Sound indicator anti-sleep. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices
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The low-voltage sound indicator circuit (Fig. 4.12) is designed to improve the safety of driving a car at night. This device prevents the driver from falling asleep while driving. The indicator, together with the battery, is made on a single-sided printed circuit board in the form of a bracket (Fig. 4.13), which allows, by turning on the SA1 microswitch, to fix it behind the ear.
Rice. 4.12. Sound indicator
With a deep tilt of the head (at the moment of falling asleep), the contacts of the tilt sensor F1 will close and the indicator will turn on - a loud signal will instantly wake the driver.
Of course, the reliability of the device will largely depend on the design of the sensor F1. Having tried various designs of the head tilt sensor, I chose the simplest one - it can be easily made without the use of machines. It consists of a spring from a ballpoint pen, an M4x5 brass screw and a contact stop (Fig. 4.14). The screw is inserted into the spring and soldered (using flux or an aspirin tablet). The second end of the spring is shortened and attached to the board.
The indicator is operable when the supply voltage changes in the range from 0,7 to 2 V and consumes no more than 5 mA.
The device circuit is a self-oscillator on transistors of different structures with a direct connection. The use of a piezo emitter allows you to make the indicator small and light. To obtain sufficient sound volume, the L1 coil is connected in parallel with the piezoelectric emitter. Together with the internal capacitance HF1, it forms a resonant circuit. This allows, due to resonant oscillations, to increase the operating voltage on the piezo-radiator, which will significantly exceed the supply voltage.
Rice. 4.13. PCB topology and layout of elements: the HF1 piezo emitter is fixed above the board elements by soldering to the contact pads
Piezo emitters of different types have values of their own sound resonant frequency, which are in the range of 2...8 kHz. Therefore, when changing the type of piezoelectric emitter for each specific case, you can choose the best combination of circuit parameters (to obtain maximum volume with minimum current consumption).
Rice. 4.14. Head tilt sensor design
The frequency of sound can be changed by capacitor C1 or by changing the number of vitkov coil L1, which, of course, is less convenient. Coil L1 contains 600 turns of PEV-0,08 wire (0,1 or 0,12 mm) wound on two rings of size K2x10x6 mm glued together with BF-3 ("Moment") glue made of ferrite 700NM1 (or 1000NN). Microswitch SA1 can be used type PD-9-2. Battery G1 type RC53M or similar. Resistors and capacitors are suitable for any type, it is permissible to replace the KT315G transistors with KT312V, KT3102E, and the KT361V transistor with KT3107.
The loudest sound will be when the frequency of the self-oscillator and the natural resonant frequency of the piezoelectric emitter coincide. The sound indicator can find other applications, for example in children's toys.
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