Schema, assembly and components of an electronic light detector

We suggest you make a small utility that will prevent installation, perhaps, to throw the contents of your refrigerator or freezer in the trash. Indeed, in these times of great heat, leave open the door of such a device usually results in disaster.
Our implementation easily habitable in a small plastic box (the size of a matchbox), and battery powered, will issue a "beep" when the door is opened. This is to avoid forgetting or not closing.
This little editing, achievable by any beginner, may save the contents of your freezer
The operating principle of our installation is very simple: open the refrigerator door causes systematically lighting a lamp, a photocell detects the light and will control the start of an oscillator loaded to generate an AC voltage. A speaker of small size will issue the beep.
Note that all freezers are not equipped with a lighting device. However, ambient light will provide sufficient light to the cell to trigger the device - the only case where it would be totally ineffective or that you would come to loot the contents of the freezer at night without turning on the light ... If that does not really normal use of the device!
Turning now to the study of our assembly. His heart, the oscillator will be done using an integrated circuit of the type NE 555 (which we are now familiar). We will find at his side resistors and the capacitor responsible for setting the oscillation frequency. To order the start, we will use the pin number 4. A transistor will amplify the voltage fluctuations resulting from changes in light intensity received by the photocell sensitive to give them sufficient amplitude. The latter will be directly connected to the leg number 4 of the integrated circuit, to ensure its startup or shutdown. The connection between the integrated circuit and the speaker will be made through a chemical capacitor. The latter's role is to remove the DC component of the signal delivered by the oscillator and, consequently, greatly reduce the power consumption of the installation; important when using batteries as a power source .

The wiring of this set, given the small number of components it requires for its realization is extremely simple. It is therefore a realization that beginners can tackle safely. However, care should be taken to cut all conductive strips of the plate wiring in the integrated circuit, and the locations mentioned on the wiring diagram.
Similarly, care should be taken to respect the pinout of the integrated circuit and the transistor, and the polarity of chemical capacitors. Once wiring is complete, the stack will be implemented. In ambient light the beep will be immediate. By masking the cell, or by passing in a dark room, it will stop. After performing this simple test, we can install it, and this, in any place in the refrigerator or freezer.
All housing will be used for its skin. If it is opaque it will be necessary to strike a hole in the cell. Similarly it is to perforate the speaker.
Finally note that most ordinary batteries do not function properly below a temperature of -18 ° C. Alkaline batteries seem to be less affected by cold, and their duration is much greater.

Electronic component

R1 = 220 kilohms (red, red, yellow, gold)
R2 = 220 ohms (red, red, brown, gold)
R3 = 220 ohms (red, red, brown, gold)
R4 = 220 ohms (red, red, brown, gold)
R5 = 22 kilohms (red, red, orange, gold)
C1 = 1 microfarad
C2 = 47 microfarad
Ic = 555 DO
T1 = 2N 3905
LDR = LDR photoresistor
HP = Speaker diameter 50 mm