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

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

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

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