB design of this microphone preamplifier are available to download at google drive.

Tuesday, October 27, 2015

48V Phantom power supply

Phantom power supply is widely used in audio industry to power the active microphones over single audio cable. Most of the phantom power supplies deliver 48V DC over the balanced audio link and power supply unit described in this article is also based on same technique.

Phantom power supply in this article is based on TL783 high voltage regulator IC and 230V - 50V step down transformer. Like most of the phantom power supplies this power supply is also design to connect in between active microphone and audio mixer devices.

PCB of 48V Phantom power supply.

Schematic of this power supply unit is self explanatory and PCB design is also based on commonly available through-hole components. For the step down transformer we use commonly available 230V - 25V-0-25V 1.5A transformer.

During the construction we recommended to use shorter wires between PCB and XLR sockets to reduce the noise.

PCB and schematic of this power supply unit is available to download at google drive.

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 google drive.

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 google drive 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 google drive.

Wednesday, July 15, 2015

Soldering station to drive HAKKO 907 handles

HAKKO 907 is popular and commonly available soldering iron in electronics field. To drive those soldering irons user needs to have HAKKO or any other compatible soldering station. The biggest problems with those soldering stations are there higher price tags and limited availability in local market.

Because of those problems we decided to create our own soldering station, and for that, we choose "Chinese" version of HAKKO 907 soldering handle. Now Chinese versions of 907 compatible irons are available in eBay for less than 5USD. As per our observations, those Chinese versions of soldering irons look and perform almost similar to genuine HAKKO 907 iron(s).

To keep it simple we choose LM358 dual operational amplifier IC to monitor the 907's thermocouple and drive the TRIAC. The entire control circuit of this soldering station is powered by single rail 5V DC power supply. To drive both handle and station we use 12V-0-12V 8A step down transformer.

Internal view of finished soldering station.

Once calibrated, we may be able to regulate temperature of the soldering iron between 210 °C to 450 °C. As shown in the photograph we construct our final version of soldering station in slightly modified old ATX power supply casing. For this entire project we spent only 25USD, which is roughly 3000LKR at the time of writing.

Schematic, PCB design and wiring diagrams related to this project are available to download at google drive.

Thursday, June 25, 2015

Contactless DTMF switch

This is contactless DTMF (Dual Tone - Multi Frequency) switch to control 16 channel relay boards. This module can turn ON/OFF relays based on DTMF singles which captured from its microphones.

This project is building using 4 commonly available ICs which including TL084, TL081, CM8870 and 74LS154. TL084 is used as microphone preamplifier and LPF. TL081 act as combiner and CM8870 is used to decode the DTMF inputs. 74LS154, demultiplexer is used as line selector. In this circuit TL084 and TL081 and replace with any compatible operational amplifier IC. During our prototyping we replace TL084 with LM324, TL074 and TL081 with TL071 and got the identical results.

For the microphones it is highly recommended to use omnidirectional condenser microphones. In our tests we get good results when both the microphones are in 90⁰ plane.

This circuit is design to work with single rail 5V DC power source and its output terminal (SV1) is compatible with most of the 16 channel relay boards in the market.

To get maximum performance it is essential to build this circuit on PCB with proper shielding. As per our observations regulated power supply is vital for this circuit and AC ripples on power lines may leads to erroneous results.

 Schematic of this contactless DTMF switch is available to download at google drive.

Monday, June 15, 2015

UC3844 base motor speed controller

UC3844 is popular current mode controller which is commonly found on DC-to-DC converter circuits and switch mode power supplies. This motor speed controller is also based on UC3844 and it is specifically design to drive 20V - 24V DC motors.

In this given configuration UC3844 produces (approx.) 50kHz to 240kHz PWM output and this range can be adjust by changing the value of C2 timing capacitor. As per the datasheet UC3844 is capable to produce PWM output frequency up to 1MHz.

UC3844 waveforms on RT/CT (Pin 4) and OUT (Pin 6).

This circuit is quiet sensitive to the noises, and to get the accurate output construct this circuit on PCB. As per our observations long wires on oscillator circuit may change the stability of the output signal.

According to the ON semiconductors, UC3844 is now discontinued product but it is still commonly available in the market for less than Rs. 40.00 (LKR). For this circuit either DIP (UC3844N) or SMD (UC3844D1) version of UC3844 can be used but the PCB design available with this article is based on DIP version of UC3844.

