Low Cost Hearing Aid

1.    INTRODUCTION

            As we all know, commercially available hearing aids are costly and only rich class can afford. Here is an inexpensive hearing aid device that uses just four transistors and a few passive components. It is very compact and no complicated procedures are involved in the working of the circuit. As a matter of fact this is an application of what we gained from our engineering knowledge.

This is a self designed product which is highly affordable and compact. As per today’s hearing aids are of high cost and their prices vary with its sensitivity and compatibility.

            According to the recent studies conducted by WHO, In INDIA 4 out of 10 people suffer from hearing problems regardless of their age. Nowadays almost everyone uses headsets for entertainment; gradually it affects the ear and causes hearing problems.

In such situation an effective hearing aid becomes inevitable, but one of the major disadvantages of today’s hearing aids is that, they are highly expensive and very much unaffordable to common people, economically speaking ‘as the price increases the demand decreases’. Hence people go for a ‘Low-cost hearing aid’.

            One of the major differences between hearing aids available in the market to our hearing aid is that, in our design we can plug it with a normal headset and use accordingly with volume control in which others don’t recognize the person wearing a hearing aid unlike other hearing gadgets in which it cannot be accessed with headsets and the gadget itself specifies the user as one with hearing problems.

            It is quite similar to a portable music device that can be operated with all headphones and music headsets.

2.    BLOCK DIAGRAM

3.    BLOCK DIAGRAM DESCRIPTION

Condenser Microphones

            Condenser means capacitor, an electronic component which stores energy in the form of an electrostatic field. The term condenser is actually obsolete but has stuck as the name for this type of microphone, which uses a capacitor to convert acoustical energy into electrical energy.

How Condenser Microphones Work

            A capacitor has two plates with a voltage between them. In the condenser mic, one of these plates is made of very light material and acts as the diaphragm. The diaphragm vibrates when struck by sound waves, changing the distance between the two plates and therefore changing the capacitance. Specifically, when the plates are closer together, capacitance increases and a charge current occurs. When the plates are further apart, capacitance decreases and a discharge current occurs.

            A voltage is required across the capacitor for this to work. This voltage is supplied either by a battery in the mic or by external  power.

Cross-Section of a Typical Condenser Microphone

AMPLIFIER SECTION

            An amplifier is a basic building block of most electronic systems just as one brick does not make a house ,a single stage amplifier is not sufficient to build a practical electronic system ,although the gain of the amplifier does depend on device parameters and circuit components. There exist an upper theoretical limit for the gain obtainable from one stage. The gain of a single stage is not sufficient for practical application.

The voltage level of a signal can be raised if you are using more than one stage. When a number of amplifier stages are used in succession (one after the other), it is called a multi-stage amplifier or a cascaded amplifier. Much higher gains can be obtained from the multi-stage amplifiers.

The overall gain of the multi-stage amplifier

A=A1×A2×A3……×An

Where ‘n’ is the number of amplifiers

HEADPHONE SECTION

            Headphones are a pair of  small loudspeakers or less commonly a single speaker, with a way of holding them closer to a user’s ears and a means of connecting them to a single source such as an audio amplifier, radio or CD player. They are also known as stereo phones headsets or colloquially cans. The in-ear versions are known as earphones or ear buds. In the context of telecommunication the term headset is used to describe the combination of headphone and microphone used for two way communication, for example with a telephone.

4.    CIRCUIT DIAGRAM

5. LIST OF COMPONENTS

No.	COMPONENT	SPECIFICATION	QUANTITY
1	Transistor	BC 549C,BC 548,BC 548, BC 558	4
2	Condenser Mic	STD.	1
3	Stereo Socket	STD.	1
4	Headset	STD.	1
5	Switch	STD.	1
6	Capacitor	0.1μ,0.1μ,1μ/10V, 100μ/10V,100μ/10V	5
7	Resistor	2.2K,680K,3.3K,220K, 1.5K,100K,220,2.2K,	8

6. CIRCUIT DESCRIPTION

            On moving power switch S to ‘on’ position, the condenser microphone detects the sound signal, which is amplified by transistors T1 and T2. Now the amplified signal passes through coupling capacitor C3 to the base of transistor T3. The signal is further amplified by PnP transistor T4 to drive a low impedance earphone. Capacitors C4 and C5 are the power supply decoupling capacitors.

