Saturday, March 17, 2018

Electrical wiring of the house

In last year we spend lot of time and effort to wire our new house by ourselves. To complete this job we took nearly 2 ½ months and it includes wiring, fixing electrical fittings, communication equipment, etc. In this post we describe how we archive this task with some technical details.

Due to large number of electrical points we decided to use use 3 phase electrical wiring in our house. To make it simpler we divide entire house wiring into 3 isolated circuits with 3 separate distribution boards. High level design of our AC wiring systems is illustrated in below diagram.

High level electrical wiring diagram up to distribution boards

In above circuit the 3 phase AC line is first fed into 4-pole 40A isolator. Then it connected to 4-pole RCCB with 3 separate indicator lights. We use indicator lights to see the status of the each phase, easily at any time. After RCCB we fed each phase into 3 separate distribution boards.

As seen in the diagram the first (phase) circuit is bit complex due to the change-over-switch. We use  change-over-switch to connect additional power source into AC line during the power failures. For the change-over-switch we use DIN-rail type, 2-way 2-pole 40A change over switch. In here also we use two indicator lights to show both mains and external (generator) line status.

4-pole isolator and RCCB mount

Sunday, March 11, 2018

Automatic fan controller for server racks

In this post we describe fan controller which we designed for our 9U wall mount server cabinet. This fan controller is designed to drive a 12V DC cooler fan with pre-configured intervals or by monitoring the temperature of the server cabinet.

Final version of fan controller with DC brushless fan and 12V - 60W PSU.

Core components of this fan controller is CD4060 binary counter, LM35 temperature sensor and LM358 operational amplifier. In this design CD4060 is used as long duration timer and it can configured to trigger cooler fan from 1-minute and up to 4-hour.

In this design LM35 temperature sensor is used to activate cooler fan in specified temperature. This sensor stage is useful to drive cooler fan when timer stage is in inactive state.

To control the cooler fan we use AP9971 dual N-channel power MOSFET transistor. We design this system to drive 12V cooler fans up to 2A of current. To test this controller we use commonly available 120mm × 120mm, 12V - 300mA brushless DC fan. In our server cabinet we mount this fan to push hot air out of the cabinet.

This fan controller is designed to work with 12V - 1A (or higher) power supply. For the testing and for the final installation, we use 12V - 60W power supply unit which is commonly used for LED lightning projects.

3D view of fan controller PCB.

Supplied PCB design of this controller is 64mm × 63.5mm and it based on standard through-hole type components. All the PCB designs and schematic of controller is available to download at google drive or from

Monday, September 11, 2017

Network cable tester

This is an automatic Cat6 / Cat5 network cable tester designed using NE555 timer and 4017 decade counters. This unit test all 8 wire lines of twisted pair network cable and indicate pass/fail status with single LED.

We design this unit to test network connectivity issues in Cat6 / Cat5 cable systems and it is capable to check both crossover and straight-through type network cables.

Prototype version of network cable tester.

The recommended way to construct this circuit is on PCB. Both Master and Salve units need to be build on two separate boards. To perform test one end of the network cable must connected to the Master unit and other end need to be connected to the Slave unit. After connecting both Master and Slave units press "START-TEST" (S1) button to perform the test. Status of the cable is indicated through LED2 (PASS indicator).

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

Sunday, August 13, 2017

Restoring Sony STR-AV780 AV receiver

Sony STR-AV780 is a combined receiver and audio/video control center. This receiver is manufactured by Sony in around 1986 and it is capable to deliver 2 × 80W audio output power with 8Ω speaker load.

The STR-AV780 which I got had two problems. First problem is in power amplifier stage and it produces distortion in high frequencies. The second problem is in FM radio receiver and it is completely dead.

Power amplifier of STR-AV780 is based on two Sanyo STK4042 XI AF power amplifier ICs. To fix the distortion issue we replace all the electrolytic capacitors in both STK4042 power amplifier boards. After replacing capacitors distortion issue got fixed and power amplifier starts to produce clear output waveform.

The FM receiver of this AVR is mainly based on 2 ICs. For IF stage it uses LA1235, and to demodulate FM stereo signal it use LA3401. Apart from that Sony CX7925 is used as PLL frequency synthesizer. The front end of FM tuner is based on couple of transistors and FETs.

To fix FM receiver we inject FM test signal into receiver and check each stage of the FM receiver. Based on VCO output (of CX7925) and FM front end output we identify LA1235 as faulty component. After replacing LA1235, FM tuner starts to work again.

Fixed AV780 mainboard - tuner stage.

Due to weak nature of PCB copper tracks we attach LA1235 into main PCB through an IC base.

After replacing all above described components we clean and repaint the entire AV receiver.

During test stages we check this receiver with several speaker setups. Out of all the speakers this receiver performs extremely well with our Pioneer CS-707 speaker system.

Due to lack of service manual / schematic Sony STR-AV780 is quiet difficult receiver to repair. We trace and fix components of this receiver based on PCB layout and IC data-sheets.