The schematic and PCB layout for this motor controller is available to download at google drive.

Sunday, June 7, 2015

EAGLE BOM generation script

This is a quick post about EAGLE parts list generation script which I was written to replace existing "part2html.ulp". This script generates more organized and detailed BOM HTML file and it can directly replace "part2html.ulp".

This script is tested with EAGLE 6.6.0, but it can also work with older versions of EAGLE software.

This script is available to download at with usage details.

Tuesday, June 2, 2015

FlashProg - USB serial flash memory programmer

FlashProg is USB base flash memory programmer to work with 3.3V serial flash memory devices. This programmer is specifically design to read, program and configure 25x series, serial flash memory devices which are commonly used to store BIOS in PC mainboards.

Originally we design this project to read and program BIOS of Foxconn G31MXP mainboard. Our version of G31MXP contains Macronix MX25L8005 8M-Bit serial flash memory and we use this programmer to load some of our custom BIOS to this serial memory.

MX25L8005 SPI flash memory on Foxconn G31MXP mainboard

FlashProg programmer is built around Atmel ATmega8A microcontroller with standard through-hole type components and it support for both 32 bit and 64 bit Windows and Linux operating systems. All the compiled binaries for both the platforms are available at project home page at

This programmer use very few components and it can be easily constructed on breadboard or on PCB. Both EAGLE and PDF versions of PCB design files are available with the project package.

Prototype version of FlashProg programmer

This project is an open source hardware project and all it’s source codes, design files and documentation are available at site.

Friday, May 15, 2015

CD2003 - yet another simple FM radio receiver

In last few days we are looking for some simple FM radio receiver to integrate into one of our ongoing project. For that we try several FM radio receiver ICs including TDA7000, CD2003/TA2003/TA8164, CXA1019 and KA22429. Out of all those chips we select CD2003 (or TA2003/TA8164) based receiver for our project because of its simplicity and outstanding performance. Except to CD2003, Sony CXA1019 also perform well but we drop it because of its higher component count.

We design our receiver based on Toshiba TA2003 datasheet and later we try TA8164 and CD2003 with same circuit. Either CD2003 or TA8164 can directly replace TA2003 IC, and as per our observations TA8164 gives excellent results out of those 3 chips.

Prototype version of CD2003 FM radio receiver

The PCB design and schematic which we used in our prototype project are available to download at google drive (including pin-outs of crystal filters and inductors).

Except CD2003 IC, this receiver consist with 12 components. Three inductors used in this receiver are home-produced and all the winding details are available with the schematic diagram. This component count can also reduce by introducing FM band pass filter module to FM RF input stage of TA2003 chip. We try BPW85 FM band pass filter with this circuit and it also give same results as original circuit.

In our testing we operate this receiver using 3.3V power supply and by average it draws 8mA - 12mA current from PSU.

To drive speaker(s), this receiver needs to connect with suitable AF power amplifier and in our testing we use TDA2050 base AF power amplifier with this receiver.

Sunday, May 3, 2015

3A Power supply for USB devices

This is 5V 3A power supply to drive two USB based devices simultaneously. This power supply is design around LM2576-5.0 switching regulator IC and the main reasons to choose this IC is its low parts count, small heat sink requirements and its inbuilt current limit protection feature.

Prototype version of 3A USB power supply

We specially design this power supply to work with development boards / single board computer (SBC) platforms such as Raspberry Pi, Banana Pi, BeagleBone, etc. This power supply can also be used to provide power to USB OTG peripherals over USB Y cables. We test this setup and got successful results with Huawei Ascend P6 mobile phone.

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 this 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 google drive.

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 google drive.

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 google drive.

Sunday, March 1, 2015

Two transistor superheterodyne shortwave receiver

This is two transistor and two IF transformer based superheterodyne receiver which is designed to receive 13m to 41m bands. With quiet good external antenna this receiver performs similar to commercially available shortwave receiver and its audio quality is in very satisfactory level.

Prototype version of two transistor superheterodyne shortwave receiver.

This receiver can be constructing on PCB, breadboard or commonly available veroboard or stripboard. For tuning use Polyvaricon or air variable capacitor. For precision tuning, connect small trimmer capacitor with parallel to variable capacitor’s gangs or use mechanical wheel assembly with the variable capacitor.