            The circuit can be easily assembled on a small, general-purpose PCB. It operates off a 3V DC supply. For this, we need to use two small 1.5V cells. Keep switch S to ‘off’ state when the circuit is not in use.

Transistor

            A bipolar (junction) transistor (BJT) is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes. Charge flow in a BJT is due to bidirectional diffusion of charge carriers across a junction between two regions of different charge concentrations.

            There are basically three possible ways to connect a Bipolar Transistor within an electronic circuit with each method of connection responding differently to its input signal as the static characteristics of the transistor vary with each circuit arrangement.

1. Common Base Configuration   -   has Voltage Gain but no Current Gain. 

2. Common Emitter Configuration   -   has both Current and Voltage Gain. 

3. Common Collector Configuration   -   has Current Gain but no Voltage Gain.

 

THE COMMON EMITTER CONFIGURATION.

            In the Common Emitter or Grounded Emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the "normal" method of connection. The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward-biased junction, while the output impedance is HIGH as it is taken from a reverse-biased junction.

THE COMMON EMITTER AMPLIFIER CIRCUIT

            In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib. Also, as the load resistance (RL) is connected in series with the collector, the Current gain of the Common Emitter Transistor Amplifier is quite large as it is the ratio of Ic/Ib and is given the symbol of Beta, (β). Since the relationship between these three currents is determined by the transistor itself, any small change in the base current will result in a large change in the collector current. Then, small changes in base current will thus control the current in the Emitter/Collector circuit.

            By combining the expressions for both α and β the mathematical relationship between these parameters and therefore the current gain of the amplifier can be given as:

Where: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into the base terminal and "Ie" is the current flowing out of the emitter terminal.

            Then to summarise, this type of bipolar transistor configuration has a greater input impedance, Current and Power gain than that of the common Base configuration but its Voltage gain is much lower. The common emitter is an inverting amplifier circuit resulting in the output signal being 180^o out of phase with the input voltage signal.

            Most common circuit configuration for a transistor is that of the Common Emitter Amplifier and that a family of curves known commonly as the Output Characteristics Curves, relates the Collector current (Ic), to the output or Collector voltage (Vce), for different values of Base current (Ib) signal. All types of signal amplifiers operate using AC signal inputs which alternate between a positive value and a negative value so some way of presetting the amplifier circuit to operate between these two maximum or peak values is required. This is achieved using a process known as Biasing. Biasing is very important in amplifier design as it establishes the correct operating point of the amplifier ready to receive signals, thereby reducing any distortion to the input signal.

BIPOLAR TRANSISTOR BIASING

            Bipolar transistor amplifiers must be properly biased to operate correctly. In circuits made with individual devices (discrete circuits), biasing networks consisting of resistors are commonly employed.

           

            The operating point of a device, also known as bias point, quiescent point, or Q-point, is the point on the output characteristics that shows the DC collector–emitter voltage (V_ce) and the collector current (I_c) with no input signal applied. The term is normally used in connection with devices such as transistors.

SIGNAL REQUIREMENTS FOR CLASS A AMPLIFIERS

            For analog circuit operation, the Q-point is placed so the transistor stays in active mode (does not shift to operation in the saturation region or cut-off region) when input is applied. Often, Q-point is established near the center of active region of transistor characteristic to allow similar signal swings in positive and negative directions. Q-point should be stable. In particular, it should be insensitive to variations in transistor parameters (for example, should not shift if transistor is replaced by another of the same type), variations in temperature, variations in power supply voltage and so forth. The circuit must be practical: easily implemented and cost-effective.

FIXED BIAS (BASE BIAS)

Fixed bias (Base bias)

           

           

            This form of biasing is also called base bias. In the example image on the right, the single power source (for example, a battery) is used for both collector and base of transistor, although separate batteries can also be used.

