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Fridge and Freezer Door Alarm
This page describes an easy-to-build alarm to warn you if your fridge or freezer door has been
left open too long.
fridge and freezer door alarm
Background Info
After our freezer door was left slightly ajar several times, resulting in all ice cream melting and meat defrosting,
I decided I wanted to build something to ensure it didn't happen again.
Some research indicated there are various off-the-shelf circuits available,
but most are based on monitoring if the temperature rises.
This isn't ideal, as it could mean the fridge or freezer door is open for a while before alarm triggers,
and no-one may be in earshot anymore.
I wanted something that would trigger immediately if door was open too long.
Silicon Chip Magazine had published a
fridge door open alarm kit
ten years ago (and I built one a long time ago for a family member), but their kit is no longer available.
I found an updated circuit that uses two CMOS 555 timers to provide better circuit operation at lower battery voltages
(an improved version of the Silicon Chip circuit),
and after some prototyping, I had a circuit that provided the functionality I wanted.
Schematic
The finalised circuit diagram of my fridge/freezer alarm is shown below.
fridge / freezer alarm - circuit diagram
(click for larger version)
Component List
R1 = 10K 1/4W resistor
R2 = light-dependent resistor (approx 2K resistance under light)
R3 = 2M2 1/4W resistor
R4 = 1M 1/4W resistor
R5 = 470 1/4W resistor
C1= 10µF 25V electrolytic capacitor
(10µF gives initial delay of approx 25 seconds; use 22µF to increase to approx 50 seconds)
C2 = 100nF 63V polyester capacitor
D1 = 1N4148 diode (or 1N4001)
Q1 = BC548 NPN transistor or similar
IC1, IC2 = 7555 or TS555CN CMOS timer ICs
BZ1 = piezo buzzer
B1 = 3-6V battery pack
(3V may not be enough for some piezos - I used 4 x AA batteries to provide 6V)
Circuit Description
With the fridge door closed, R2 provides a high resistance, clamping IC2 by holding C1 fully charged across
R1 and D1.
When the fridge door is opened, R2's resistance drops to around 2K, stopping C1 from charging. This causes IC1,
configured as an astable multivibrator, to start oscillating at a low frequency.
After about 25 seconds, its output (pin 3) goes high, triggering IC2.
Also configured as an astable multivibrator, IC2 drives the piezo buzzer at about 5Hz
for about 18 seconds. After this time, the piezo is silenced, and the cycle starts again.
Closing the fridge door clamps IC1 again, silencing the piezo.
The initial delay before the piezo sounds can be adjusted by changing C1 and/or R3,
and the piezo frequency can be adjusted by changing C2 and/or R4.
With R3 = 2.2 megaohms and C1 = 10µF, the initial delay is about 25 seconds.
Replacing C1 with a 22µF electrolytic capacitor increases the initial delay to just over 50 seconds.
The circuit is based on one that is widely published on the internet,
which uses 2 x AA batteries to provide a 3V power source, and uses one of the 555 timers (IC2) to directly drive a piezo buzzer.
I found that the 555 timer output, as specified in the original circuit, provided insufficient voltage to drive the
piezo buzzers I was using.
I added in an NPN transistor (Q1) to drive the piezo,
and raised the voltage by using 4 x AA batteries to provide a power source of 6V.
While this circuit can be boxed and placed inside a fridge, it should not be placed inside a freezer,
as the colder temperatures will affect the operation of the circuit and the battery life.
Instead, extend the wiring to R2, mount R2 inside the freezer, but keep the circuit and batteries outside the freezer.
Construction Details
Rather than etching a circuit board, I opted for the easier option of using vero board.
Below is the vero board layout I used, where blue lines indicate wire links, red dashes indicate cut traces,
and purple lines indicate the traces on the bottom of the vero board.
vero board layout (top view)
(click for larger version)
vero board with all components installed, except battery pack and LDR (R2)
cuts in vero board traces (bottom view)
wire links on vero board (top view)
fridge and freezer door alarm - completed circuit, ready for installation in a case
As the circuit is not suitable for installation inside a freezer, I extended the wiring to the light-dependent
resistor via some bell wiring, using a 3.5mm mono plug and socket, to allow it to be unplugged from the rest
of the circuit.
light-dependent resistor wired to a 3.5mm mono plug
Although a little larger than required, the only suitable case I could find in my shed was an
electrical junction box.
A piece of acrylic was glued into the case, to keep the battery case separate from the vero board.
The power switch, piezo buzzer and 3.5mm mono socket were mounted on the sides of the case.
circuit installed in a case with battery pack
Once the base was screwed to the rest of the case, I glued a few small magnets to the base, to ensure the case
would stay in its location on top of the freezer.
The light-dependent resistor was mounted inside the freezer with some gaffer tape, near the interior light,
and plugged into the case.
completed device - with light-dependent resistor plugged in on the right,
power switch (left) and piezo buzzer (centre)
Initially, I used a 10µF electrolytic capacitor for C1, which provided an initial delay of around 25 seconds.
After my wife complained that this wasn't long enough when she was looking for something buried in the back of the
freezer, I replaced C1 with a 22µF electrolytic, which extended the initial delay to about 50 seconds.
References
last updated 8 Jun 2014
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