This is only a preview of the June 2004 issue of Silicon Chip. You can view 17 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Dr Video Mk.2: An Even Better Video Stabiliser":
Items relevant to "An RFID Security Module":
Items relevant to "Fridge-Door Open Alarm":
Items relevant to "Courtesy Light Delay For Cars":
Items relevant to "Upgraded Software For The EPROM Programmer":
Purchase a printed copy of this issue for $10.00. |
Is your fridge or freezer
door often left open for
too long? Or does it
sometimes not
close properly?
Ensure it’s closed
when it should be
by building this nifty
Fridge Alarm.
FRIDGE
DOOR-OPEN
ALARM
By JOHN CLARKE
A
REFRIGERATOR OR freezer
door that is left open or ajar
may cause the food contents to
spoil. In some cases, the internal temperature of the fridge or freezer will be
maintained if the refrigeration system
can cope with the open door.
But without the door sealing in the
cold air, it may be a losing battle. Running costs will certainly rise.
Typically, refrigerators and freezers are in constant use in the summer
months and so it is important to ensure
that the door is not open for any longer
than is necessary. Otherwise the fridge
or freezer will not be able to keep the
siliconchip.com.au
contents cool. And it will cost more
money to needlessly run the fridge’s
compressor in a futile effort to keep
the contents cool.
Even the most diligent fridge user
may sometimes leave the door of the
fridge or freezer open without realising it. And tilting the fridge or freezer
slightly backward so that the door will
fall shut is not completely fool proof as
there may be an obstruction inside the
door. The obstruction could be because
an item inside the compartment has
moved or fallen over or because the
compartment is too full.
This is where the Fridge Alarm is
useful. It warns when the door of the
refrigerator or freezer is left open for
longer than a preset time period. 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.
The fridge alarm operates by detecting when any light enters the
compartment area. Therefore it is just
as useful for freezers (which normally
do not have a light) as it is for fridges
(which normally do). As long as there
is some ambient light which the alarm
can react to, it will operate.
June 2004 59
door is left ajar since the internal light
is switched off via the door switch
before the door closes.
The circuit
You don’t have to house it in a transparent box, as we did . . . but if you don’t,
you’ll need another hole in the appropriate place on the box wall so light can
strike the LDR inside.
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.
Commercial coolrooms
and freezers
While the Fridge Alarm is primarily
intended for domestic fridges, it has
its applications for large (ie walk-in)
commercial coolrooms and freezers.
If you think that your fridge at home
costs a lot of money to run, try paying
the bill for one of those walk-in models
that clubs and restaurants use. And in a
busy club or restaurant, it is very common for staff to leave the door open.
Because the door is so large, bulk cold
escapes very quickly.
If the walk-in coolroom or freezer
has a door-operated light, the Fridge
Alarm will work in exactly the same
way as in a domestic fridge. If the light
switch is manual (as many are), it will
warn that the light has been left on.
And if it doesn’t have a light inside,
you could set it up near the doorway
and have the alarm triggered by natural
light from outside.
Note that the alarm cannot be used
60 Silicon Chip
with display refrigerators or freezers
that have a glass door.
Does the light really go off?
Do you or members of your family
have doubts whether the fridge light
really goes off when the door is closed?
Does the little man in the fridge really
do his job? Or is he sitting in there
goofing off?
This Fridge Alarm will finally dispel any doubts on this score. If you
open the door and can hear the alarm
sounding immediately, it means that
the light has remained on while the
door was closed. Disbelievers will say
it’s a fault in the alarm unit itself rather
than the light remaining on. Perhaps
we will never know.
The Fridge Alarm is battery operated
and so does not need to be connected
to any wiring inside the compartment.
It comprises a small transparent box
with the alarm circuit and battery
housed inside. The box is placed
within the freezer or refrigerator near
to the door opening. In this way it can
monitor both the light from the internal lamp and also light entering from
the outside. Monitoring light from the
outside is important since it allows
detection of the door being left only
slightly ajar. Monitoring the internal
light only will not indicate when the
Circuitry for the Fridge Alarm
comprises a single IC package, a Light
Dependent Resistor (LDR), a siren plus
a few resistors, diodes and capacitors.
The low temperature operation has
meant that all components need to
be rated for sub zero temperatures.
