This is cheap hearing aids circuit project. Or a small listen booster is a simple function.

They are similar to the common small audio amplifier.

Why use it?

Imagine your dad is old. Sure, he hears anything weak sound.

How do you do?

You are an electronics inventor, right?

Yes, you can try to make a tool. To help him back to listen to great music again.

This project is small, use only one 1.5V battery. Using normally components you can buy in stores near you.

Let’ s getting to learn.

the working starts with a preamplifier circuit.

We receive audio signals from a microphone. Then, it increases a lower signal to a higher volume up before.

Next, a power amplifier boosts power up again. To drive headphones to we hear that sound.

Is it easy? Yes, you can read more in deep.

Dynamic Microphone Amplifier For Earphone

Look at the circuit below.

It is a simple two-stage transistor amplifier.

All of all, a dynamic microphone gets a sound to convert into a small signal.

Then, the C2-capacitor gets only AC to two 2N3053 NPN transistors have a particularly high gain.

Next, the higher signal comes out of C3 to an earphone output.

But this circuit is not suitable for true using.

So, look at the second circuit below.

Related: Low noise microphone preamplifier circuit

Hearing Aids Circuit Project

The specialty of this preamplifier circuit is an automatic level control circuit or ALC.

Which it will operate at a fixed control volume automatically.

Suppose that the sound is very loud. This circuit will turn the volume down.

On the other hand, if soft sound. It will increase the volume up.

Make us hear all the levels. Both many and the less sound.

And we can adjust the volume as needed. To set the volume to a level that is appropriate for each person.

Read Also:Active microphone preamplifier using LF356

How it works

Look in the circuit below. It is a complete circuit of a hearing aids project.

Here is the step-by-step process:

Microphone preamplifier

Start with the microphone MIC1, condenser microphone. This MIC requires a DC voltage supply.

So, It gets DC current by a battery through resistor R1(10K).

Then, a sound signal from MIC comes to C1- 0.1uF capacitor. It passes only AC to amplify into the base of transistor-Q1, BC547.

Which serves as a first audio preamplifier set.

The output signal at collector via C2 connected to Q2. It acts as the second preamplifier.

This is a very high gain. There are resistors R3, R5 are a load, the R4 is a feedback and bias base to Q2 runs.

Next, The signal via C3 to the coupling signal in two ways:

First way:

Automatic Level Control

To automatic level control circuit. It includes transistor-Q3. It makes a feedback bias current to a base of Q1 to control the volume of the amplifier.
Which this voltage occurs from a DC voltage by rectifier diode D1. This voltage will be filtered to a stable level before.

And a capacitor C4 delays a step of volume control, fast or slow. If small value makes the rhythm control up and down of sound faster. But if the value is high, it will slow the sound control.

Next, send a signal to the base of Q1 through R2

If MIC1 gets a loud voice. There is a many bias current to Q3. It causes a collector voltage of Q3 is lower. And, makes a voltage of C4 is lower, too.

This level volume control voltage will via R2 into the base of the transistor Q1. If the voltage is high (MIC1 is low volume) will have a bias current to the Q1 works more and louder sound.

In contrast, if the level volume control voltage is low (MIC1 is louder).

It would make current input to the transistor decreases. As a result, the audio signal will be reduced accordingly. Thus, the sound level has nearly the same all the time. Whether loud or soft.

So what?

Headphone Amplifier

Then, the signal flows through to a power amplifier circuit to drive headphones to hear.
They include:

Transistor Q4-BC337 increases a sound signal.

Potentiometer VR1 adjusts the volume.

Connect signals through the capacitor C9 to the base of transistor Q4.

And R9 resistor feedback a signal and bias to the transistor works in class A.

Reduce the gain in high-frequency range by capacitor-C7. To reduce noise and maintain the stability of the circuit.

The sound signal from the collector is driven through headphones connected directly with the power supply. The headphone is a load of the amplifier circuit. Use only dynamic headphones with an impedance range from 16 to 32 ohms.

Direct Current pass Load

While turning ON-OFF this circuit. We will hear the sound “thub”. It is normal for the output that uses load pass current to the circuit.

So, before and after use this circuit you should not listen to your ear.

And it is not good for the headphone. But this circuit use 1.5V 27mA or 1.3mW only. It is so low, safe for all parts.

How to makes and adjusting

This circuit is a small size. So, you can build it easily on the Perforated PC board. Sure some this normal PCB look at the image below.

PCB layout for build this circuit

Parts you will need

0.25W Resistors, tolerance: 5%
R1, R4: 470K
R2: 1M
R3: 4.7K
R4, R7: 100K
R5: 3.9K
R6: 1.5K
R8: 100Ω
VR1: 50K (A) Potentiometer

Ceramic Capacitors
C1,C2: 0.1µF 50V
C7: 470pF 50V

Electrolytic Capacitors
C3, C6: 1µF 50V
C4: 10µF 25V
C5: 470µF 16V

Semiconductors:
Q1,Q2,Q3: BC547, 45V 100mA NPN Transistor
Q4: BC337,45V 800mA NPN Transistor

Others
Headphone
B1: 1.5V battery
S1: On-Off switch

Check out these related articles, too:

• Dynamic Microphone Preamplifier circuit
• Dynamic and electret condenser microphone preamplifier
• A Microphone From Normal Speaker

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