Homemade hearing aid. Circuitry of hearing aids

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, he was engaged in the maintenance of jet aircraft, and in his youth he did not pay attention to hearing protection. 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. The input amplifier is assembled on two transistors T1 and T2 according to a circuit with direct connections between the stages and is covered by a common negative DC feedback 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 signal amplitude 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.

Hearing aid This device is intended for people with poor hearing (congenital or acquired with age).
In essence, this is simply an ordinary microphone amplifier in a miniature design, intended for everyday use.

Hearing aids are generally available for sale and their range is quite large, but you can make your own hearing aid- it will be much cheaper.

Now on sale there are electret microphones (with a built-in field-effect transistor amplifier) ​​from cell phones or modern telephones. Such microphones have a smooth frequency response and high sensitivity, and headphones from a player or phone can be used as headphones.

Diagram of a homemade hearing aid

As the basis for the circuit, I chose an amplifier for eavesdropping (“spy equipment”). Having simplified it a little, I got a completely working diagram of a hearing aid (Fig. 1), which fits into a standard case with dimensions of 128x66x28 mm.

Resistor R1 sets the sensitivity of the microphone VM1 of the hearing aid. Capacitors SZ and C4 form the frequency response in the high-frequency region (prevent self-excitation in ultrasound and prevent overloading of the amplifier at higher audio frequencies). Capacitor C5 forms the frequency response at low frequencies (removes the “mumbling” of the microphone). Resistor R8 sets the operating point of the output stage: the voltage at the emitters VT4 and VT5 should be half the supply voltage.
The GB1 battery status indicator is assembled on transistor VT6. Resistor R12 sets the ignition voltage of LED VD2 at 4 V, which corresponds to the minimum permissible battery voltage. A green LED with a diameter of 2 mm of increased light output from the “Piranha” series is used as VD2. The battery consists of four cells with a capacity of 500... 1000 mAh. LED VD3 indicates charging (goes off after it is completed). Red AL307 is used as VD3. Zener diodes VD4 and VD5 are selected to limit the voltage (with the charging unit connected) at 7.3. ..7.4 V. A simple plastic stereo jack is used as the output connector X1 for installation on the board. The right and left channels in it are paralleled on the printed circuit board, as this improves the output of the headphones. Since such sockets do not last long, I recommend installing two of them in parallel. This will allow you not to waste time on repairing (replacing) one socket - you just need to insert the earphone into another socket.
The shape, arrangement of parts on the board and drawing of the printed circuit board are shown in Fig. 2-4. The VM1 microphone is installed in a soft rubber clip and is fixed inside the case with silicone adhesive-sealant.

The battery charging unit is made from a universal power supply ("Chinese") for electronic equipment (Fig. 5). It uses the third (bottom) tap of the secondary winding of the transformer for operation. The open circuit voltage at the output is about 9.7 V, the charging current at the specified R1 rating is approximately 50 mA. One battery charge is enough for 3...5 days of hearing aid operation. The device allows simultaneous operation and charging.

The sound pressure created by this hearing aid (I did not have the appropriate measuring instruments) is so high that it creates pain in a person with normal hearing and subsequent temporary (several minutes) deafness. My father, who has profound hearing loss, received almost complete hearing compensation with good intelligibility with this hearing aid.
When repeating the design, special attention should be paid to the headphones. Some of them are not capable of creating a sufficiently high sound pressure either due to high ohmic resistance or due to low efficiency (read quality). On-ear headphones with a headband and soft ear pads for Hi-Fi equipment can give a good effect. However, the use of such headphones is only possible if the ear pads fit well.
It is useful to install a latch on the front wall of the hearing aid housing for attaching to the breast pocket flap. It makes sense for experienced radio amateurs to work on reducing the size of the hearing aid by switching to microcircuits and miniature batteries.

V.ZAKHARENKO. UA4HRV, Samara.

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 first stage transistor 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 from 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

Many people today experience hearing problems and the scale of this disaster is impressive. In addition to the elderly, many younger generations will also face hearing loss in the future, fueled by the rampant use of headphones and youth's love of discos.

Consequently, the question of how to make a hearing aid with your own hands will always be relevant, because the cost of such branded hearing aids is often beyond the means of many people.

In fact, making a hearing aid with your own hands is quite simple, for this purpose, available tools are used that anyone can easily find.

The resulting hearing aid is compact in size and can easily fit into an ordinary Bluetooth headset.

To get started, you will need a microphone - a regular microphone from a mobile phone will do. If this is not the case, then you can successfully use a microphone from a tape recorder. The tape recorder is quite ordinary, Chinese - the main thing is that the microphone has greater sensitivity.

Now let's move on to considering the circuit of the hearing aid. As you can see, the scheme is quite simple.

You should also use an earphone from your mobile phone as a speaker. The earphone must have quite high resistance, about twenty-five to forty ohms.

To power the device, a lithium tablet is used (voltage three volts). If you couldn’t find a lithium tablet, you can use three batteries from a regular wristwatch. The battery connection configuration is in series, and the total voltage should be 4.5 volts. When assembling, special attention should be paid to the microphone and its polarity - the microphone must be connected in the correct way.

If you have the desire and opportunity, you can use a lithium-ion battery from a Bluetooth headset instead of the option discussed above. With a capacity of 80-120 milliamps and a voltage of 3.7 volts, the lithium-ion battery will allow the hearing aid to work longer, and it can be charged. The following types of transistors can be used for the device: S9014 and S9018, as well as transistors KT315 and KT368.

Let's go further in studying the question of how to make a hearing aid with your own hands. To reduce the size of your device, you should use SMD components. In order to increase the sensitivity of the hearing aid, you can replace the non-polar capacitor according to the microphone used by 0.01 microfarad.

Assembling the hearing aid.

When assembling a hearing aid, you need to ensure high-quality isolation of the microphone from the speaker - otherwise, background will form during use.

Another version of the device has two cascades that enhance the operation of the microphone. Since the microphone tablet itself has a built-in amplifier (single-stage), the result is a hearing aid with increased sensitivity, about 9-10 meters. You simply need add a simple amplifier, operating on a single transistor (similar to the amplifier used in the previous stage).

The first type of hearing aid has a current of 5 milliamps per hour, the second - about 10 milliamps/hour.

Such a hearing aid will work continuously and you won't need to turn it off, so there's no need for a switch.

Similar factory-produced devices are quite expensive, while the option discussed in this article will be inexpensive and will not be inferior in quality to factory samples.

This circumstance is especially important for pensioners and people with low incomes who cannot afford to purchase a branded hearing aid. You can please your grandparents, or help a colleague or friend overcome hearing problems. It is enough just to take all the necessary elements listed above and independently construct a device that can help people fully perceive the world around them and enjoy sounds and communication with loved ones.

As you can see, a DIY hearing aid quite easy to do, there is nothing complicated about it, but the benefits are quite tangible.