I am a technical person. I have been involved with computers most of my adult life so naturally when computerization came along in model railroading I decided to go future as much as possible. I jumped into DCC and along with DCC came sound. The whole experience in sound in a model railroad is having localized sounds that add realism to the overall scene. That's to say, as a train passes you play a sound in that area or have crickets come out at night as the light changes or the sound of a crackling fire for either a house or campfire.
To this end I looked for an inexpensive circuit that would handle my needs. I found one based on the 28 pin ISD Sound Recorder/Playback chip. The circuit I have developed is a variation of one presented by Dick Roberts in Railroad Model Craftsman magazine, issues January and February 2001. This chip provides record times of 10 to 120 seconds. Utilizing the looping capabilities of the chip and circuit virtually unending sounds can be created.
To utilize all this easily I created a PC board that includes the sound chip and associated circuitry, a 500mw amplifier and a photocell trigger option. All this on a board 3" x 2 1/8". These are quality professionally made boards with solder mask and silk screening. The bare board is visible on this page and the various versions or degree of population can be viewed here.
I have created this board so that all or part of the circuitry may be used depending upon your needs and budget. I know that in the area of model railroading we are all on a tight hobby budget. Only 13 parts are needed for the basic playback unit at a cost under $20 for the board and components. (See the errata sheet for errors and ommissions.)
The best part of this is the ability to record your own sounds. You can customize sounds to your layout and scenes. Canned sounds will be available from here as well as the terabytes of sounds available on the internet.
Although I have generated this item for use in my model railroad there are other applications available. One would be a narrator for static displays playing a canned speech when activated by a viewer. If you build dioramas this would be a great addition.
A number of features were designed into the board with cost savings in mind. Since the circuitry on the board requires +5VDC, a regulated power source is required. An onboard regulator, fed by an external supply, can handle multiple units by back feeding the +5VDC from the provide output. The onboard regulator is designed to be supplied by +12VDC found in most model railroads. The basic sound unit with amplifier and photocell draws 25ma. The 7805 regulator used is capable of 1000ma (1 amp) when used with a heatsink. We recommend drawing no more than 200ma from one board as there is no heatsink. This, however, still provides the capability of powering 7 additional units from one regulator. For those in need of less power capacity you can utilize the 78L05 low power regulator. This item provides 100ma output and is in a smaller package. It is recommended that you not attempt to backfeed any additional boards with this regulator.
The board has 3 main circuits. One, two or all three can be populated depending upon your needs. The photocell trigger circuit and amplifier can be eliminated if not needed and the speaker driven directly from the sound chip, although not very loud, or it can feed an amplifier of your own if more volume is needed.
Any number of trigger devices can be used to initiate the recording. All that is required is to present the common ground (GND) to either the Cont(inuous) or Mom(entary) trigger inputs.
Recording times from 10 seconds to 2 minutes are possible depending upon the chip used. With the looping capabilities of the board this provides great possibilities. Most circumstances will allow the cheaper 10 sec chip.
The normal "setup" would consist of one board populated to provide recording capabilities and any additional number of basic playback boards. The recording board incorporates a ZIF (Zero Insertion Force) socket allowing easy insertion and removal of the sound chip. (All playback boards must incorporate a DIP socket to eliminate damage to the sound chip due to soldering. Failure to use a socket degrades or eliminates many of the features of the board.) You can view the assembly instructions, schematic and connections here.
In addition to the recorder board you'll need an audio cable for the audio input from your computer.
The recording process involves finding the sound(s) you want, editing the sound(s) and recording them to the sound chip. The most difficult or critical actions will be finding the right sounds and performing the editing. Looping sounds require some patience to blend the loop. The actual recording process is simple and takes little more time than the length of the recorded sound.
The actual playback operation is very simple. You have two trigger modes and two playback modes. The trigger modes are either play while active or play until end. The play while active input plays the recording so long as the input is active low or gnd. The play to end trigger mode plays the recording to completion regardless of the status of the input during the playback. A truth-table lists the various combinations of trigger and playback functions.
Actual trigger devices can be anything that presents a gnd signal to either trigger input. Tortoise switch machine auxiliary relay contacts could be utilized. Other trigger devices, such as an infrared emitter/detector pair (Basic Infrared Across Track Detector ), can be used to sense trains and activate the sound. The playback module with a photocell circuit provides an onboard trigger device based on light levels. After thoughts have developed an easy modification that creates a timer from the photocell circuit. (Future revisions will include this modification into the PCB) This enables the audio to be played at selected intervals.