International Journal of Scientific & Engineering Research, Volume 4, Issue 9, September-2013 1999

ISSN 2229-5518

DESIGN AND CONSTRUCTION OF A LOW-COST HEARING AID

Domtau, D.L.1, Chagok, N.M.D.2, Akparanta, P.H.3, Fom, T.P.4, Mado, S.D.5

1, 2, 3, 4&5Department of Physics, University of Jos, Nigeria

*E-mail of the corresponding author: nchagok@yahoo.com

Abstract

A low-cost hearing aid device was designed and constructed to produce an amplified sound for people with hearing loss. A 9V dc was used as the power supply. The condenser microphone was used as input transducer to pick up sound from the environment for conversion to electrical signal, NPN transistor (BC548C) along with three capacitors and five resistors were used as pre-amplifier.to amplify. The integrated circuit (IC) TDA2822M, available in 8-pin mini chip package and specially designed for portable power amplification was used for the amplification function. A 32 ohms ear phone was used as the output transducer to convert the amplified electrical signals back to sound. The computed current

gain was 100. Rc was obtained as 3.3Ω, 𝐼𝐵 = 0.025𝐴, 𝐼𝐸 = 2.5𝐴, 𝑅𝐸 = 1.5Ω, the impendence 𝑍0 = 1.9Ω.

Index Terms: Design, construction, hearing aid, hearing loss, integrated circuit, low-cost, npn transistor.

Introduction

Hearing is one of the five senses along with vision, taste, smell and touch. The ear serves as a receiver of incoming sounds. Although the ear receives sound and is the sense organ of hearing, it is the brain and central nervous system that interpret the sound. The human auditory system can generally hear sounds within the frequency range of 20 Hz and 20 kHz but the frequency range between
100Hz and 6 kHz contains most of the information
of human voice (Kinsler et al., 1982). Although the sensation of hearing requires an intact and functioning auditory portion of the central nervous system as well as working ear, human deafness (extreme insensitivity to sound) and hearing loss
most commonly occur because of damages of the
ear, rather than the central auditory system.
Hearing loss is usually reserved for people who have relative insensitivity to sound in the speech frequency range. A hearing loss can happen when any part of the auditory system is not working in the usual way (Oyler, et al., 1988). The constructed device can improve the quality of sound for people with hearing loss. The heart of the circuit is an integrated circuit which amplifies all the signals picked by the condenser microphone and converts them back into sound and sent to the ear. It is a battery-powered electronic device that can amplify sound for people with hearing loss. The amplification function of the hearing aid is made possible with several components; a microphone (input transducer) which picks up sound and
converts it into electrical signals, an amplifier

International Journal of Scientific & Engineering Research, Volume 4, Issue 9, September-2013 2000

ISSN 2229-5518

which increases the volume or the sound, a
receiver (output transducer) which changes the electrical signal back into sound and sends it to the ear (Mills, 2011; Holstrum, et al., 2008).

Materials and Methods

The computation involved in arriving at the component values used in the implementation of the device is based on the equations governing the behavior of electronic circuit devices. The hearing device is made up of the power supply (DC9V), input transducer (microphone), pre-amplifier, amplifier and the output transducer (earphone). The system is presented in figure 1.
Fig 1: Block diagram of a hearing aid device system
Electronic circuits generally require direct current (dc) to power components. The battery supplies the power to turn the electronic components on
and off. The 9 volt battery was used for the power
supply. The condenser microphone was used in
this work. The primary function of a pre-amplifier (fig. 2) is to pick up signals from its primary source (microphone) and then operate on it in preparation for passage into the amplifier section for further amplification (Akande et al., 2007). Typically, a pre-amplifier amplifies the signal, controls its volume, and perhaps changes its input impedance (Horowitz and Hill, 2010; Lowenberg, 1976). The pre-amplification unit used in this work was designed using an NPN transistor BC547 along with some capacitors and resistors. In this circuit, transistor Q 1 and its associated components form the audio signal pre-amplifier for the signals picked up by the condenser microphone and the audio signal is converted into corresponding electrical signals. Resistor R1 biases the internal circuit of the low-voltage condenser microphone for proper working. The audio output from the pre-amplifier stage is fed to the input of the amplifier circuit via capacitor C3 and volume control VR1 .