The IC is rated to –40° C, while the
piezo siren is rated to –20°C. Other
components such as the capacitors,
diodes, LDR and resistors will operate
to below -20°C.
The battery is specified as an alkaline type to provide the necessary
current at lower temperatures. And
current drain is not very high. When
the circuit is in the dark, quiescent
current is typically less than 6µA and
this low current will prevent the battery discharging before the end of its
shelf life. Current consumption when
the alarm is sounding is a mere 2mA.
Operation of the alarm relies upon
light detection using the LDR. This
device has low resistance below 10kΩ
when there is sufficient light on its
surface and a high resistance of more
than 1MΩ when in darkness.
We use this change in resistance in
a voltage divider with a 1MΩ trimpot
and a 150kΩ resistor across the 9V
supply. Voltage across the LDR is
monitored at the pin 1 input of Schmitt
trigger IC1a.
IC1a has two threshold voltages
which are nominally 1/3rd the supply
and 2/3rd the supply. These thresholds are 3V and 6V with a 9V supply.
If voltage at pin 1 is 6V or more then
the output at pin 2 will be 0V. If the
pin 1 voltage falls below 3V, then the
output at pin 2 will be at 9V.
In the dark
When the fridge or freezer door is
closed, the LDR is in complete darkness and so it has a high resistance.
The total resistance of the 150kΩ
resistor and VR1 is now smaller than
the LDR resistance and this causes the
voltage at pin 1 to rise above the upper
threshold of the Schmitt trigger. As a
result, the output at pin 2 will be at 0V.
Capacitor C1 is held at 0V via diode
D1 and the series connected 2.2kΩ
resistor. Schmitt trigger IC1b monitors
the voltage across C1 at its pin 3 input.
Since pin 3 is at 0V, pin 4 is at 9V.
siliconchip.com.au
Fig.1: the circuit is basically a light trigger, timer, oscillator and piezo driver. It’s all based on one low-cost IC.
Diode D2 and the series 2.2kΩ resistor pull the pin 5 input to IC1c close
to 9V and so pin 6 is at 0V. The output of IC1c drives paralleled Schmitt
triggers IC1d, IC1e and IC1f and since
IC1c’s output is at 0V, the paralleled
Schmitt outputs are at 9V. Outputs of
IC1d, IC1e and IC1f at pins 8, 10 and
12 respectively drive the (-) side of the
piezo siren. At this stage the siren will
not be driven since the (+) terminal
of the piezo siren connects to the 9V
supply and the (-) terminal is at 9V.
This is the Fridge Alarm’s normal
state when in darkness. Current drain
from the battery is very low and is
caused by several current paths.
The first is the current flow through
the LDR, VR1 and the 150kΩ resistor.
The LDR will be about 2MΩ or more in
darkness and the current will be less
than 4.5µA for this part of the circuit.
Another current path is through diode
D2, and the series connected 2.2kΩ resistor and the 10MΩ resistor connected
between pins 5 and 6 of IC1c. Current
flows because pin 4 of IC1b is at 9V
and the pin 6 output of IC1c is at 0V.
Current drain here is less than 1µA.
The final current drain is the supply
to IC1 itself and the 100µF capacitor
across the supply (after D4). For that
reason we specify that both 100µF capacitors should be low-leakage types.
IC1 is a CMOS device that has a very
low supply current of typically below
.05µA. The total current drain is therefore expected to be around 6-7µA.
Door open
When the fridge or freezer door
is opened, the resistance of the LDR
drops and this pulls pin 1 of IC1a below its lower threshold and pin 2 goes
to 9V. Diode D1 becomes reverse biased and so capacitor C1 now begins to
charge via the 9V at pin 2 and through
the 100kΩ resistor and VR2 trimpot.
Charging time for C1 can be adjusted
using VR2 which allows timing values
from around 10s through to 100s.
When the capacitor voltage reaches
about 6V, the voltage becomes more
than the positive going threshold for
IC1b, and the output goes to 0V.
