Recently, someone close to me needed a hearing aid. In many countries, such hearing aids are provided free of charge to people with hearing loss, but free cheese...
Here is one of the free devices, these can be bought in China for 2-3 dollars.
There are good devices, but their price is clearly more than 2-3 dollars.
The factory device was not satisfactory because it had low sensitivity and unstable gain, and it was also battery powered, which meant extra costs for purchasing batteries.
Having opened the Chinese device, it became clear why it has low sensitivity. As you can see, the circuit is primitive, built on a pair of transistors, although the sound quality is not bad.
As a result, I made several similar devices.One of these devices is quite bulky, but has very good sensitivity and auto gain control.The final stage is built on the MC34119 chip, which can deliver up to 250 milliwatts of power to the load, so literally any headphones can be connected to such a device.This device was equipped with a fairly capacious lithium-ion battery; one charge is enough for several days of operation. Unfortunately, I didn’t save any diagrams or photographs.
The second device is no less good; it is built entirely on transistors.
A pair of pre-amplification stages, then an automatic volume adjustment stage and a final stage in the collector circuit of which a high-impedance earphone is connected.
How does automatic volume control work? At the initial stage, the voltage from capacitor C5, which is charged by resistor R8, is supplied to the base of the transistor of the first stage in order to provide maximum gain.
As the input signal increases, for example during a loud conversation near the device, the signal at the output of the second preliminary stage also increases, and as soon as the voltage reaches the unlocking value of the silicon transistors, which is 0.6 - 0.7 Volts, the transistor is triggered through its open transition and resistor R6, capacitance C5 will be discharged, this lowers the voltage at the base of the first transistor and reduces the sensitivity of the device, the gain gradually increases after a few seconds, as capacitor C5 charges.
This function should be present in any normal hearing aid; it provides stable sound in the earphone, regardless of the volume of the conversation.
The disadvantage of this device is that it is critical to headphones; here it is needed with high impedance. Headphones from smartphone headsets have an impedance of 20 to 35 ohms, but I had to work hard to find a headphone with a coil impedance of 50 ohms, but with such a headphone the sound is simply perfect, although the device can work with lower-impedance headphones.
There are many advantages: high sensitivity, loud noise suppression function, low-voltage power supply and low current consumption.
Such a device can be powered by just one battery, and if you manage to find a small-sized nickel metal hydride battery, then your device may well compete with its Chinese counterpart in terms of ergonomics.
I used a small-sized lithium-ion battery from a Bluetooth headset. Another feature of the circuit is the wide range of input voltages. In the range from 0.8 to 4 volts the device works perfectly.
The printed circuit board of this option was laid out for DIP components, the board turned out to be quite compact.
Later I decided to try with SMD mounting
One of my friends shared his problem with me - he began to hear poorly and began to notice that when watching TV, speech was unintelligible, he had to increase the volume, which created inconvenience for others. Previously, his service took place at the airfield, engaged technical maintenance jet aircraft, did not pay attention to hearing protection in his youth. As a result, hearing loss is 40%, especially in such cases, the perception of high sound frequencies of the speech spectrum from 1000 Hz and above is lost. Industrial hearing aids are very expensive, and I decided to help him - I assembled a hearing aid with my own hands from simple and affordable parts. A diagram of the assembled device is shown below.A hearing aid is a simple sound amplification device consisting of a microphone, an input amplifier, a final amplifier and a telephone. Input amplifier assembled on two transistors T1 and T2 according to a circuit with direct connections between cascades and covered by a common negative feedback By DC, in order to stabilize the gain and improve the amplitude-frequency characteristics. Setting the modes of transistors T1 and T2 is carried out using resistors R3 and R6. It is important to use the low-noise transistor P28 in the first stage of the amplifier. In addition, the operating mode of this transistor (Ik = 0.4 mA, Uke = 1.2 V) also ensures minimal noise. The amplifier provides uniform signal amplification in the frequency band of the conversational spectrum 300...7000 Hz. From the collector of transistor T2, the signal goes to potentiometer R7, which acts as a gain regulator. Instead of the P28 transistor, you can use: MP39B, GT310B, GT322A, silicon KT104B, KT203B, KT326B, but low-noise transistors of the KT342, KT3102 and KT3107 series give especially good results. The final stage is assembled on transistor T3 according to an amplifier circuit with a floating operating point, which makes it possible to sharply reduce the current consumed by the stage in silent mode.
This hearing aid amplifier circuit is characterized by an effective shift of the cascade operating point and, accordingly, small nonlinear distortions. When a signal is applied to the input from resistor R7 through capacitor C6, the signal is sent to the base of transistor T3. The signal amplified by the transistor from the collector T3 through capacitor C8 is sent to a doubler rectifier on diodes D1 and D2. The rectified voltage accumulates on capacitor C7 and is applied to the base of transistor T3, shifting its operating point towards opening.
