Refrigerator open door alarm. Encyclopedia of radio electronics and electrical engineering

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The proposed signaling device will beep if the door of the household refrigerator is left open for more than a few minutes.

The device is connected to the power supply circuit of an incandescent lamp illuminating the refrigerating chamber with the door open, and is placed behind the protective cover of the lamp in the chamber itself or, for example, in the upper instrument compartment of the refrigerator. As a result of connecting the signaling device, the effective value of the voltage supplied to the lamp decreases by only 2 V, which does not affect its brightness in any way.

The scheme of the device is shown in fig. one.

(click to enlarge)

The SF1 switch and the EL1 lamp are structural elements of the refrigerator. Its closed door keeps the switch contacts open. When the door is open, the contacts are closed and current flows in the EL1 lamp circuit. The signaling device is powered by the voltage falling across the resistor R1. It is limited by the zener diode VD1 and rectified by the diode VD2.

Immediately after opening the door, the capacitor C2 is discharged, so the transistors VT1 and VT2 remain closed. This capacitor is being charged through the resistor R3. If the door is not closed for too long, the growing voltage at the gate of the field-effect transistor VT1 has time to exceed the threshold (taking into account the voltage drop across the VD5 diode), as a result, transistors VT1 and VT2 begin to open. Thanks to positive feedback through the VD4 diode, the process ends like an avalanche.

With fully open transistors, a voltage close to the nominal voltage (6 ... 1,5 V) is supplied to the power outputs of the electronic module of the electromechanical alarm clock through the VD2 diode. Those outputs of the module, to which the contacts that closed when the alarm clock was originally connected, are connected by a jumper, so the emitter HA1 in the module emits an intermittent sound signal.

The closed door of the refrigerator opens the contacts of the switch SF1, as a result, the current in the lamp circuit EL1 stops, the voltage across the capacitor C1 drops rapidly and the signaling device that has lost power goes silent. Thanks to the VD3 diode, the capacitor C2 quickly discharges, after which the device is ready to re-read the shutter speed.

The device is mounted on a 113x20 mm foil fiberglass board, shown in fig. 2. If it is supposed to be placed inside the refrigerating chamber, the installation must be covered with several layers of moisture-resistant varnish, for example, ML-92T or FL-98.

Resistors MLT, C2-23, C1-4 can be installed on the board. Capacitor C2 should be selected with the lowest possible leakage current, for example, K53-4. You can try to install here an imported oxide capacitor from the companies "PHILIPS", "RUBYCON", "DON", "XENIA" for an operating voltage of 35 ... minutes is not possible.

Diodes - any series KD521, KD522, KD103, KD503, KD512, D223, 1N4148. The KS433A zener diode is replaced by KS407A, BZX55C3V3, 1N4728A and other domestic and imported ones with a stabilization voltage of 3,3 V at a current of at least 100 mA. The KP501V transistor can be replaced with a similar one with a different letter index or one of the ones included in the KR1064KT1 A, K1014KT1, KR1014KT1 microcircuits. The KT3107G transistor, if necessary, is replaced by any low-power silicon p-n-p structure.

The electronic module removed from the Chinese-made quartz electromechanical alarm clock "KANSAI" is glued to the signaling device board in the position indicated in Fig. 2nd place. The service life of the mechanical components of such alarm clocks usually does not exceed a year, but their electronics are still able to function for a long time. Modules from other alarm clocks, including domestic ones, will also work. Please note that many of them are limited to one minute of continuous beeping.

Setting up the signaling device consists in setting the desired delay for turning on the sound by selecting the values ​​​​of the resistor R3 (it can be replaced by several connected in series) and capacitor C2. In order not to expose yourself to the danger of electric shock, it is recommended to do this without connecting the alarm device to the refrigerator. It is enough to apply to the terminals of the capacitor C1, observing the polarity, a voltage of 3 ... 3.5 V from any source.

It should be noted that it is very difficult to achieve a delay of more than 5 minutes. Installing an alarm adjusted at room temperature inside the refrigerator will usually decrease the shutter speed slightly.

Author: A.Butov, village of Kurba, Yaroslavl region

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