            One application of fixed bias is to achieve crude automatic gain control in the transistor by feeding the base resistor from a DC signal derived from the AC output of a later stage. It is simple to shift the operating point anywhere in the active region by merely changing the base resistor (R_B).Main advantage is that a very small number of components are required.

 In the given circuit,

V_CC = I_BR_B + V_be

Therefore,

I_B = (V_CC - V_be)/R_B

            For a given transistor, V_be does not vary significantly during use. As V_CC is of fixed value, on selection of R_B, the base current I_B is fixed. Therefore this type is called fixed bias type of circuit.

Also for given circuit,

V_CC = I_CR_C + V_ce

Therefore,

V_ce = V_CC - I_CR_C

            The common-emitter current gain of a transistor is an important parameter in circuit design, and is specified on the data sheet for a particular transistor. It is denoted as β on this page.

Because

I_C = βI_B

            we can obtain I_C as well. In this manner, operating point given as (V_CE,I_C) can be set for given transistor.

 

COLLECTOR-TO-BASE BIAS

Collector-to-base bias

            This configuration employs negative feedback to prevent thermal runaway and stabilize the operating point. In this form of biasing, the base resistor R_B is connected to the collector instead of connecting it to the DC source V_CC. So any thermal runaway will induce a voltage drop across the R_C resistor that will throttle the transistor's base current. Circuit stabilizes the operating point against variations in temperature and β (ie. replacement of transistor)

From Kirchhoff's voltage law, the voltage across the base resistor R_b is

By the Ebers–Moll model, I_c = βI_b, and so

From Ohm's law, the base current , and so

Hence, the base current I_b is

            If V_be is held constant and temperature increases, then the collector current I_c increases. However, a larger I_c causes the voltage drop across resistor R_c to increase, which in turn reduces the voltage across the base resistor R_b. A lower base-resistor voltage drop reduces the base current I_b, which results in less collector current I_c. Because an increase in collector current with temperature is opposed, the operating point is kept stable.

 Usage: The feedback also decreases the input impedance of the amplifier as seen from the base, which can be advantageous. Due to the gain reduction from feedback, this biasing form is used only when the trade-off for stability is warranted.

SZIKLAI PAIR

            In electronics, the Sziklai pair (also known as a "compound transistor") is a configuration of two bipolar transistors, similar to a Darlington pair. In contrast to the Darlington arrangement, the Sziklai pair has one NPN and one PNP transistor, and so it is sometimes called the "complementary Darlington". Current gain is similar to that of a Darlington pair, which is the product of the gains of the two transistors. The configuration is named for its inventor George C. Sziklai.

           

            One advantage over the Darlington pair is that the base turn-on voltage is only about 0.6V or half of the Darlington's 1.2V nominal turn-on voltage. Like the Darlington, it can saturate only to 0.6V, which is a drawback for high-power stages.

            As with a Darlington pair, a resistor (e.g., 100Ω–1kΩ) is usually connected between Q2 emitter and base to improve its turn-off time (i.e., its performance for high frequency signals)

SZIKLAI-BASED OUTPUT STAGES

            Sziklai pairs are sometimes used in the push–pull output stage of power amplifiers (e.g., for audio) when the designer wants to use devices of the same type (e.g., NPN), instead of complementary types, which rarely match accurately. That is, rather than using a Darlington NPN push pair (i.e., two NPN transistors) and a Darlington PNP pull pair (i.e., two PNP transistors), the designer uses Sziklai pairs for both the upper push pair and the lower pull pair. By using two Darlington pairs, the designer is hoping that the combination of two NPN transistors will have similar characteristics of two PNP transistors. By using two Sziklai pairs which both have mixed NPN–PNP type, the designer hopes the push–pull matching will be better.

