RE: block diagram of smart hearing aid
||SMART HEARING AID
SMART HEARING.docx (Size: 310.23 KB / Downloads: 18)
This project is very useful to people with difficulty in hearing.
This single instrument can be used by people having hearing problems in all frequencies by simply adjusting its volume nobe manually.
Normally, hearing aid circuits
consume battery power continuously once they are switched on. The circuit given here saves
battery power by switching on the sound amplifier section only when sound is detected.
The sensitivity of the detection section and the ‘on’ time duration of the sound amplifier
circuit can be set by the user. Also the circuit uses only a single condenser mic for
sound detection and amplification.
This hearing aid consists of a condenser microphone, earphone, and a sound
detection and amplification section. The sound detection section consists of an opamp and a timer, which will determine the sensitivity and on time of the amplifier section in the hearing aid. The amplifier section will amplify the signal which is obtained at the microphone.
In general, the function of a preamp is to amplify a low-level signal to line-level. A list of common low-level signal sources would include a pickup, microphone, turntable or other transducer. Equalization and tone control may also be applied.
In a home audio system, the term 'preamplifier' may sometimes be used to describe equipment which merely switches between different line level sources and applies a volume control, so that no actual amplification may be involved. In an audio system, the second amplifier is typically a power amplifier (power amp). The preamplifier provides voltage gain (i.e. from 10 millivolts to 1 volt) but no significant current gain. The power amplifier provides the higher current necessary to drive loudspeakers
A standard op-amp operating in open loop configuration (without negative feedback) can be used as a comparator. When the non-inverting input (V+) is at a higher voltage than the inverting input (V-), the high gain of the op-amp causes it to output the most positive voltage it can. When the non-inverting input (V+) drops below the inverting input (V-), the op-amp outputs the most negative voltage it can.
CIRCUIT DIAGRAM DESCRIPTION
An amplifer is a device which increase or change the amplitude of a signal which is applied to the input.The relationship of the input to the output of an amplifier usually expressed as a function of the input frequency is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain. The essential role of this active element is to magnify an input signal to yield a significantly larger output signal. The amount of magnification is determined by the external circuit design as well as the active device.
The most common type of amplifiers are the bipolar junction transistor amplifiers or the BJT transistor amplifiers.The circuit used in this amplifier section is the BJT amplifer in the common emitter configuration, in this configuration the input is applied to the base with respect to the emitter and the output is taken across the collector with respect to the emitter,in this configuration the emitter is common to both the input and the output so this configuration is reffered as the common emitter configuration.The main advantages of this configuration high voltage and current gain.
This pre amplifier designed to operate with the collector to base bias.in this circuit there is negative feedback effect which tend to stabilize the collector current against any change either as a result of change in temperature or as a result of change in the components. So this circuit will increase the stability of the pre amplifier stage and increase the amplitude of the input signal.
The sensitivity selector is basically a comparator circuit using a op amp ic lm324.The comparator is basically a circuit which compares the two input voltages that are applied to the inverting and non inverting terminals of the op amp .
A standard op-amp operating in open loop configuration (without negative feedback) can be used as a comparator. When the non-inverting input is at a higher voltage than the inverting input , the high gain of the op-amp causes it to output the most positive voltage it can. When the non-inverting input drops below the inverting input , the op-amp outputs the most negative voltage it can.
In this comparator section the reference voltage is provided by the potentiometer and the Vcc. On adjusting the potentiometer we can vary the value of the reference voltage to a desired value there by varying the sensitivity of the hearing aid. The input for the op amp is supplied by the pre amplifier stage .If the value of the input is less than the reference voltage then the output will be positive voltage.
