A simple hearing aid circuit is a low power amplifier designed to boost weak sound signals for someone with hearing loss.
This compact amplifier circuit can pick up quiet sound and make them audible through a small earphone.
It is powered by a single 1.5V battery.
Circuit Working:
Parts List:
| Category | Component | Quantity | Details |
|---|---|---|---|
| Resistors | 10k | 2 | 1/4 watt |
| 220k | 1 | 1/4 watt | |
| 330k | 1 | 1/4 watt | |
| 3.3Ω | 1 | 1/4 watt | |
| Capacitors | |||
| Ceramic | 22nF | 1 | |
| Electrolytic | 10µF 25V | 1 | |
| Semiconductors | Transistor BC547 | 2 | |
| Transistor BC557 | 1 | ||
| Other Components | Electret Mic | 1 | |
| Earphone 8Ω | 1 | ||
| ON/OFF Switch | 1 | ||
| Battery 1.5V | 1 |
This circuit is a simple hearing aid circuit which is powered with 1.5V battery the working of this circuit is mentioned below.
The electret microphone picks up sound waves and converts them into a small voltage signal.
Transistor T1 BC547 amplifies this weak signal.
Resistor R3 330k and capacitor C2 10uF set the bias point for T1.
This means they control the amount of current constantly flowing through the transistor which in turn affects how much the transistor amplifies the incoming sound signal.
Capacitor C1 22nF blocks DC voltage from reaching the base of transistor T1.
Resistor R1 10k limits the current flowing into the base of T1.
Transistor T2 BC557 provides further amplification of the signal from T1.
Resistor R4 10k set the bias point for T2.
Transistor T3 BC547 serves as an output amplifier.
It takes the amplified signal from T2 and increases its voltage and current so it can drive a earphone.
Resistor R5 3.3 ohm limits the current flowing into the base of transistor T3.
The 10k resistor and 10uF capacitor together perform two functions in this circuit.
First, they stabilize the bias point for the three transistors, which helps to ensure consistent amplification.
Second, they create a simple automatic gain control AGC circuit.
When a loud sound is detected the voltage across resistor R3 increases.
This increased voltage is then passed to capacitor C2, which causes the voltage on the base of T1 to decrease.
This reduce the gain of the first stage of amplification helping to prevent distortion from loud sound.
Formula:
The circuit design of a preamplifier for a hearing aid that uses an electret microphone and a 1.5V battery must take minimal power consumption and enough gain to amplify the microphones weak signals into account.
Here is a simple summary of the formula to think about:
Gain of Preamplifier Av:
The preamplifier circuits gain controls the amount of microphone signal amplification.
The voltage gain Av for a simple common emitter transistor amplifier may be roughly calculated as follows:
Av = −RC / re
where,
- The transistors intrinsic emitter resistance is represented by re, while the collector resistor is represented by RC.
More exact computations and optimizations based on particular needs like gain, frequency response, and power consumption may be obtained using simulation tools, precise component values and comprehensive circuit designs.
How to Build:
To build a simple hearing aid circuit using 1.5V battery below are the steps mentioned for connections:
- The positive terminal is connected to 1.5V battery.
- The negative terminal of the battery connects to the ground.
- The positive terminal of the electret microphone connects to one end and other end of Mic connects to collector of transistor T3 .
- The base of T1 connects to the two series of resistors R2 and R3 from positive supply to negative supply.
- The collector of T1 is connected to base of T2 transistor and emitter of T1 is connected to ground through capacitor C2
- Resistor R1 is connected from positive supply to one end of electret Mic and ground.
- Transistor T2 base is connected to collector of transistor T1 and emitter is connected to positive supply, and collector is connected to base of transistor T3.
- Resistor R4 is connected between the emitter of transistors T1 and T3.
- Transistor T3 emitter id connected to ground through resistor R5, base of T3 is connected to collector of transistor T2 and collector is connected to positive supply of battery 1.5V through a earphone.
- On/Off switch is connected to positive supply of a 1.5V battery.
Note:
- This is a very basic circuit and may not be suitable for people with severe hearing loss.
- It is important to use components with the correct ratings to avoid damaging the circuit and to properly assemble the circuit on a PCB.
- Safe practices such as wearing appropriate ESD protection should be followed while building this circuit.
Conclusion:
A simple hearing aid circuit using 1.5V battery offers a low power way to amplify sounds.
While it can be a learning experience to build these circuits have limitations.
They may not work for all hearing loss needs and offer less functionality compared to professional hearing aid.
Consulting a hearing professional is best for proper evaluation and a hearing aid tailored to your specific needs.