Note: This article describes a hearing aid circuit is simple, readily available materials, suitable for beginners to learn how to make the reference, but also can be used for mild hearing loss who wear temporary, it is recommended deaf patients to specialized institutions with their hearing aids fit, so as not to lose the greater ! First, the working principle of hearing aid circuit shown in Figure 1, which is essentially a multi-stage transistor VT1 ~ VT3 audio amplifier composed of crystals. VT1 and the peripheral RC components of a typical resistance-capacitance coupled amplifier circuit, as a pre-amplified audio voltage; VT2, VT3 composed of two direct-coupled power amplifier circuit, wherein: VT3 output form an emitter connected to its output low impedance to match the low-resistance 8Ω earphones. After the electret microphone B receives the acoustic wave signal, the corresponding micro-electrical signal is output. This signal is coupled via a capacitor C1 to the base of VT1 amplifies the amplified signal output by the collector, and then through C2 is coupled to a second stage amplifying VT2, VT3 last signal generated by the emitter follower, and by feeding the jack XS ear to playback machine. Circuit, C4 is a bypass capacitor, whose main role is to bypass the output signal form the various harmonic components of noise to improve the sound quality of the earphones. Smoothing capacitor C3, mainly for reducing the AC resistance of the battery G (actually provide a good path for the current audio machine), which can effectively prevent the battery is almost self-oscillation circuit generates the scrap, and the sound emitted earphones more loud and clear.
Second, component selection VT1, VT2 selected 9014 or 3DG8 silicon NPN low power, low-noise transistor, the current amplification factor required β≥100; VT3 type or the like should be selected small 3AX31 germanium PNP power transistor, the current required to penetrate as little as possible ICeo some, β≥30 can. B selection of CM-18W type (φ10mm × 6.5mm) with high sensitivity electret microphone, its sensitivity is divided into five gears, are represented by color point: red is -66dB, -62dB as small yellow, rhubarb is -58dB, blue -54dB, white> -52dB. This should be used in making a white dot products to obtain a higher sensitivity. B may also be a blue dot, high-sensitivity electret microphone CRZ2-113F directly instead.
XS selection uses CKX2-3.5 (φ3.5mm caliber) earphones, two-core jacks that are commonly used in earphones, and it is necessary to use it to use it. Restructuring method, see FIG. 2, the spring forceps insertion hole is bent slightly downwardly, the inner and outer spring normally closed state into the original state can be a normally open. Good restructuring jack, the headphone plug is inserted claim, inner and outer spring can be reliably turned on, and then pull out the plug can be reliably separated, in order to use the power switch also serves. The headset is a 8Ω low-resistance earbeliner with CSX2-3.5 (φ3.5mm) two-core plug. R1 ~ R5 are made of RTX-1 / 8W carbon film resistors. C1 ~ C3 all use CD11-10V electrolytic capacitors, C4 CT1 type ceramic capacitors. G No. 5 with two batteries in series, voltage of 3V. Third, the production and use of the hearing aid of FIG. 3 is a wiring diagram of the printed circuit board shown in FIG. The actual size of the printed circuit board is approximately 60 mm × 50mm. This printed plate does not have to be corroded, as long as the knife will cut the unwanted copper foil to cut off. Available cell clip size of about 20mm × 8mm rectangular phosphor sheet 4, made of bent "L" shape, each marked with a small hole in the foot, with copper rivets riveted directly formed on the circuit board.
The welded circuit board is equipped with a fine plastic or organic glass cartridge with a size of about 64 mm54 × mm × 18 mm. On the side panels and box, respectively, prior microphone B, XS out jack receiving sound hole and the mounting hole. Hearing aid assembled shape as shown in FIG.
Native debugging is very simple: First, by adjusting the resistance of the resistor R2, so that the collector current VT1 (DCMA table series circuit R3) at about 1.5mA; then, by adjusting the resistance R4, the total quiescent hearing aid current (dc milliammeter supply circuit connected in series to the battery G), to about 10mA. Because everyone electret microphone B parameters used are so different, the value of R1 may also need to make appropriate adjustments, should be adjusted to the most sound loud and clear so far. In use, the hearing aid will generally be placed in the user's shirt pocket, noted by the microphone sound hole B to be outwardly. The wearing earphones, hearing aids and the plug is inserted into jack XS, i.e., automatic circuit power work; pull the plug, i.e. the hearing aid off automatically stopped.
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