Resistor R8 sets the initial cascade current. The hearing aid is powered by a voltage of 9 volts from the Krona element. LED D3 serves to indicate power on. Any miniature dynamic or condenser microphone can be used as a microphone. If you are using a condenser microphone, you must supply power to it through a 3–5 kOhm resistor. You can use TM-3, TM-4 as a telephone. A suitable plastic case was selected for the hearing aid, which houses the printed circuit board and power supply. When setting up, you must first set the currents of all transistors. resistors R4 and R6 current T1 and T2, then resistor R8 with the microphone turned off, set the quiescent current of Transistor T3 to 2-2.5 mA. A signal with a frequency of 1000 Hz and an amplitude corresponding to the maximum amplitude of the signal at the collector of transistor T3 is supplied to the base of transistor T3 from the generator. Use resistor R9 to achieve undistorted signal amplification. In this case, the collector current of the transistor should have a value of 15-17 mA. Select the capacitance of capacitor C3 according to the best sound, the absence of harsh sounds. Author: Shimko Sergey.
Answer to the question:
Hearing aids, like any other electronic devices, may break. Some faults cannot be repaired, while others can be easily resolved on your own.
In the table we tried to list some of the most common problems that may arise during the operation of the SA. And also their decision.
| What's happened | What could have caused this to happen? | How to fix |
| The hearing aid does not turn on | Perhaps the reason is that the battery is weak, or it is completely discharged | Replace the dead battery with a new one |
| The battery may be installed incorrectly in the device (reverse polarity) | Check that the battery is connected correctly and reinsert it | |
| SA makes a humming or buzzing sound | The battery is almost empty | Try replacing the nutrient with a fresh one. |
| The battery box is not closed correctly, the lid does not fit tightly | Close it, but you shouldn’t be too zealous in this process. To avoid damaging anything | |
| The hearing aid produces a weak signal or is completely silent | Perhaps the CA is not enabled | Power on your device |
| Hearing aid volume setting is very low | Turn up the volume | |
| It is also possible that your in-ear device (or, if it is external, then the earmold) is not inserted correctly. It does not fit snugly into the ear canal | Reinsert the CA | |
| The cause may also be clogged earwax. | The CA needs to be cleaned. And then it is advisable to show it to your audiologist for proper adjustment | |
| The hearing aid whistles | Unpleasant whistling is possible if the volume is set too high | Turn down the volume on your CA using the dedicated control |
| This is also possible if there is a large accumulation of ear wax in your ears, or if the device is contaminated with wax | Clean your hearing device as well as your ears. Remember the hygiene and operating rules of the SA. If you cannot clean the device yourself, an audiologist will help you with this. | |
| CA transmits intermittent or distorted sounds | Hearing aid volume is too loud | Turn or press the volume control on the CA downwards |
| Check, maybe the reason is a low battery charge | Replace the nutrient element with a new one |
Yu. A. Shtan, V. Yu. Shtan, Berdyansk
functionally it consists of a highly sensitive electret microphone and a low-noise low-frequency amplifier (LFA), loaded onto headphones (see figure).
The hearing aid amplifier must have a gain of more than 10,000 times the voltage, increase the frequency response in the range of 300-300 Hz and provide sufficient output power. Low-voltage power supply (2-3 V) forces you to carefully consider the selection of power modes based on the direct current of the transistors, the quality of the transistors themselves and other parts. Despite the reduced power supply, the problem of dealing with amplifier excitations both in terms of sound and high frequency remains.
Details and design. The case under the Chinese VHF micro-receiver contains headphones, a socket for connecting them, a volume control with a switch, and a power-on LED.
During development printed circuit board it is necessary to place these parts so that they coincide with the holes in the body of the former receiver. Naturally, this design option for a hearing aid is not the only one.
Details. Small-sized electret microphone
MKE-ZZ2; transistors KT3102D, E with gain 500-800, KT31 5b, G, E with gain 100-150; resistors type MLT-0.125; capacitors various types, the main requirement for them is to be as small as possible. Headphones are small-sized headphones made in China. Power supply is from galvanic cells. The current consumed by the hearing aid is almost 2 times less than that of VHF microreceivers.
The setup consists of selecting resistor R1 within the specified limits for the maximum sensitivity of the device. The maximum current consumption with fresh batteries is 9-10 mA. Evidence of a properly tuned ULF is that it remains operational at a supply voltage of 1.5 V, although the gain is significantly reduced compared to power supply from two elements.
