R1______________10K 1/4W Resistor R2_____________Photo resistor (any type) R3_______________2M2 1/4W Resistor R4_______________1M 1/4W Resistor 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)
The main purpose of this design was to obviate a small defect of the very popular Fridge Door Alarm circuit, available on this website since 1999 and built by a lot of hobbyists.
Unfortunately, that circuit stops operating when the battery voltage falls below about 2.6 - 2.7 Volts. This is due to the 4060 CMos IC used. In some cases, devices made by some manufacturers (but not Motorola's) fail to operate even at nominal 3V supply voltage.
A simple cure to this shortcoming could be the substitution of the original IC specified with a 74HC4060 chip: this should allow circuit operation down to 2V but, unfortunately, this IC is not easy to locate.
For this reason, an equivalent circuit using about the same parts counting was developed, in order to allow safe operation even when battery voltage falls down to about 1.3V.
The circuit, enclosed in a small box, should be placed in the fridge near the lamp (if any) 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 C1 charging current. Therefore IC1, wired as an astable multivibrator, starts oscillating at a very low frequency and after a period of about 24 sec. its output pin (#3) goes high, enabling IC2. 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.
- Delay time can be varied changing C1 and/or R3 values.
- Beeper repetition rate can be varied changing C2 and/or R4 values.
- Stand-by current drawing: 150µA.
- Place the circuit near the lamp and take it away when defrosting, to avoid circuit damage due to excessive moisture.
- Do not put this device in the freezer.