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

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