For normal operation of the refrigerator, hold the door open as little as possible. So many of today's refrigerators is set to display the open door, which, if the refrigerator is opened more than 10...15 C, begins to beep. In refrigerators made before, as a rule, no such device. About the design of "electronic Butler", which sound intermittent sound reminds you to close the refrigerator described in this article. Diagram of the device an audible indication is given in Fig. 1.
Device works well. When opening the refrigerator door closed button contacts, supplying the alternating voltage of 220 V to the lamp. These same contacts and connects "electronic Butler". It utilizes a transformerless power supply a capacitive ballast C1. Low AC voltage is rectified by diodes VD1 and VD2. The capacitor C2 eliminates pulsations and the Zener diode VD3 protects the electronic components of the circuit from voltage spikes and stabilize the voltage level V. 13 transformerless power source for such devices is better because he is not afraid of short circuits. With short-circuit load current is limited to a relatively small value, so that the danger of spontaneous combustion is minimized. Fire safety for working around the clock without supervision device is the main requirement. Yes, and the dimensions of the power source capacitor with damping is much less than the transformer.
Fig. 1. A schematic diagram of an audible indication (click to enlarge)
A voltage of 13 V is applied to chip DD1 and DD2. As DD1 used CRP. This chip is the dial-tone for telephones that can operate directly on the piezoelectric transducer. Switching voltage IC is within 12,1 13,1... In the voltage off - 7,9 8,9...V. When voltage is applied on DD1 it produces two tones of frequencies F1 and F2 with a switching frequency of F3. Tone frequencies and the switching frequency are set by external elements R3 and NW. Capacitor Sz) sets the value of F3, a resistor R3 (8,2.. .56 ohms) determines the tone of the sound frequency. The change of the values of NW and R3 in a wide range allows to obtain at the output a signal that is close in sound to the siren.The frequency can be calculated by the formulas:
F1=32200/R3; F2=0,72*F1; F3=1000/C3
The value of R3 in kiloomah, NW - almost never. Then F is in Hertz.
The audio signal, which produces DD1, is supplied to the piezoelectric emitter through the diode VD5. Diodes VD4, VD5 and resistor R5 form a logical element I. Thus, the audio signal is transmitted to BF1, provided that at the cathode of the diode VD4 filed a logic 1. Chip DD2 contains two 4-bit binary counter. Setting triggers the counters in the initial (zero) condition occurs when a signal R is a logical 1. The expense comes from the decay of pulses of positive polarity at the input CP at "0" input CN. To increase the division ratio of two counters are connected in series.
At the time of occurrence of the food chain R5-C5 produces a reset pulse which resets the counter to the zero state. A logical "0" from the terminal 12 or 13 DD2 served on VD4, barring the passage of a sound signal piezoceramic emitter BF1. This "0" is fed to the inputs of CN (pins 1 and 9 DD2), allowing the meter counting of pulses coming from the generator at the input of SR (pin 2) DD2.1. As the pulse generator uses a flashing led light HL1. The frequency of flashes HL1 - approximately 2.5 Hz. Therefore, the "1" at pin 12 DD2 appears after 12.5 C, if soldered jumper E1, or vyvoda through 25 C, if set E2. This signal disables the account and permit the passage of a sound signal with DD1 chip on BF1. Thus, the alarm siren will sound using 12.5 or 25 after opening the doors and will sound as long as the fridge will not close.
Resistors for construction can take any appropriate power - MLT, S2-23. Capacitor C1 - K73-17 at a voltage of at least 400V, Sz - K10-17, oxide capacitors K50-35 or imported counterparts. The Zener diode VD3 will fit any for voltage stabilization from 13 to 15 IN - BXZ55C14V, XA, XA. Diodes VD1 and VD2 can be replaced with a single diode Assembly KDSW. Diodes VD4, VD5 - any of the series KD521, KD522, CD, CD. Flashing led light will suit you, for example, L-816DRSRSC-B, L-56BYD, L-795BGD. Analogues of the chip CRP produce different firms, both in Russia and abroad: CRP, CRHP, IL2418N, CA, GL6840A, L3240. In chips CA and IL2418N missing inverted output voltage of audio frequency (pin 6). This conclusion is derived additional signal level adjustment voltage enable. This adjustment in the schema can not be used. If there is an opportunity to reduce the switching voltage of the IC to 8...9 To include a resistor (about 1K) from pin 6 to pin 7.
When you use chips CA or IL2418N and lower voltage switching, it is necessary to reduce the voltage of the entire device from 13 to 9...10 V. For this we need to install the Zener diode VD3, the voltage stabilization 9...10V. Chip counter CIE is replaced by CREE or its imported counterpart CD4520. As a piezoceramic emitter BF1 can be applied ZP-1, ZP-5, ZP-22. Temperature mode of operation of all these emitters - from -30 to +60°C sound pressure level at 1 meter distance - not less than 75 dB, resonant frequency from 1.8 to 3 kHz. External view of the device shown in Fig. 2.
Fig. 2.
The dimensions of the PCB and its fasteners depend on the design of upgraded refrigerator.
For fridge STINOL-110 PCB has dimensions 32x51 mm. As the figure shows, the Board has the edges of the two "antennae". With these "whiskers" the device is installed in the dome light lamp interior lights without additional fasteners, as shown in Fig. 3, and is connected parallel to the lamp of the refrigerator.
Fig. 3.
The device operates in harsh climatic conditions: cold, frost, high humidity, therefore the cost of the device, excluding the piezoelectric transducer is preferably covered with several layers of Tapolca.
Author: A. Pavlov, St. Petersburg, Russia, This email address is being protected from spambots. You must have JavaScript enabled to view it. ; Publication: www.cxem.net