General circuit of Hearing Aid available over internet used normal transistorize circuit thus it consumes more power. So, here we are concentrated to design a circuit which consumes less power. For that the circuit present here of DIY Hearing Aid Using 555 have automatic triggering mode which consumes 96 mA at the time of amplifier trigger or you can say when sound is detected else it consumes only 6 mA which is very less. Other than less power consumption the circuit have advantage like user can select sensitivity as well as amplifier ‘on’ time duration.
Circuit Description of DIY Hearing Aid using 555
The circuit of DIY Hearing Aid using 555 is shown in figure 1. The components used in DIY Hearing Aid is most common and easily available in market. A quad op-amp IC (LM324) which has four individual op-amp circuit integrated in it, out of four we had only used two op-amp (one as comparator whereas other as unity follower). A timer IC NE555 is configured in monostable mode with timer constant component R4 (resistor) and C2 (capacitor). The value we have selected for these components is for 4.9 or we can say 5 seconds on timer.
A microphone is used to detect a sound and convert it into electrical signal which need further amplification. For amplification, we had used pre-amplifier as well as amplifier circuit. Pre-amplifier circuit is built around transistor T1. When sound is detected by microphone it converts it into electrical signal and fed to base of transistor T1. The amplifier output is available at collector which is fed to inverting input (pin 2) of IC1:A which is compared with reference voltage available at non-inverting input (pin 3) of IC1:A (configured in comparator mode). When the pulsating voltage at collector is more in compare to reference voltage at pin 3 the output at pin 1 of IC1:A become low. This low output trigger timer IC2 (NE555). The output from pin 3 is high which provide power source for amplifier section.
One output from microphone is also fed to non-inverting input of IC1:B which is configured in unity follower mode as its inverting pin is connected to output (pin 7). This is done in order to overcome the problem of impedance mismatch. The output from pin 7 is given to base of transistor T2 which amplify the weak signal and fed to next stage of amplifier (T3. T4 and T5 respectively). A PNP transistor (T5) is used in last stage which drive the earphone.
BEP NOTE: The earphone used in the circuit DIY Hearing Aid using 555 must be mono type earphone with voice control facility.
PARTS LIST OF DIY HEARING AID CIRCUIT USING 555
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1, R5, R11 = 2.2 KΩ
R2, R7 = 3.3 KΩ
R3, R6 = 680 KΩ
R4 = 49 KΩ
R8 = 220 KΩ
R9 = 1.5 KΩ
R10 = 100 KΩ
VR1 = 1 MΩ PRESET
|C1, C4 = 0.1 µF (Ceramic Disc)
C2 = 0.01 µF (Ceramic Disc)
C3 = 100 µF/25V (Electrolytic Capacitor)
C5 = 1 µF/25V (Electrolytic Capacitor)
C6 = 470 µF/25V (Electrolytic Capacitor)
|IC1 = LM324 (Quad op-amp)
IC2 = NE555 (Timer IC)
T1, T2 = BC549
T3, T4 = BC548
T5 = BC558
|M1 = Condenser Microphone
SW1 = ON/OFF Switch