This receiver is design to work with 9V regulated power supply and it requires external AF amplifier to drive the headphone or speaker. In our testing we use TDA2050 audio power amplifier with this receiver.

With small wire antenna this receiver is capable to receive most of the stations which including CRI, BBC World service, All India Radio, etc. in Sri Lanka. To receive other stations which including VoA, Radio France, Deutsche Welle, ABC we use our 2 × 12 feet T antenna.

For initial IF transformer tuning we recommended to use quiet large antenna and some powerful station. For example in Sri Lanka, CRI (17855.00kHz, 17730.00kHz, etc.) or AIR (11710.00kHz, 13710.00kHz, 9690.00kHz, 11620.00kHz, etc.) are quiet easy to locate and tune.

The complete schematic of this superheterodyne shortwave receiver is available at google drive.

Saturday, February 21, 2015

Simple Telephone Locator Light

This is simple telephone locator-light which is used to identify the ringing telephone at the night / low light conditions. This unit only activates at low light conditions and it automatically turns off lights after 12 - 15 sec. from last ring.

This telephone locator is mainly based on NE555 time IC and it is design to work with 6V 0.5A - 1A power source. This given schematic support maximum of two 3W power LEDs. During the assembly it is important to place both LEDs and LDR in two separate positions to avoid any interference to LDR from LEDs. 

Also note that in some counties it is illegal to connect custom circuitry into phone lines, please check your local regulatory bodies if concerned.

Schematic and PCB designs of this unit are available to download at google drive.

Tuesday, February 10, 2015

SL414 Vintage power amplifier IC

SL414 is combined audio preamplifier and power amplifier IC manufactured by National semiconductors in early days. This amplifier is designed to operate in 10V to 20V range and deliver 3W (R.M.S) audio output power into 7.5Ω load. The frequency bandwidth of this IC is between 30Hz to 18kHz and with 8Ω load it produces 0.5% T.H.D on 3W.

The most uncommon feature of this IC is its EP-10 package. This EP-10 package is common to most of SL4xx series amplifiers and thanks to its inbuilt screw it is really easy to attach heat-sinks to these ICs.

EP-10 package of SL414 audio amplifier IC.

During our prototyping, we try several SL414 amplifiers and most of them give quiet satisfactory results to us. Link to most stable SL414 design is available at the bottom of this article. As per our experiments, to archive best results it is mandatory to build this amplifier on PCB with screened wires. Also to get the good results make sure to drive this amplifier with regulated DC power supply. In our tests we use LM317 base PSU to drive this amplifier.

SL414 Amplifier prototype.

SL414 is quiet hard to find and no longer manufactured by National (now TI). For those who having SL414’s can try our design/ PCB which is available at google drive.

Tuesday, February 3, 2015

Digital data capture unit with EMF reader

This is PIC16F73 base digital data capture unit with optional EMF reader. This data capture utility is design to work with most of the analog sensor inputs and by default it is equipped with EMF reader module. This unit use PIC16F73's internal 8bit ADC to capture the analog signals.

EMF reader unit in this schematic is useful to detect variations of EM field in environment and it is mainly build around uA741 operational amplifier and 12mH detector coil. Normally EMF meters are used to trace electric wires, find hidden transmitters, and diagnose electric wiring problems (and to find ghosts too ☻).

Sample reading from EMF detector when mobile phone is ringing

This digital data capture unit can operate in 2 modes. In first mode it directly convert analog signal into digital data and transferred to host via serial interface. In second mode it display captured signal in 5 LED bar graph either slow or high speed capture modes.

This unit is design to work with 5V 500mA power source and to get optional EMF readings it is highly recommended to use battery with regulator to drive this system.

This is an OSHW project and all the schematics, compiled software binaries and source codes are available to download at google drive.

Sunday, January 18, 2015

Low power FM stereo transmitter

BA1404 is quiet famous FM stereo transmitter IC manufactured by ROHM Semiconductor. This 18pin chip contain FM stereo multiplexer, 19 kHz pilot tone generator and RF output stage with oscillator. The FM transmitter in this article is also based on BA1404 and 78L33 voltage regulator.

The FM transmitter in this article is specifically designed to transmit audio signals generated by PC/Laptop sound cards. To archive this goal, this transmitter is design to drive using 5V DC power source.

