When you’ve popped in your fridge alarm, getting peckish in the middle of the night and forgetting to close the door – along with escalating electricity bills – will be a thing of the past. Italian inventor Flavio Dellepiane has designed a 3V battery-powered fridge alarm that beeps if you leave the door open for more than 20 seconds. When the fridge lamp illuminates, the alarm’s photo resistor lowers its resistance, the IC starts counting down and after a preset delay the piezoelectric buzzer beeps for 20 seconds.
This circuit, enclosed into a small plastic box, is intended to be placed in the fridge near the inner lamp. With the door closed the interior of the fridge is in the dark and the photo resistor R2 presents a high resistance (>2M): therefore, R1 clamps IC1’s internal oscillator by holding pin 12 high.
When the fridge lamp illuminates, the photo resistor lowers its resistance (<2K), pin 12 goes low, IC1 starts counting and, after a preset delay (20 seconds in this case) the piezoelectric buzzer beeps for 20 sec, then stops for the same lapse of time and the cycle repeats until the fridge door closes.
D2 connected to pin 6 of IC1 allows the piezoelectric buzzer to beep about three times per second.
Transistor Q1, wired as an emitter follower, acts as a current driver for the buzzer.
• Connecting D1 to pin 2 of IC1 will halve the delay time.
• Delay time can be varied changing C1 and/or R3 values.
• Quiescent current drawing is negligible, so the usual On/Off switch was omitted.
• 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.
You can mount all the parts on a printed circuit board having the dimensions and layout as shown above in full size. Or you can use the suggested layout to accommodate the parts on a pre-drilled board of about the same size with holes spaced 2.54mm, as shown in the prototype photographs.
In this case, the connections of the components can be made on the underside of the board using pieces of wire (e.g. resistors, diodes and capacitors leads) cut to the right length and carefully soldered to the appropriate pins.
In any case, you must at first insert and solder the two diodes (D1 and D2) after bending their leads at 90 degrees, paying attention to their polarity: the coloured band indicates the Cathode. Then you should insert the three resistors in the same way of the diodes.
There is no polarity to respect, but you must pay attention to the colour code of the main three coloured bands, indicating the value of each resistor.
R1 has brown-black-orange bands plus a golden band indicating the tolerance (5 per cent for all resistors used in this circuit). R3 and R4 have brown-black-yellow bands plus the usual golden band.
Next step will be the insertion of IC1: please note that the IC must be inserted with pin 1 at the top left, viewing the circuit from above as shown in the Parts Layout drawing. Some ICs have an indentation notch on the body near pin 1, or a semicircle notch at the top of the case, as shown in the Parts Layout drawing.
C1 insertion is straightforward: you can insert it in any way. C2 is a polarized type, so you must pay attention to its polarity.
The Parts Layout drawing shows where the positive lead of this capacitor must be inserted.
Please note that most electrolytic capacitors currently have the negative lead marked on their body instead of the positive.
Q1 transistor has three pins but, due to the particular shape of its case, it should be an easy task to insert the pins correctly.
Now it’s time to place the piezoelectric buzzer: this part is also polarized. Its positive pin is clearly marked on the body by a + sign.
The photo resistor R2 should not be inserted flush to the board, but its leads should be cut at a length of 1cm or 1.5cm, in order to left more than 1cm of room between photo resistor and board after the device is soldered in place.
Next, the two holders for the N cells will be soldered to the board, respecting polarities.
Finally, after encasing the pcb into the plastic box, two holes of about 5mm diameter each must be drilled on the box lid, one exactly above R2 and the other above the centre of the buzzer.
N.B. As shown in the photographs of the prototype, the photo resistor R2 is mounted through a hole drilled on a lateral side of the box. This choice is optional and is depending mainly on the actual place you want to position this gadget into the fridge in respect to the internal lamp.
Choosing this option, R2 leads must be cut at a length of about 1.5cm and bent at 90 degrees at a distance of about 1cm from the photo resistor body.
|Quantity||RS Part #||Part description|
|2||279-4971||PCB mount holders for 1xN cell|
|1||131-378||10k 0.33W Carbon Film Resistor|
|1||596-141||Photo resistor (ldr)|
|1||131-491||100k 0.33W Carbon Film Resistors|
|1||537-3038||10nF 100V Polyester Capacitor|
|1||475-8769||100?F 10V Electrolytic Capacitor|
|1||436-7341||1N4148 75V 150mA Diodes|
|1||632-679||74HC4060 14-bit binary counter & oscillator IC|
|1||131-1430||BC337 45V 800mA NPN Transistor|
|1||535-8253||Piezoelectric buzzer, 13.7mm|
|1||593-451||1.5V N Alkaline batteries|
|1||314-5213||Black ABS potting box with lid, 64x43x25mm|