Fridge Door Alarm Report l1u3m

Fridge Door Alarm Circuit Diagram:

Components:

Working of Circuit:

R1______________10K 1/4W Resistor R2_____________Photo resistor (any type) R3_______________2M2 1/4W Resistor R4_______________1M 1/4W Resistor

The circuit, enclosed in a small box, should be placed in the fridge near the lamp or close to the opening. With the door closed, the interior of the fridge is in dark, the photo resistor R2 presents a high resistance (>200K) thus clamping IC1 by holding C1 fully charged across R1 and D1. When a beam of light enters from the opening, or the fridge lamp lights, the photo resistor lowers its resistance (<2K) stopping C1charging current. Therefore IC1, wired as an astable multivibrator, starts oscillating at a very low frequency and after a period of about 20 seconds its output pin (#3) goes high, enabling IC. This chip is also wired as an astable multivibrator, driving the Piezo sounder intermittently at about 5 times per second. The alarm is activated for about 17 sec. then stopped for the same time period and the cycle repeats until the fridge door closes.

C1______________10µF 25V Electrolytic Capacitor C2_____________100nF 63V Polyester Capacitor D1____________1N4148 75V 150mA Diode IC1,IC2_________7555 or TS555CN CMos Timer ICs BZ1___________Piezo sounder (incorporating 3KHz oscillator) B1____________3V Battery (2 x 1.5V AA, AAA or smaller type Cells in series) Introduction: A refrigerator or freezer door that is left open or ajar may cause the food contents to spoil. Moreover it also reduces the efficiency of the fridge by needlessly running the compressor in a futile effort to keep the contents cool. The perishable goods like fishes, vegetables etc. are more at risk if the fridge door is left open. This is where the Fridge Alarm is useful. The fridge alarm operates by detecting when any light enters the compartment area. The alarm will sound if the light is present for longer than the preset period and will continue to sound until the door is closed. In practice, the preset period is adjusted so that in normal use the alarm will not sound. It will sound when the door is left wide open for too long or if left slightly ajar. It is great for indicating when someone is standing with the door open for too long and a real asset in warning when the door looks shut but is still partially ajar.

Photo resistor: A photo resistor is a sensor whose resistance varies with light intensity. Most decrease in resistance as the light intensity increases. In a typical microcontroller application, this resistance must be converted to a voltage so that an A2D converter can measure it. The easiest way to do this is with a voltage divider circuit. Working of Photo resistor: A voltage divider is just two resistors in series connected between a voltage supply and ground. If R1 is connected to the voltage supply and R2 is connected to ground then the voltage at the junction between the two resistors is: V = V supply * (R2 / (R1 + R2)). If R1 is the photo resistor, the voltage will increase with increasing light intensity. If R2 is the photo resistor, the voltage will decrease with increasing light intensity.

Sample Photo resistor Circuits

Theory:

Here, we use R2 as photo resistor and hence the voltage decreases with increasing light intensity. Thus when the door of the fridge is closed, the light intensity is zero and the buzzer does not sound, but sounds only when the door is open i.e. when the voltage is minimum. To understand what happens when a semiconductor absorbs a photon material it is necessary to brief review energy band gap theory. Each electron in the orbit in an atom has a certain allowable discrete energy level. Values other than these specific levels are forbidden energy levels. The two interest bands in a semiconductor material, are the valence band and the conduction band, which are separated by the forbidden gap, as shown in figure.

IC7555CN:

The valence band is the highest energy band that is filled by electrons. The conduction band is partially filled with electron that moved across the forbidden gap from one band to another. Light, of energy equal to or greater than the energy gap between the valence and the conduction bands incident upon a semiconductor material, can force electrons from the valence to the conduction band and thereby change the conductivity. Light dependent resistor (LDR) is made from cium sulphide. Cium Sulphide is a II-VI semiconductor. (It is so called because Cium is in group II and Sulphide is in group VI.) LDR is commonly used in auto-focusing camera for light metering.

Description: The ICM7555 is a CMOS timer providing significantly improved performance over the standard NE/SE555 timer, while at the same time being a direct replacement for those devices in most applications. Improved parameters include low supply current, wide operating supply voltage range, low THRESHOLD, TRIGGER, and RESET currents, no crowbarring of the supply current during output transitions, higher frequency performance and no requirement to decouple CONTROL VOLTAGE for stable operation. The ICM7555 is a stable controller capable of producing accurate time delays or frequencies. Features of IC 7555 CMOS Timer: • Exact equivalent in most applications for NE/SE555 • Low supply current: 80mA (typ) • Extremely low trigger, threshold, and reset currents: 20pA (typ) • High-speed operation: 500kHz guaranteed • Wide operating supply voltage range guaranteed • Can be used with higher-impedance timing elements than the bipolar 555 for longer time constants • Timing from microseconds through hours • Operates in both astable and monostable modes • Adjustable duty cycle • High output source/sink driver can drive TTL/CMOS • Typical temperature stability of 0.005%/°C at 25°C • Rail-to-rail outputs.

APPLICATIONS of IC: • Precision timing • Pulse generation • Sequential timing • Time delay generation • Pulse width modulation • Pulse position modulation • Missing pulse detector Troubleshooting: The commonly available photo resistor does not show much difference in its resistance in light as well as dark. Hence the resistance keeps on varying and is not constant. Due to this IC1 remains in ON state even in dark which eventually triggers IC2 to keep the buzzer ON. Thus the photo resistor used should be of high sensitivity. Applications: Since the above circuit is a light operated device, it can be used for security of lockers. In this case the Delay time can be varied by varying capacitors C1 and/or resistance R3. Limitations: The above circuit cannot work in high moisture areas; hence it should be switched off and taken out during defrosting.