7. CIRCUIT DESIGN

SELECTION OF RESISTOR R1

Condenser mic current=0.500 mA

Internal impidence of cond. Mic=2.2K

Select R1=2.2K

DESIGN OF AMPLIFIER SECTION T1

As per data sheet BC 549C has gain 420-800 at Ic = 2mA

Take gain=300

Vcc=3V,Vce=50%Vcc=1.5V

Select Q-POINT at Ic=400μA, Vce=1.5V

Vcc=Vce+(Ic+Ib)R3

So R3=(Vcc-Vce)/(Ic+Ib)

Ib=400μ/300=1.3μA

So  R3=(3-1.5)/(400μ+1.4μ)

R3=3.5K,select 3.3K std

Vcc=Ic R3+Ib R2+ Vbe

R2=(Vcc-Vbe-Ic R3)/Ib

=(3-0.6-400μ×3.3K)/1.3μ

=720K, use 680k std

Select C1=0.1μF

DESIGN OF AMPLIFIER SECTION T2

As per data sheet BC548 has gain 110-800 at Ic=2mA

Take gain=100

Vcc=3V,Vce=50%Vcc=1.5V

Select Q-POINT at Ic=1mA,Vce=1.5V

Vcc=IcR5+Vce

R5=(Vcc-Vce)/Ic

R5=(3-1.5)/1m

R5=1.5K std

Ib=1m/100=10μ

Vcc=IbR4+Vbe

R4=(Vcc-Vbe)/Ib

R4=(3-0.6)/10μ

R4=240K, use 220K std

Select C2=0.1μF

DESIGN OF AMPLIFIER SECTION T3

As per data sheet BC548 has gain 110-800 at Ic=2mA

Take gain=50

Vcc=3V,Vce=50%Vcc=1.5V

Select Q-POINT at Ic=0.7mA, Vce=1.5V

Vcc=IcR8+Vce

R8=(Vcc-Vce)/Ic

=(3-1.5)/0.5mA

=2.5K, use 2.2K std

Ib=0.7m/50=14μ

Vcc=IbR7+Vce

R7=(Vcc-Vbe)/Ib

=(3-0.6)/14μ

=108K, use 100K std

Select coupling capacitors=100μF,10V

8. PRINTED CIRCUIT BOARD

            In a printed circuit board, as the name indicates, the circuit is printed on a board cladded with copper. The main need for PCB is to avoid the complexity and unwanted wires in the circuit. It makes the circuit compact. The following steps are followed in order to design the PCB.

(1)               The circuit layout of the circuit for printing the circuit on the board is designed. The circuit layout should be as compact as possible.

(2)               The layout of the circuit is drawn on the copper clad board using a permanent marker or pointer. We must bear in mind about the width of the track.

(3)               This board is dipped in a solution of ferric chloride of required concentration and is stirred continuously. The concentration should not be more than a particular limit since it may remove the coating.

(4)               The copper which exposed to solution reacts to form copper chloride.

(5)               The coating is then removed and drilled in proper points needed with required size bits.

(6)               The components are then soldered in proper positions. Try to use flux for best results.

(7)               Varnish the soldered face. This is to prevent the circuit from reacting with atmospheric air, water vapour and other chemicals suspended in the air.

9. P.C.B LAYOUT

10. ADVANTAGES

·         Inexpensive and less complex

·         Can be used with music headsets, headphones etc

·         Easy to fabricate and comparatively less complex

11. DISADVANTAGES

·         Being an analog circuit noise effect is bound to happen

·         Can only be operated with standard headsets

12. CONCLUSION

            This project deals with the design and fabrication of the hearing aid circuit, which can be easily used with a headset. It is inexpensive and is very easy to construct. Its maintenance cost is very cheap. No special environmental conditions are required for its operation. By connecting a standard headset one can use the device. It is very accurate, simple and inexpensive.

13. FUTURESCOPE

            One in seven of the population have hearing loss and this is likely to become one of the biggest health and social issues of our times, as noise pollution soars and live longer, millions of people who could benefit from wearing a hearing aid or hearing protection are reluctant to do so. With the rising of Bluetooth technology we can make wireless hearing aids that can look like jewellery or ear studs.

The European market for hearing aids is worth €2.9 billion, yet with less than 30% of penetration is remarkably low, and literally millions of people who could benefit are missing out. So the need for low cost hearing aid is inevitable and the future of this device looks bright.