This hearing aid has a lower noise level than hearing aids produced in the Soviet Union in the 80s; Its sensitivity and output sound pressure level are higher than those of behind-the-ear hearing aids or glasses placed in the earpiece.
The circuit of a hearing aid can be considered as basic. Although the design takes some measures to narrow the frequency band, its sound is much more natural and pleasant than that of industrial hearing aids.
parathas. However, further narrowing of the ULF frequency band may be necessary when designing devices for people with severe hearing loss.
To reduce current consumption, a “floating point” mode, etc. can be introduced into the final stage of the ULF. Literature
1. Amateur Radio Handbook/Ed. G.M. Tereshchuk, K.M. Tereshchuk, S.A. Sedo-va.-K.: Vishcha school, 1981.
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A simple hearing aid with automatic gain control in a 3D printed housing
Since the case is not miniature, you can use a more capacious (compared to miniature models) Li-Ion battery, which allows the device to work autonomously for more long time. Another advantage compared to miniature Chinese models is the independence from the type of headphone. Since the earphone here is the external part, you can choose the earphone individually without changing the hearing aid itself, since the performance of the entire device greatly depends on the characteristics of the earphone itself. It is advisable to select headphones depending on the user’s hearing characteristics. Of course, if you purchase an expensive hearing aid in a specialized store, a qualified worker will make audiograms and adjust the device to the patient’s hearing characteristics, but if you purchase cheap models in Chinese online stores, this cannot be done.
The body of the device has a volume control, a headphone jack, a microphone electret capsule and a jack for connecting a charger. The hearing aid is charged using a five-volt cell phone charger. The case is printed on a 3D printer. You can download the 3D models from the link at the end of the article along with the printed circuit board and other files for this project. To charge the Li-Ion battery, a miniature cheap controller board, ordered from Aliexpress, is used.
!!! Do not try to charge a Li-Ion battery directly from a voltage source without a controller board! This is dangerous for the battery and may cause it to catch fire!
Schematic diagram of a homemade hearing aid with automatic gain control. Click on the diagram to enlarge it
An electret microphone (not shown in the diagram) is connected to the mic and GND_mic contacts. The positive terminal of the capsule must be connected to the mic contact, and its second, negative terminal must be connected to the GND_mic contact. Typically, the capsule has this pin connected to its body. Used electret condenser capsule type WM-61A from China from Aliexpress:
The supply voltage is supplied to the capsule through resistor R1. Next, through capacitor C2, the signal is supplied to the first amplification stage, made on transistors Q1 and Q2. The automatic gain control unit is assembled on transistor Q3 and field-effect transistor Q4. Field effect transistor controls the gain of the first stage, shunting alternating current resistor R13 in the collector circuit of transistor Q2, reducing the stage gain as the input signal level increases. Thus, a relatively constant signal level is maintained at the output of the cascade when the input level changes over a wide range. Here, diode D3 rectifies the alternating voltage of the audio signal, converting it into a pulsating voltage, which is amplified by transistor Q3 and then smoothed by electrolytic capacitor C7.
From the output of the pre-amp stage, the signal is fed to the volume potentiometer. The potentiometer is not shown in the main diagram. How to connect it to the board contacts is shown in the small diagram below. A 10 kOhm potentiometer is used.
Connection diagram of the volume control potentiometer to the board contacts
From the volume potentiometer slider, the signal is supplied to the final amplifier assembled on the MC34119 () chip. The chip is an audio power amplifier capable of operating from a very low supply voltage, starting from 2 volts and is ideal for operation in our hearing aid powered by a Li-Ion battery. In fact, the chip contains two output power amplification stages that operate in antiphase to implement bridge mode. The load is connected between two outputs final amplifiers chip, and not between the output and ground, as in most other integrated amplifiers. That is, we connect the headphones to pins 5 and 8 of the microcircuit. None of the headphone pins should be connected to ground; you should pay attention to this if you are using a headphone jack in which one of the pins is connected to a metal case. such a socket must be isolated from the common wire of the device.
Powering the device.
As a power source, you can use any small Li-Ion battery with a voltage of 3.7 V, suitable in size. I made a case for a “pinky” Li-Ion battery, I was too lazy to go buy one, and in the end I used a small battery from a children’s toy - a helicopter.
To charge the battery, I used this board, ordered from Aliexpress. 10 pieces of such boards cost around 150 rubles, I ordered a lot of 10 pieces, the boards are useful in amateur radio and cheap.
Since the controller board has a standard micro USB connector, you can use a regular USB connector to charge the hearing aid. charger from any cell phone.
Diagram of connecting batteries to the hearing aid board
PCB created in the DipTrace program. You will find the PCB drawings in the archive with the project files.