The timer section is one of the most important part of the smart hearing aid, which is responsible for controlling the on time of the amplifier section .The timer section is formed by the 555timer IC in this the timer ic is designed to work as a monostable multivibrator. Monostable multivibrator often called a one shot multivibrator is a pulse generating circuit in which the duration of this pulse is determined by the RC network connected externally to the 555 timer. In a stable or standby state, the output of the circuit is approximately zero or a logic-low level. When external trigger pulse is applied output is forced to go high. The time for which output remains high is determined by the external RC network connected to the timer. At the end of the timing interval, the output automatically reverts back to its logic-low stable state. The output stays low until trigger pulse is again applied. Then the cycle repeats.
An amplifier is a device which increase or change the amplitude of a signal which is applied to the input.The relationship of the input to the output of an amplifier usually expressed as a function of the input frequency is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain. The essential role of this active element is to magnify an input signal to yield a significantly larger output signal. The amount of magnification is determined by the external circuit design as well as the active device.
PCB PREPARATION TECHNIQUES
You need to generate a positive (copper black) UV translucent artwork film. You will never get a good board without good artwork, so it is important to get the best possible quality at this stage. The most important thing is to get a clear sharp image with a very solid opaque black. Nowadays, artwork is drawn using either a dedicated PCB CAD program or a suitable drawing/graphics package. It is absolutely essential that your PCB software prints holes in the middle of pads, which will act as center marks when drilling. It is virtually impossible to accurately hand-drill boards without these holes. If you’re looking to buy PCB software at any cost level and want to do hand-prototyping of boards before production, check that this facility is available. If you’re using a general-purpose CAD or graphics package, define pads as either a grouped object containing a black-filled circle with a smaller concentric white-filled circle on top of it, or as an unfilled circle with a thick black line (i.e. a black ring). When defining pad and line shapes, the minimum size recommended for vias (through-linking holes) for reliable results is 50 mil, assuming 0.8mm drill size; 1 mil = (1/1000)th of an inch. You can go smaller with smaller drill sizes, but through-linking will be harder. 65mil round or square pads for normal components and DIL ICs, with 0.8mm hole, will allow a 12.5 mil, down to 10 mil if you really need to. Centre-to-centre spacing of 12.5mil tracks should be 25 mil—slightly less may be possible if your printer can manage it. Take care to preserve the correct diagonal track-track spacing on mitered corners; grid is 25 mil and track width 12.5 mil. The artwork must be printed such that the printed side is in contact with the PCB surface when exposing, to avoid blurred edges. In practice, this means that if you design the board as seen from the component side, the bottom (solder side) layer should be printed the ‘correct’ way round, and the top side of a double-sided board must be printed mirrored.
Artwork quality is very dependent on both the output device and the media used. It is not necessary to use a transparent artwork medium—as long as it is reasonably translucent to UV, its fine-less translucent materials may need a slightly longer exposure time. Line definition, black opaqueness and toner/ink retention are much more important. Tracing paper has good enough UV translucency and is nearly as good as drafting film for toner retention. It stays flatter under laser-printer heat than polyester or acetate film. Get the thickest you can find as thinner stuff can crickle. It should be rated at least 90 gsm; 120 gsm is even better but harder to find. It is cheap and easily available from office or art suppliers.
Laser printers offer the best all-round solution. These are affordable, fast, and good-quality. The printer used must have at least 600dpi resolution for all but the simplest PCBs, as you will usually be working in multiples of 0.06cm (40 tracks per inch). 600 dpi divides into 40, so you get consistent spacing and line width. It is very important that the printer produces a good solid black with no toner pinholes. If you’re planning to buy a printer for PCB use, do some test prints on tracing paper to check the quality first. If the printer has a density control, set it to the blackest. Even the best laser printers don’t generally cover large areas well, but usually this isn’t a problem as long as fine tracks are solid. When using tracing paper or drafting film, always use manual paper feed and set the straightest possible paper output path to keep the artwork as flat as possible and minimize jamming. For small PCBs, you can usually save paper by cutting the sheet in half. You may need to specify a vertical offset in your PCB software to make it print on the right part of the page. Some laser printers have poor dimensional accuracy, which can cause problems for large PCBs. But as long as any error is linear, it can be compensated by scaling the printout in software. Print accuracy is likely to be a noticeable problem when it causes misalignment of the sides on double-sided PCBs—this can usually be avoided by careful arrangement of the plots on the page to ensure the error is the same on both layers; for example, choosing whether to mirror horizontally or vertically when reversing the top-side artwork.
