Hence, this Fridge Door Alarm Circuit is a good solution which will indicate the user about the door in prolonged open condition. We can also set different pre-set time after which the audible indication has to be given. This is done here by using the versatile 555 timer IC under astable multivibrator mode and LDR. As soon as we open the Door of refrigerator, LDR senses it and start the countdown using 555 Timer, and after a preset time the buzzers starts beeping as alarm signal.
- ·555 timer IC – 2Nos（datasheet for reference: http://www.kynix.com/uploadfiles/pdf8798/NE555DR.pdf）
·5mm LDR – 1No.
·Buzzer – 1No.
·Diode (1N4007 or 1N4001) – 1No.
·Capacitor, 47uF(Electrolytic) – 1No.
·Capacitor, 0.1uF(Ceramic) – 1No.
·Resistors (10kὨ - 1; 470kὨ -1; 150kὨ -2; 100Ὠ -1)
The two 555 timers are connected in Astable multivibrator mode. The key components in the circuit are LDR (Light Dependent Resistor) and 555 Timer IC.
LDR (Light Dependent Resistor):
LDR works under the principle of Photo Conductivity. The conductance of the material inside the element increases when light falls over it. In terms of resistance, the value of resistance decreases when light falls over it and resistance will be large in dark surrounding. The resistance is directly proportional the light over the material, check the table below:
There are several types of LDR like 3mm LDR, 4mm LDR, 5mm LDR, 7mm LDR and etc. The part used here is 5mm LDR. Using the above data we have considered the resistance divider as 10k with 5mm LDR.
555 Timer in Astable operation:
Astable multivibrator has no stable states. The output swings between high and low based on the timing resistor and capacitor.
The formulae to calculate the time delay is as below,
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Time (Sec) = 1.1 x (R2 + R3) x C1
Below we have calculated Time delay for Buzzer to be triggered and selected the resistor values accordingly. Here the Time delay means the duration for which the Refrigerator Door is left open. This is done by first 555 IC in the circuit.
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Time (Sec)=1.1 × (620kὨ ± 5%) × 47uF Time =30.4 secs Hence, R2=150kὨ, R3=470kὨ in series and C1=47uF
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Time (Sec) =1.1 × (470kὨ ± 5%) × 0.1uF Time = 0.5 secs Hence, R5=470kὨ and C2=0.1uF (The buzzer turns ON and OFF at this time frame)
The whole circuit is powered by a 9V battery. When the fridge door is closed, it is dark and the resistance of the LDR is nearly 1MὨ as given in datasheet. The output voltage of the potential divider appears across the capacitor and it remains in charged condition (Voltage higher than 2/3Vcc) making the output LOW. When we open the Fridge, the light falls over LDR which lowers down the resistance of LDR and causing the capacitor to discharge which in this RC combination it is 30 secs. After this (Voltage lower than 2/3Vcc), the output starts to oscillate at certain frequency and output is HIGH. Again, the capacitor charges and reaches a threshold continued by discharge of the capacitor. This continues till the LDR resistance goes high which will happen in the absence of light (door is closed).
This makes the second 555 timer to oscillate and the output becomes HIGH and LOW causing the buzzer connected to output to beep in a pattern which is combinational cause of the first timer oscillations and the second timer internal oscillation. During HIGH condition of first timer output, the second timer master reset will happen. Thus, the capacitor C2 charges (Voltage higher than 2/3Vcc) and output goes LOW. In a short span the capacitor starts to discharge (Voltage lower than 2/3Vcc) causes the output HIGH. Hence, the buzzer connected to output becomes pulsed beep sound.
Here is the video of this project: https://youtu.be/xeZBvZaAM4E
Thanks for reading and welcome any suggestion or questions.