Except Q1 (38 kHz crystal) and L2 (oscillator inductor), this transmitter uses commonly available electronic components. 38 kHz crystal is vital to generate 19 kHz pilot tone and this crystal is available to purchase in several web stores.

For L2, wind 3 turns of 22 S.W.G enameled copper wire on 5mm bobbin with adjustable ferrite core.  This variable inductor is used to select the output frequency of the transmitter.

To archive good quality output and frequency stability it is necessary to construct this transmitter on PCB.

As per our tests the maximum range of this transmitter is 20m - 25m with obstacles and 50m - 60m in open area. In shorter distances this transmitter performs very well and produces really good audio output. Due to legal reasons we didn't test this transmitter beyond 50m.

PCB design for BA1404 FM stereo transmitter.

As per the time of writing, BA1404 is discontinued product, but there are lots of places to buy this chip in either DIP or SOP form.

The schematic and PCB of this transmitter is available to download at google drive.

Friday, January 9, 2015

100W RF dummy load

This is quick post about 100W RF dummy load which we build to use with HF and VHF transmitters and transceivers. This dummy load uses two non-inductive 100Ω 50W thick film power resisters manufactured by Vishay Sfernice and both these resisters are connected in parallel to archive 50Ω impedance.

Final version of 100W RF dummy load.

Outer-casing (which also act as heatsink) of dummy load is constructed using four bonded fin 50mm × 115mm × 20mm heatsinks.

During our testing we got flat 1:1.2 SWR with this dummy load.

Internal view of 100W RF dummy load.

Above mentioned 100Ω 50W power resisters are obtained through Farnell element14.

Monday, January 5, 2015

ImageHide: Digital image based steganography tool

Steganography is method of hiding information and one of the techniques is hide an image inside a "cover image" by without distorting the supplied "cover image". ImageHide is command-line driven console application to perform this task and it is capable to hide/embed colored or gray-scale image inside another color or gray-scale cover image and/or extract embedded image from the supplied "cover image". 

This application is developed using FPC/Lazarus and it can execute on any FPC supported operating system which including Linux, Windows, Mac OS, etc. ImageHide is an open source project and its source code is available at under the terms of GNU General Public License version 3.0. Compiled binaries of ImageHide are also available for both Linux and Windows operating systems. As an input this application accept bitmap, PNG or JPEG image formats and it can export processed image as bitmap or PNG file. Because of lossy compression techniques, JPEG exports are not supported by ImageHide.

ImageHide documentation and examples are available at

Saturday, January 3, 2015

Homebrew Audio Signal Generator

Audio Signal Generator (or AF Signal Generator) is often used in RF and AF base designing and experiments. In this article we introduce less complex AF signal generator based on commonly available electronic components. The heart of this AF signal generator is Intersil ICL8038 precision waveform generator/voltage controlled oscillator IC. ICL8038 is a function generator capable of producing sine, square and sawtooth waveforms at same time. In this project we combine ICL8038 with inverting and non-inverting amplifiers to get above mentioned 3 waveforms with 0° and 180° phase differences at the same time. Those inverting and non-inverting outputs help to use this AF signal generator with balanced audio systems.

Final version of AF signal generator.

The given schematic and PCB design of this AF signal generator contains ICL8038 waveform generator, inverting and non-inverting amplifier stage, output buffer and 12V regulated power supply. All the above mentioned modules are integrated into 65mm × 110mm PCB with standard through-hole type components.

This AF signal generator is design to work with 18V 500mA AC input. To increase the stability, we recommended using "audio grade coaxial cables" for all external connections except to power indicator LED.

This given design is capable to generate 200Hz to 10kHz sine, square sawtooth waveforms. It is possible to have some slight frequency drifts due to temperature variations in the operating environment.

Generating 1.415kHz sawtooth waveform using AF signal generator.

As per our measurements and observations warm-up time of this AF signal generator is around 3 to 5minutes, but this can be varied based on the type of enclosure, component arrangements, external temperature, etc.

Schematic, PCB design and wiring diagrams of this AF signal generator is available to download at google drive. All these design files of this project are distributed under the terms of Creative Commons Attribution 4.0 International License (CC-BY-4.0). High resolutions photographs of this project are available at this flickr page.