This cheap hearing aids project or the small listen booster is a simple function similar to the function of the common audio amplifier.
The work starts with a preamplifier circuits, the light and amplifier. To be the stronger. To receive audio signals from a. microphone to be larger to increase the volume up, Before it is sent to a power amplifier to power up again, To drive headphones to we hear.
The specialty of this preamplifier circuit at The Automatic level control circuit-ALC. Which will operates at a fixed control volume automatically. This part of the circuit, the sound has been very loud, the circuit is to reduce the volume down. On the other hand, if soft the circuit is increased the volume up. Make us hear all the levels. Both lot and the less sound. Adjust the volume as needed. To set the volume to a level that is appropriate for each person.
How this circuit works
-In the Figure 1 is a complete circuit of the cheap hearing aids project. The works of circuit will starts with the microphone MIC1, that is a condenser microphone, This MIC will be need has a voltage supply, It get a current with a battery pass through the resistor R1 – 10K.
-A sound signal from MIC will be connected through capacitor C1 value 0.1 uF, to amplify into the base pin(?B) of transistor Q1 number BC547. Which serves as a pre-amplifier audio amplifier in the first set.
-The output signal at the pin collector-C is connected through a capacitor C2 connected to the transistor Q2 acts as the second preamplifier. The circuit setting this manner a very high gain, by has the resistors R3,R5 are a load, the R4 is a feedback and bias base to the transistor Q2 works.
-The sound signal that passed the preamplifier, will be connected via the capacitor C3, and to the control volume level that acts by the transistor Q3, The Mission of the transistor Q3, the building is a voltage bias to the base of transistor Q1 to control the amplifier. By voltage that occurs from the introduction of the sound signal. from the output of the preamplifier circuit go to the rectifier to make the DC voltage by have the diode D1 work together.
-The voltage that is filtered to the stable level before moving up, by the capacitor C4 is to delay the rhythm in volume control to fast or slow. If the driver makes a slow pace before the base of transistor Q1 pass way the resistor R2.
-The MIC has been loud, will has a bias signal the transistor Q3 a lot. As a result, the voltage at collector pin-C of Q3 will decreased. As a result, the voltage across the capacitor C4 is very close from the power supply (due to resistor R7 is connected) is pulled to decrease, too.
-This control volume level voltage will send pass through the resistor R2 into the base of the transistor Q1. If the voltage is high (Low MIC) It would make have current input to the transistor Q1 makes the sound louder. And If the level volume control voltage have low level (MIC loud). It would make current input to the transistor decreases. As a result, the audio signal will be reduced accordingly. As a result, the noise level have nearly the same all the time. Whether loud or soft.
-The sound signal through the control volume. Is sent to the power amplifier circuit to drive headphones to hear. Served by the transistor Q4 number BC337 has the potentiometer VR1 to adjust the volume. Connection signal through the capacitor C9 serves the signal feed back and bias to the transistor works in class A. The capacitor C7 will reduce the growth rate in the high frequency range. To reduce noise and maintain the stability of circuit.
The sound signal from the collector pin is driven through headphones connected directly with the power supply. The headphones that will act as a DC voltage to the load of the amplifier circuit. It must be dynamic headphones with impedance of the power supply as well. No more, no less, in the range from 16 to 32 ohms. Which will be the typical impedance of headphones already.A crystal earphones or a pizo ceramic headphones. Can not be used in this project. Due to the value high resistance power. Will not load current circuit to operate.
The headphone is used, can be used both headphones Mono and stereo. With the jumper JP1 is an option that will used with stereo or mono headphones. Because this project uses a stereo jack socket. The internal structure of the jack may have caused a short circuit at the jack male mono. This jumper must be set aside to float circuit. If you intend to use with any stereo headset only. It can be Connection short jumper was not permanent.
How to makes and adjustable
In figure 2 is PCB layout for build this circuit.
The circuit can will “throb” sound when turned on. Is typical of the output are used to current the self through headphones. Applications should be turned on before you wear headphones. And first off was to remove the ear is removed. The current flowing through the load. (Headphones or speakers) all time is not good. In the case of high-watt power amplifier. Because it can cause have current to flow through lot headphones. In the earphones and the wire burn. But in this case, the power supply is 1.5V. The circuit use low current consumption 26-27mA (Use stereo headphones Impedance 16 ohm.) The headphone jack is not less than 1mA current during the power flow headphones have a 7-8mA per watt is equivalent to approximately 1-1.3mW per side, which is still in the safe range. It is not fair.