Improvement of the HPA24AX sound emitter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur
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Not so long ago, relatively inexpensive miniature piezoceramic sound emitters with a built-in generator appeared on sale, which were used as tone sound annunciators. Connecting to a power source with just two wires, maintaining performance at a voltage of less than a volt, consuming a current of only a few milliamps at a rated voltage, these devices emit a very loud, piercing sound with a frequency of 1000 ... 3500 Hz.
At the first time of operation of such emitters, it seemed that they were perfection itself and that there was no need to modify anything in them. But the moment came when one of the emitters, namely HPA24AX (case diameter 24 mm, thickness 9 mm), was opened.
As expected, it was assembled according to an extremely simple oscillator circuit (see figure), consisting of several SMD elements: resistors R1-R3, transistor VT1 and the piezoelectric resonator itself - the sound emitter ZQ1. The numbering of elements is conditional. A similar scheme has long been used, for example, in the calling nodes of the simplest wired telephones.
After connecting to the emitter in accordance with the polarity of the 10 V DC voltage source indicated on its case, the operating modes of the transistor VT1 were determined using an oscilloscope. It turned out that the amplitude of the voltage amplitude of the emitter-collector does not exceed 8 V, and the emitter-base - +0.6 ... -3.5 V.
The installation of a silicon diode transistor VD1 between the base and the emitter made it possible to reduce the voltage amplitude at the base to 1,3 V and simultaneously increase the emitter-collector voltage amplitude to 10 V. The volume of the emitter increased markedly, and the stability of the pulse amplitude also increased.
If you have such or a similar emitter, you can open it with a thin screwdriver, with which you can carefully squeeze out the printed circuit board through a special hole in the side wall. If the board from the side of installation into the case is filled with epoxy glue, then experiments and refinement should be refrained from.
Diode - any of the series KD102, KD103, KD512 or other small-sized. If there is enough free space in the case, you can install the missing blocking ceramic capacitor with a capacity of 0,047 ... 0,47 μF inside it, for which contact pads are provided on the printed circuit board of the HPA24AX. The parts installed on the board after its placement in the case should not come into contact with the piezoelectric element.
Author: A.Butov, village of Kurba, Yaroslavl region
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