Saturday, September 24, 2016

μPC1237 based 2 channel speaker protector

When working with expensive speaker system, speaker protector is an essential item to avoid any damages to speakers. In this project we build low cost speaker protector by using NEC's μPC1237 IC. The circuit in this project is based on μPC1237 datasheet and it is specifically modified to work with 24V AC power source and with 24V relay.

Prototype version of μPC1237 - 2 channel speaker protector system.

PCB design of this project is limit to 90mm × 43mm and based on through hole components. Depending on supplied voltage and relay, value of R7 resistor is need to be change. The specified value in schematic is for 24V AC power source and for Omron 24V G2R-2 relay. Start-up delay of this unit can be adjust by changing the values of R5 and C5.

Schematic and PCB design files of this project are available to download at

Tuesday, August 30, 2016

CoCA Emerge + Tech - Musical plants project

CoCA "Emerge + Tech" - Musical plants is an electronics, software and plants base exhibit we developed for Cinnamon Colomboscope 2016 exhibition. This installation will feature a touch sensitive sensor that will generate a soothing sound from each plant. By interacting with individual plants, the audience can create musical compositions.

Sensor unit of this project consists with 14 high sensitive, configurable level detectors and USB interface. In this sensor unit we use Samsung's KA2284 as amplifier, level detector, and PIC18F4550 8-bit MCU as data multiplexer and USB I/O controller. PC817 opto-isolators are used to interface between analog front-end and digital data processor. In this design both analog and digital circuits are physically and electronically isolated to minimize the noises and interferences.

In this design we use KA2284 because it is commonly available in local market (in Sri Lanka) and costs less then 0.1 USD. To reduce the interferences we use shielded cables between plants and sensor ICs. With the special wire we are using we may be able to extend the sensor connectivity up to 3m.

Sensor board of musical plants project.

PC application of this project is developed by using HIDAPI and irrKlang libraries. This application is designed to work with Microsoft Windows operating systems and we tested it on Windows 7 and Windows 10.

This project is an open source hardware project released under terms of MIT and CC BY-SA licenses. All the content of this project are available to download at or Conceptual details of this project is available at CoCA homepage.

Sunday, August 28, 2016

Sony TC-500 reel to reel player restoration

TC-500 is 4 track stereo reel to reel player which is build around 1960 - 1970 by Sony Corporation. This is vacuum tube base reel to reel player with 2 × 3W audio output power. TC-500 is dual speed (3¾ and 7½ ips) tape recorder and it support reels up to 7 inches (18cm). Amplifier stage of this tape recorder is build around 12AX7, 6AU6 and 6AQ5 vacuum tubes and 12BH7 tube is used in biasing oscillator. Other than above tubes, 6CA4 is used in PSU for rectification.

Restored Sony TC-500 reel to reel tape recorder.

Recently we got a chance to restore TC-500 reel to reel player which we got from one of our friend. The player which we got is heavily damaged and we spend nearly 2½ months to restore this player.

In first stage of restoration we disassemble mechanical parts of this player and clean everything. In this player capstan idler wheel and take-up idler wheel is damaged beyond repair and we reconstruct those wheels using rubber sheets.

Saturday, May 28, 2016

Modified Heathkit HD-1410 electronic keyer

HD-1410 is quiet popular iambic keyer which is manufactured around 1970's by Heathkit. The original HD-1410 keyer is designed using 5 commonly available 74LS series TTL ICs and 8 transistors. Other than iambic operation it has an option to adjust sidetone frequency and support for (external) single-paddle key unit.

In this project we redesign original HD-1410 electronic keyer with today's commonly available components. In this new design the key components which we replace are transistors, diodes and power supply section of original HD-1410 circuit. Most of the NPN transistors in original design are replaced with KSP42 and KSP10 transistors, and all the PNP transistors are replaced with KSP92 transistors. To keep everything simple we also use same TTL chip-set with this new design.

PCB of modified Heathkit HD-1410 electronic keyer.

Tested PCB layout for this new keyer is available in the project archive and this PCB layout is based on standard though hole type components. Dimensions of this PCB is 110mm × 102mm.

We test this modified HD-1410 design with several transceivers and it performs extremely well. This new design also behaves similar to original HD-1410 unit and the parameters which got change are DC input voltage range and power consumption only.

Schematic, PCB and wiring diagram of this modified HD-1410 keyer are available to download at The original Heathkit HD-1410 schematic and details are available at