A Low Cost Hearing Aid

A Low Cost Hearing Aid

Circuit Description

This low-cost, general-purpose electronic hearing aid works off 3V DC
(2×1.5V battery). The circuit can be easily assembled on a veroboard.
For easy assembling and maintenance, use an 8-pin DIP IC socket for
TDA2822M.

Circuit Diagram:

Circuit

Parts:

  • P1 = 10K
  • R1 = 2.2K
  • R2 = 330K
  • R3 = 680R
  • R4 = 33R
  • R5 = 100R
  • R6 = 4.7R
  • R7 = 4.7R
  • R8 = 220R
  • C1 = 0.01uF-10V
  • C2 = 100nF-63V
  • C3 = 47uF-10V
  • C4 = 10uF-10V
  • C5 = 0.01uF-10V
  • C6 = 100uF-10V
  • C7 = 100nF-63V
  • C8 = 100nF-63V
  • D1 = Red LED
  • Q1 = BC547
  • IC1 = TDA2822M
  • EP1 = Mono Earphone 32R
  • SW1 = On-Off Switch

Circuit Operation:

In this circuit, transistor Q1 and associated components form the
audio signal preamplifier for the acoustic signals picked up by the
condenser microphone and converted into corresponding electrical
signals. Resistor R5 and capacitor C3 decouple the power supply of the
preamplifier stage. Resistor R1 biases the internal circuit of the
low-voltage condenser microphone for proper working. The audio output
from the preamplifier stage is fed to the input of the medium-power
amplifier circuit via capacitor C2 and volume control P1.

The medium-power amplifier section is wired around popular audio
amplifier IC TDA2822M (not TDA2822). This IC, specially designed for
portable low-power applications, is readily available in 8-pin mini DIP
package. Here the IC is wired in bridge configuration to drive the
32-ohm general-purpose monophonic earphone. Red LED (D1) indicates the
power status. Resistor R8 limits the operating current of D1. The audio
output of this circuit is 10 to 15mW and the quiescent current drain is
below 1 mA.

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