Wireless home bell. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Wireless home bell. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Calls and audio simulators

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I propose a simple device for an electronic call that transmits a signal over a radio channel.

Wireless home bell

The receiving node for such a transmitter is any FM radio tuned to 87,9 MHz. The transmitter frequency can be easily tuned over the entire bandwidth by changing the ratio of the capacitances of capacitors C4 and C6 to a small extent, as well as slightly stretching and compressing the turns of the inductor L1. This must be done carefully, since even a slight change in the distance between the turns of the coil L1 (as well as the capacitance of the capacitors C4 and C6 relative to that indicated in the diagram) leads to a change in the frequency of the transmitter signal. Therefore, when setting up the device, it is easier to tune the FM receiver frequency to the transmitter signal than vice versa.

The generator on the popular KR1006VI1 chip automatically starts when the supply voltage is applied. The pulse frequency of the generator depends on the capacitance of the capacitor C1 and the resistance of the resistor R3. With the elements indicated in the diagram, the alternation frequency of the output pulses will be approximately 400 Hz. It can be slightly adjusted with a trimming resistor R1. The operation of the pulse generator can be checked without the presence of additional devices by connecting any telephone capsule with a winding resistance of at least 1 ohms to the output of the DA3 chip (pin 50) and the common wire. It will make a sound.

It is important that the elements of the device are mounted by printed wiring and their leads have a minimum length. The transmitter power is small and is only 0,01 watts. This is quite enough to transmit a radio signal over a distance of up to 50 m.

The most optimal operation of the transmitter is carried out at a node supply voltage of +5 V. The power supply must be autonomous or stabilized with a step-down output transformer. Current consumption 28...30mA. With an increase in the supply voltage, the operating mode of the bias of the transistor VT1 changes, which negatively affects the output amplitude and power of the transmitter, and hence the distance of reliable signal reception. Therefore, the operating point of the bias of the transistor VT1 can be corrected by changing the resistance of the resistor R5.

Since the output signal from pin 3 of the DA1 chip is very large in amplitude, a regulator is introduced into the circuit. The variable resistor R4 (type SPO-1) serves as a regulator of the output signal level of the low-frequency generator assembled on the DA1 chip. When establishing a node, it is necessary to set the level of the generator output signal so that the signal received by the portable receiver can be heard without distortion such as "step" or "echo". Isolating capacitor C3 (type KM-6B) does not pass the DC component of the voltage from the generator to the input of the transistor transmitter. An antenna is connected to capacitor C5, which is a piece of stranded unshielded mounting wire with a cross section of 1,2 mm and a length of 20 cm. This antenna wire can be laid around the assembly elements in a spiral, inside an unshielded case.

The elements of the device are installed compactly in an unshielded plastic case, which is located, if possible, away from the mains wiring.

Details. The SA1 button is a regular call button installed at the entrance to the apartment. All fixed resistors type MLT-0,125. Capacitors type KM-6B. Transistor VT1 can be replaced by KT368A, KT355A, KT355B. Coil L1 contains 5 turns of PEV-1 wire with a diameter of 0,5 mm, wound in a frameless way or on a wooden frame with a diameter of 5 mm. For the frame, a simple wooden pencil (with the graphic rod removed) or an old-style spool of thread is suitable.

The disadvantage of this scheme is the single-tone nature of the signal. You can supplement the device circuit with an intermittent signal generator or use a microassembly of the UMS08 type as a sound generator with a recording of a musical fragment of a melody.

Author: A.P. Kashkarov

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