Normally we just push the fridge door close, hoping that the door gasket magnets will hold it. Often the door does not stay close tight in many households. Children do not generally bother to keep the door properly closed. If the fridge door is not kept closed properly, the cooling will be lost and fridge tends to work more, thus increasing your power bill. The circuit here reminds you with an alarm if the door is left open. Alarm continues to sound as long as the fridge door is open, telling you to do your job fast and close the door.
We now make use of CMOS IC CD 4060. It is an interesting IC, which has a built in oscillator and divider. The basic oscillator can be configured either by RC or crystal network. Then CD4060 divides this oscillator frequency into binary divisions of 4,5, 6, 7, 8,9,10,12,13, and 14 which are available as outputs. This means that IC can divide the basic frequency up to a maximum of 16384 times. Because of the division of basic frequency in large numbers, the timings are more stable. For normal operation reset pin is pulled low. If the reset pin is held high, all the counters are reset and stops oscillator. Oscillator Block diagram is shown in Figure 20 and pin out of the IC is given in Figure 21.
Basic frequency is determined by the following formula.
F = 1 /2.2 R1 X Cx at VCC=10V
where R2 must be greater than 2R1 to 10R1
The circuit is shown in Schematic 15. When the fridge door is closed, the interior will be dark. Hence the LDR has a high resistance and the pin 12 (RESET) of the IC is held high. But if the door is left open, outside light enters the fridge or the fridge light itself comes up. This effectively lowers the resistance of LDR and Pin 12 goes low. Then IC 4060 starts oscillating and counting. With the present timing components at pins 9,10,11, the output timing is two seconds. Hence pin 3 goes high for two seconds and goes low for another two seconds. Three terminal piezo buzzer there is activated for every two seconds and alarm sounds until the door is closed. The circuit operates at 3V can even be used at 6V. You may use two pen cells in two cell plastic holder or four in a four cell holder. But if you wish to use NiCad cells you have to use at least three giving 3.6V. Cell voltage of NiCad cells is only 1.2.
Make this circuit on a small Vero board; fix it in a small box leaving a little window for the LDR and a small opening for the buzzer sound. LDR should point towards the light source. Time of oscillation can be adjusted by changing the values of CI, Rl and reduced by half by connecting the outputs to Pin 2. Switch is avoided to reduce the size. After all, the current consumption is low and batteries can be removed when not in use. Please do not place this in the freezer and make sure to remove this while defrosting. Connect the trigger terminal of the three terminal Piezo buzzer to Pin 3 of CD 4060.
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