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ISSN 2229-5518

ℎ𝑓𝑒=𝐼𝐸 = 2.5
= 100

𝐼𝐵

2.5𝑥10−2

𝑍0 =

𝑅𝐸 𝑥𝑅𝑓 =

𝑅𝐸 +𝑅𝑓

3.3𝑥4.7

3.3+4.7

= 1.94Ω
𝛽 = ℎ𝑓𝑒 = 𝐴𝑖 = 100

Fig 3: Circuit diagram of pre-amplification stage

Fig 2: Circuit Diagram of the Pre-Amplifier

Data

𝑉𝑐𝑐 = 9𝑉
𝛽 = 100
𝑅𝑓 = 4.7Ω
𝑅𝑐 = 3.3Ω
𝑉𝐵𝐸 = 0.7 (𝑡𝑎𝑛𝑠𝑖𝑠𝑡𝑜𝑟 𝑓𝑖𝑥𝑒𝑑 𝑣𝑎𝑙𝑢𝑒)
Where 𝑅𝑐 𝑖𝑠 𝑡ℎ𝑒 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑜𝑟 𝑟𝑒𝑠𝑖𝑠𝑡𝑜𝑟
𝑅𝐸 𝑖𝑠 𝑡ℎ𝑒 𝑒𝑚𝑖𝑡𝑡𝑒𝑟 𝑟𝑒𝑠𝑖𝑠𝑡𝑜𝑟
𝐼𝐵 𝑖𝑠 𝑡ℎ𝑒 𝑏𝑎𝑠𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡
𝑅𝑓 𝑖𝑠 𝑡ℎ𝑒 𝑓𝑒𝑒𝑑𝑏𝑎𝑐𝑘 𝑟𝑒𝑠𝑖𝑠𝑡𝑜r
𝛽 𝑖𝑠 𝑡ℎ𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑔𝑎𝑖𝑛
𝐴𝑖 = ℎ𝑓𝑒 𝑖𝑠 𝑡ℎ𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑔𝑎𝑖𝑛

The amplification function is carried out by the
integrated circuit (IC) TDA2822M. This IC, specially designed for portable low power applications, is available in 8-pin mini DIP package. Here, the IC was wired to drive the 32 ohm earphone. In this design, TDA2822M which
operates with only a single power supply voltage
𝑉𝑐𝑐 − 𝑉𝐵𝐸
9 − 0.7 −2
has two differential inputs and two outputs. The

𝐼𝐵 =

𝑓

+ 𝛽𝑅𝑐

= = 2.5𝑥10 𝐴
4.7 + 100(3.3)
parameters of the TDA2822M are:
The emitter current, 𝐼𝐸 = 𝛽𝐼𝐵 = 100𝑥2.5𝑥10−2 =
2.5𝐴
VCC MAX=15V
VCC TYP=6V VCC RNG=1, 8...15V
𝑉𝑐𝑐 9
𝑉𝐸 =

− 𝑉𝐵𝐸 = − 0.7 = 3.8𝑉
2 2
𝑅𝐸 =

𝑉𝐸 =

𝐼𝐸

3.8

2.5

= 1.5Ω
PO=2X0, 65W
RL=4 Ohm
𝑅𝑐 =

𝑉𝑐𝑐−𝑉𝐵𝐸 =

𝐼𝐸

9−0.7

2.5

= 3.3Ω
TDH=10%

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ISSN 2229-5518

(iii) Soldering the components on the Vero
1CCO TYP=6mA
ICCO MAX=9mA I MAX=1A
RIN=100K GAIN=39 dB Noise=1,5mk/V RTHJ-C=70C/V Case MINIDIP
The output transducer changes the enhanced signals back into sound waves that the brain can properly perceive. Here electrical signals are converted into acoustical output signals or sound waves and are directed into the ear canal. The
32ohm earphone was used as the output transducer. The design and construction of this device involved:
(i) Designing and choice of components bearing in mind the availability and cost of components.
(ii) Arranging the components on the bread
board.
board.
(iv) Designing and cutting of the plastic that houses the components. The plastic material that houses the device was chosen according to its mechanical strength, thermal conductivity and ease of installation. A thin plastic sheet was cast into a box case and an outlet from the circuit serves as port for interaction with the user and the device. The case houses the various components such as power switch, LED (light emitting diode) which shows when the power is on or off, the microphone and the ear jack.

Result and Discussion

The results obtained from the various test performed show that the system units and the system as a whole work effectively and efficiently as amplified sound was produced by the output transducer (earphone). However, a noisy output is produced when the hearing aid is close to an electrical wave signal and when close to noisy environment. This is because the diaphragm of the

International Journal of Scientific & Engineering Research, Volume 4, Issue 9, September-2013 2003

ISSN 2229-5518

microphone is very sensitive to noise and to
electrical waves.

Conclusion and Recommendation

A hearing aid capable of amplifying sound was designed and constructed using a 9V DC power supply, a condenser microphone, a transistor (BC548C), an IC TDA2822M, a 320hm earphone together with some passive components (resistors and capacitors).
The device was realized with economic considerations so that it is cost-effective, could be made readily available and with easy maintenance. It is recommended that this hearing aid could be subjected to some modifications by using filters to
enhance efficiency and better quality.

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REFRENCES

[1] Akande, S.F., Kwaha, B.J., and Alao, S.O. (2007). Fundamentals in Electronics. Jos: Jos University Press Ltd, 191p

[2] Holstrum, W. J., Gaffney, M., Gravel, J. and Oyler, R. (2008). Early Intervention for children with unilateral and mild bilateral degrees of hearing loss. Trends in Amplification, 12: 35- 41.

[3] Horowitz, P. and Hill, W. (2010). The art of electronics second edn. New York: Cambridge University Press,

1125p.

[4] Kinsler, L.E. Frey, A.R., Coppers, A.B.

and Sanders, J.V. (1982).

Fundamentals of Acoustics 3rd edn. New

York: John Wiley and Sons, 480p. [5] Lowenberg, E.C. (1976). Schaum’s

Outline of Theory and problems of Electronic Circuits. New York: Mcgraw- Hill Book Company, 274p.

[6] Mills, M. (2011). "Hearing Aids and the

History of Electronics Miniaturization." IEEE Annals of the History of Computing 33(2): 24-44.

[7] Oyler, R., Oyler, A., and Matkin, N. (1988). Unilateral hearing loss: Demographics and educational impact. American Speech-Language-Hearing

Association. 19, 201-210.