The proposed detector will beep if the door everyday the refrigerator is left open for more than a few minutes.
The device include break the supply circuit of an incandescent lamp, illuminating the refrigerating chamber when an open door, and placed behind a protective shield for the lamp in the camera itself or, for example, in the upper dash compartment of the refrigerator. The connection of the signaling the effective value of the voltage supplied to the lamp, decreases by only 2, that does not affect its brightness.
Diagram of the device shown in Fig. 1.
Switch SF1 and the lamp EL1 - elements the design of the refrigerator. It holds closed door contact switch open. When the door is open, the contacts are closed and the circuit of the lamp EL1 current flows. To power the detector used, the voltage falling across the resistor R1. It limited by the Zener diode VD1 and rectified by diode VD2.
Immediately after the door is opened, the capacitor C2 is discharged, therefore, the transistors VT1 and VT2 remain closed. Charging of the capacitor through the resistor R3. If the door is not closed for too long, increasing the voltage on the gate field transistor VT1 manages to exceed the threshold (taking into account the voltage drop on the diode VD5), as a result, the transistors VT1 and VT2 are starting to open. Thanks positive feedback through the diode VD4, the avalanche process is terminated.
When fully open the transistors on the power pins of the electronic module Electromechanical alarm through the diode VD6 receives voltage close to nominal (1,5...2). The conclusions of the module, which were originally connected semicassis when the alarm contacts are connected jumper, therefore, available in the module emitter emits an intermittent HA1 the audio signal.
Closed the fridge door opens the contacts of switch SF1, the current in the circuit of the lamp EL1 terminated, the voltage across capacitor C1 decreases rapidly and lost power alarm falls silent. Thanks to the VD3 diode capacitor C2 quickly discharged, whereupon the device is ready to re delay countdown.
The device is mounted on Board, of foiled fiberglass dimensions 113x20 mm, shown in Fig. 2. If you intend to put inside refrigeration, installation should be covered with several layers moisture-proof varnish, for example, ML-T or PL-98.
On the Board can be installed resistors MLT, C2-23, C1-4. The Capacitor C2 you should choose smaller leakage current, for example, K53-4. Can to try to establish here imported oxide capacitor firms "PHILIPS", "RUBYCON", "DON", "XENIA" on the operating voltage of 35...63 V. Using capacitors K50-35 or K53-19, to obtain a sufficiently stable exposure duration of several minutes impossible.
Diodes - any series KD521, KD522, CD, CD, CD, D, 1N4148. Zener diode KSA replace XA, BZX55C3V3, 1N4728A, and other domestic and imported with voltage stabilization 3.3 V at a current of at least 100 mA. Transistor CPU can be substituted with another letter this index, or for one of the available in the composition of the chips KRKT AND, CCT, KR1014KT1. Transistor when CTG necessary, replace any low-power silicon structures with p-n-R.
Electronic module, extracted from quartz Electromechanical alarm "KANSAI" in China, stick to the PCB of the detector shown in Fig. 2 place. The service life of mechanical components such alarms usually does not exceed years, but their electronics are still able to function for a long time. Suitable modules from other alarms, including domestic production. Note that the continuous beep many of them is limited to one minute.
The establishment of the indicator is to set the desired on delay sound selection of the values of resistor R3 can be replaced with several, connected in series and the capacitor C2. To avoid danger of electrocution, it is recommended to do it without attaching the device to an the fridge. Enough to submit to the terminals of the capacitor C1, observing polarity, voltage 3...3.5 In from any source.
It should be noted that to achieve a delay of over 5 minutes is very difficult. When installation inside the fridge alarm, adjusted at room temperature, exposure is usually slightly reduced.
Author: A. Butov, S. CORBA Yaroslavl region