8 CHANNEL SOUND EFFECT UNIT

This project was designed to replay up to eight different recorded sound effects on radio controlled model boats, but will doubtless find application in other radio control projects. So far it has been used to great effect in several battleships to create various types of gunfire, sound horns and klaxons, make ASDIC 'pings', sound 'General Quarters', make aircraft and helicopter fly-bys etc. All the hard work is done by an embedded mp3 player module (mp3 Trigger v2) and a microcontroller chip (PICAXE 18X) chip simply serves to decode a spare (proportional) radio control channel to trigger the eight sounds.

The mp3 Trigger unit plays mp3 files stored on a micro SD card in high quality stereo. It has 18 pairs of pads, which if bridged play the track associated with the pad pair (ie shorting pads "TRIG3" will play the file "TRACK003.MP3" etc. It also has a serial interface, which if used can select up to 256 different tracks and a fast real-time volume control can be accessed too. However, this project confined itself to use of 8 of the trigger inputs. There is also a "navigation" switch on the module which can step forwards and backwards, stop and start ANY of the tracks on the card (over and beyond the eight radio controlled ones) - this could be useful in exhibitions to launch a lengthy "documentary" track describing the model. The smallest capacity micro SD card I could buy cheaply (less than £5) has 2Gbyte of storage which allows many hours of playing. The SD card has the files loaded onto it via a card reader (internal or USB gizmo) on your computer before fitting to the mp3 trigger unit.

There are a great many sound effect libraries on the web from which sounds may be downloaded, usually in WAV format, so a good sound editing package like Audacity (free!) is needed to convert them to mp3 format - it can also be useful to clean up lead-in or trail-out artefacts etc.

The PICAXE 18X chip used here has 8 outputs, which fixed the design at an 8 channel unit - though the user method of track selection I have developed makes 8 a realistic limit in any case. The (not very original) idea is to quickly 'jab' the joystick on a spare (proportional) channel N times and then hold it to get sound number N. When the joystick has been released for a period somewhat longer than the usual pause between jabs the jab counter is reset for decoding the next command. I chose to make the 8th sound a very brief period of silence asserted by a single jab of the stick in the opposite direction thus creating a 'cancel' command to instantly terminate any lengthy sounds playing. The first jab and hold sound should be assigned to your most frequent or important sound - like a horn for warning of collision.

As can be seen from the picture, the unit is easily constructed on stripboard and those of you wishing to build one for yourselves should follow the link http://www.technobotsonline.com/8-ch-mp3-rc-sound-unit-by-alan-bond.html where you will find additional technical detail, a stripboard layout diagram and parts list (all parts are available from Technobots with the exception of the micro SD card).