Do not use sodium hydroxides for developing photo resist laminates. It is a completely and utterly dreadful stuff for developing PCBs. Apart from its causticity, it is very sensitive to both temperature and concentration, and made-up solution doesn’t last long. When it’s too weak it doesn’t develop at all, and when too strong it strips all the resist off. It is almost impossible to get reliable and consistent results, especially when making PCBs in an environment with large temperature variations. A much better developer is a silicate-based product that comes as a liquid concentrate. You can leave the board in it for several times the normal developing time without noticeable degradation. This also means that it is not temperature critical—no risk of stripping at warmer temperatures. Made-up solution also has a very long shelf-life and lasts until it’s used up. You can make the solution up really strong for very fast developing. The recommended mix is 1 part developer to 9 parts water. You can check for correct development by dipping the board in the ferric chloride very briefly—the exposed copper should turn dull pink almost instantly. If any shiny copper-colored areas remain, rinse and develop for a few more seconds. If the board is under-exposed, you will get a thin layer of resist which isn’t removed by the developer. You can remove this by gently wiping with dry paper towel, without damaging the pattern. You can either use a photographic developing tray or a vertical tank for developing.
Ferric chloride etchant is a messy stuff, but easily available and cheaper than most alternatives. It attacks any metal including stainless steel. So when setting up a PCB etching area, use a plastic or ceramic sink, with plastic fittings and screws wherever possible, and seal any metal screws with silicone. Copper water pipes may get splashed or dripped-on, so sleeve or cover them in plastic; heat-shrink sleeving is great if you’re installing new pipes. Fume extraction is not normally required, although a cover over the tank or tray when not in use is a good idea. You should always use the hex hydrate type of ferric chloride, which should be dissolved in warm water until saturation. Adding a teaspoon of table salt helps to make the etchant clearer for easier inspection. Avoid anhydrous ferric chloride. It creates a lot of heat when dissolved. So always add the powder very slowly to water; do not add water to the powder, and use gloves and safety glasses. The solution made from anhydrous ferric chloride doesn’t etch at all, so you need to add a small amount of hydrochloric acid and leave it for a day or two. Always take extreme care to avoid splashing when dissolving either type of ferric chloride, as it tends to clump together and you often get big chunks coming out of the container and splashing into the solution. It can damage eyes and permanently stain clothing. If you’re making PCBs in a professional environment, where time is money, you should get a heated bubble-etch tank. With fresh hot ferric chloride, a PCB will etch in well under five minutes. Fast etching produces better edge-quality and consistent line widths. If you aren’t using a bubble tank, you need to agitate frequently to ensure even etching. Warm the etchant by putting the etching tray inside a larger tray filled with boiling water.
Soldering is the joining together of two metals to give physical bonding and good electrical conductivity. It is used primarily in electrical and electronic circuitry. Solder is a combination of metals, which are solid at normal room temperatures and become liquid at between 180 and 200°C. Solder bonds well to various metals, and extremely well to copper.
Soldering is a necessary skill you need to learn to successfully build electronics circuits. It is the primary way how electronics components are connected to circuit boards, wires and sometimes directly to other components.
To solder you need a soldering iron. A modern basic electrical soldering iron consists of a heating element, a soldering bit (often called the tip), a handle and a power cord. The heating element can be either a resistance wire wound around a ceramic tube, or a thick film resistance element printed onto a ceramic base. The element is then insulated and placed into a metal tube for strength and protection. This is then thermally insulated from the handle. The heating element of soldering iron usually reaches temperatures of around 370 to 400°C (higher than needed to melt the solder). The soldering bit is a specially shaped piece of copper plated with iron and then usually plated with chrome or iron. The tip planting makes it very resistant to aggressive solders and fluxes.