Dilshan R Jayakody’s Web Log

This post is about beat frequency oscillator (BFO) module which is used to receive SSB and CW signals from the 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 a dead link. ) This module consists of two general purpose NPN transistors and we select 2SC828 because of its availability in the 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 to place output wire of BFO near to the IF amplifier section of the target receiver....

This is two transistors and two IF transformer based superheterodyne receiver which is designed to receive 13m to 41m bands. With quite good external antenna this receiver performs similarly to the commercially available shortwave receiver and its audio quality is at a 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 designed to work with 9V regulated power supply and it requires an external AF amplifier to drive the headphone or speaker. In our testing, we use a TDA2050 audio power amplifier with this receiver. With small wire antenna, this receiver is capable to receive most of the stations which in...

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 the last ring. This telephone locator is mainly based on NE555 timer IC and it is designed to work with 6V 0.5A - 1A power source. This given schematic support a 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 is concerned. Schematic and PCB designs of this unit are available to download at google drive .

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 a 20V range and deliver 3W (R.M.S) audio output power into the 7.5Ω load. The frequency bandwidth of this IC is between 30Hz to 18kHz and with the 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 the 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 quite 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 to make sure to drive this amplifier with a regulat...

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 the 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 the environment and it is mainly built 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 ☻ ). This digital data capture unit can operate in 2 modes. In the first mode, it directly converts an analog signal into digital data and transferred to the host via a serial interface. In second mode it displays captured signal in 5 LED bar graph either slow or high-speed capture modes. This unit is designed to work with 5V 500mA power source and to get optional EMF reading...

BA1404 is quiet famous FM stereo transmitter IC manufactured by ROHM Semiconductor . This 18pin chip contains FM stereo multiplexer, 19 kHz pilot tone generator and RF output stage with the 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 designed to drive using a 5V DC power source. Except for 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 an adjustable ferrite core .  This variable inductor is used to select the output frequency of the transmitter. To archive good quality output and frequency stability ...

This is a 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 resistors manufactured by Vishay Sfernice and both these resisters are connected in parallel to achieve 50Ω impedance. The final version of 100W RF dummy load. Outer-casing (which also act as a 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 resistors are obtained through Farnell element14 .