Refrigerator door open alarm 10-15 seconds. Encyclopedia of radio electronics and electrical engineering

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For normal operation of the refrigerator, you should keep its doors open as little as possible. Therefore, in many modern refrigerators, an open door indication device is installed, which, if the refrigerator is open for more than 10 ... 15 seconds, starts beeping. In refrigerators made earlier, as a rule, there is no such device. The design of the "electronic butler", which with an intermittent sound reminds you to close the refrigerator, is described in this article. The diagram of the sound indication device is shown in fig. 1.

The device works like this. When the refrigerator door is opened, the contacts of the button are closed, which supplies an alternating voltage of 220 V to the lighting lamp. The "electronic butler" is connected to the same contacts. It uses a transformerless power supply with capacitive ballast C1. The reduced alternating voltage is rectified by diodes VD1 and VD2. Capacitor C2 eliminates ripples, and the VD3 zener diode protects the electronic components of the circuit from power surges, and also stabilizes the voltage at 13 V. A transformerless power supply for such devices is preferable, since it is not afraid of short circuits. During short circuits, the current in the load is limited to a relatively small value, so that the risk of spontaneous combustion is minimized. Fire safety for a device operating around the clock without supervision is the most important requirement. And the dimensions of the power supply with a quenching capacitor are much smaller than those of a transformer.

Rice. 1. Schematic diagram of sound indication (click to enlarge)

A supply voltage of 13 V is supplied to the DD1 and DD2 microcircuits. KR1PP1064 is used as DD1. This is a dial tone chip for telephones, which can work directly on a piezoceramic emitter. The turn-on voltage of the IC is in the range of 12,1 ... 13,1 V, the turn-off voltage is 7,9 ... 8,9 V. When voltage is applied to DD1, it generates two tonal frequencies - F1 and F2 - with a switching frequency of F3 . Tone frequencies and switching frequency are set by external elements - R3 and C3. Capacitor C3 sets the value of F3, resistor R3 (8,2.. .56 kOhm) determines the tone of the audio frequency. Changing the values ​​​​of C3 and R3 over a wide range allows you to get an output signal that is close in sound to a siren.

Frequencies can be calculated using the formulas:

F1=32200/R3; F2=0,72*F1; F3=1000/C3

The value of R3 is in kiloohms, C3 is in nanofarads. Then F is in hertz.

The sound signal generated by DD1 is fed to the piezoelectric emitter through the diode VD5. Diodes VD4, VD5 and resistor R5 form a logic element AND. Thus, the audio signal is transmitted to BF1, provided that a logical 4 is applied to the cathode of the diode VD1. The DD2 chip contains two 4-bit binary counters. The counter triggers are set to the initial (zero) state when a logical 1 is applied to the R input. The counting occurs by the decay of the positive polarity pulses at the CP input at "0" at the CN input. To increase the division factor, two counters are connected in series.

At the moment of power supply, a reset pulse is generated by the R5-C5 chain, which resets the counter to zero. Logic "0" from pin 12 or 13 DD2 is fed to VD4, prohibiting the passage of an audio signal to the piezoceramic emitter BF1. The same "0" is applied to the CN inputs (pins 1 and 9 of DD2), allowing the counter to count the pulses coming from the generator to the input of the SR (pin 2) of DD2.1. A blinking LED HL1 is used as a pulse generator. HL1 flashing frequency is approximately 2,5 Hz. Therefore, "1" at pin 12 of DD2 appears after 12,5 s if jumper E1 is soldered, or at pin 13 after 25 s if E2 is installed. This signal prohibits counting and allows the passage of an audio signal from the DD1 chip to BF1. Thus, the siren will sound 12,5 or 25 seconds after the door is opened and will sound until the refrigerator is closed.

Resistors for the design, you can take any of the appropriate power - MLT, C2-23. Capacitor C1 - K73-17 for a voltage of at least 400 V, C3 - K10-17, oxide capacitors - K50-35 or imported analogues. The VD3 zener diode is suitable for any stabilization voltage from 13 to 15 V - BXZ55C14V, KS215A, KS515A. Diodes VD1 and VD2 can be replaced with one diode assembly KDS111V. Diodes VD4, VD5 - any of the KD521, KD522, KD102, KD103 series. A blinking LED will fit any, for example, L-816DRSRSC-B, L-56BYD, L-795BGD. Analogues of the KR1064PP1 chip are produced by different companies, both in Russia and abroad: KR1085PP1, KR1091GP1, IL2418N, KA2418, GL6840A, L3240. The KA2418 and IL2418N microcircuits do not have an inverted audio frequency voltage output (pin 6). An additional signal for adjusting the turn-on voltage level is output to this pin. This adjustment in the circuit can not be used. If it is available, it is possible to reduce the turn-on voltage of the IC to 8 ... 9 V by turning on a resistor (about 1 kOhm) from pin 6 to pin 7.

When using KA2418 or IL2418N microcircuits and reducing the turn-on voltage, it is necessary to reduce the supply voltage of the entire device from 13 to 9 ... 10 V. To do this, install a VD3 zener diode, designed for a stabilization voltage of 9 ... 10V. The counter chip K561IE10 is replaced by KR1561IE10 or its imported analogue CD4520. ZP-1, ZP-1, ZP-5 can be used as a piezoceramic radiator BF22. The operating temperature of all these emitters is from -30 to +60°C, the sound pressure level at a distance of 1 m is at least 75 dB, the resonant frequency is from 1,8 to 3 kHz. The appearance of the device is shown in fig. 2.

Fig. 2.

The dimensions of the printed circuit board and its fasteners depend on the design of the upgraded refrigerator.

For the STINOL-110 refrigerator, the printed circuit board has dimensions of 32x51 mm. As can be seen from the figure, the board has two "antennae" along the edges. With the help of these "whiskers" the device is installed in the interior lamp shade without additional fasteners, as shown in fig. 3, and connected in parallel with the refrigerator lighting lamp.

Fig. 3.

The device operates in difficult climatic conditions: cold, frost, high humidity, so the board of the device, excluding the piezoelectric emitter, should preferably be covered with several layers of zaponlak.

Author: A. Pavlov, St. Petersburg, pavlov@lmail.loniis.ru; Publication: cxem.net

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