Diode D2 is now reverse biased
and the already charged capacitor C2
now discharges via the 10MΩ resistor
between pin 5 and pin 6. When C2
discharges to about 3V, it reaches the
lower threshold voltage for Schmitt
trigger IC1c and its output at pin 6
goes to 9V. Capacitor C2 now charges
Fig.2: there’s not much you can get back-to-front
on the PC board – just the IC, diodes, electrolytic
capacitors and the piezo siren (and of course the
battery snap wires). The LDR is not polarised. Use
this component layout along with the photo at right
when putting it together.
siliconchip.com.au
June 2004 61
Power for the circuit is obtained
from a 9V battery. Diode D4 provides
reverse polarity protection if the battery is connected in reverse. A 100µF
capacitor decouples the supply and
provides energy for the piezo siren
when it draws bursts of current.
Construction
The plastic box needs to have two holes drilled in the bottom (for the mounting
pillars) and one in the top (to let the sound out).
Here’s how it all goes together in the box. It’s a nice snug fit with the battery held
in place by the PC board.
up via the 1MΩ resistor and diode D3.
This charge time is about 10 times
faster than the discharge time and
when the voltage reaches the upper
threshold of IC1c’s input the output
at pin 6 goes to 0V.
IC1c thus forms a burst oscillator
where the output is at 9V for only a
short time compared to its low output
period.
When IC1c’s output is at 9V, the
resulting 0V output of IC1d, IC1e and
IC1f drive the piezo siren with a 9V
supply and the siren sounds. When
IC1c’s output goes to 0V, the IC1d, IC1e
and IC1f inverter outputs are at 9V and
the siren is off. This sequence of signal
drives the siren with bursts of sound.
When the refrigerator or freezer door
closes again, the LDR goes to a high
value of resistance. Thus pin 1 of IC1a
62 Silicon Chip
rises toward the upper threshold of the
Schmitt trigger. This may take several
seconds because the dark resistance
of the LDR slowly increases over time
until it reaches its ultimate value.
It is a rather slow responding device to low ambient light levels. VR1
is included to adjust the sensitivity
to darkness. It is adjusted so that the
alarm will still operate even with very
low light levels which are typical
when the door of the fridge or freezer
are left ajar.
Ultimately, when in complete
darkness, pin 1 of IC1a will reach 6V
and the IC1a output will go low to
discharge C1. The resulting 9V at pin
4 of IC1b charges capacitor C2 via D2
and the 2.2kΩ resistor. This holds
the burst oscillator off with the pin 6
output at 0V.
Parts for the Refrigerator Alarm
are assembled on a PC board coded
03206041 and measuring 78 x 32mm.
The PC board is mounted inside a
translucent box measuring 83 x 54
x 31mm. The box can either be uncoloured or tinted. We used the new
blue style case available from Jaycar
and Altronics.
Begin construction by checking the
PC board for any shorts between tracks
or breaks in the copper. Check hole
sizes and file out the corner section
of the PC board on two corners if not
already removed. These cutouts are required to allow access for the internal
pillars in the box. The mounting holes
need to be 3mm in diameter.
Now install the resistors, diodes and
IC1. This IC and the diodes must be
oriented as shown.
Resistors are marked with a colour code and these are shown in the
accompanying resistor code table.
You can use this table as a guide to
selecting each value. Also it is a good
idea to check the value with a digital
multimeter. Install the two trim pots
VR1 and VR2. These have a 1MΩ resistance and may have a 105 marking
on the side.
The two 100μF electrolytic capacitors should be low leakage types, as
previously mentioned, and must be
oriented with the polarity shown in
the overlay diagram.
Place the PC stakes at the 9V battery lead connection points and in
the holes allocated for the piezo siren.
The siren is mounted by soldering its
leads to the PC stakes. Note that the PC
stakes and siren leads will need to be
shortened so that when installed the
top of the siren is 14mm above the top
of the PC board.
The LDR is mounted by inserting
its leads into the PC board leaving a
10mm length between the LDR and
PC board. After soldering, the LDR is
carefully bent over at right angles to
face the edge of the PC board.
The PC board is mounted within
the case using two 10mm long spacers
to support the outside edge of the PC
siliconchip.com.au
Parts List
1 PC board coded 03206041,
78 x 32mm
1 UB5 translucent box, 83 x 54 x
31mm
1 panel label
1 piezo siren, 12mm diameter,
7.6mm pin spacing (-20°C
operation) (Jaycar AB3459)
1 9V alkaline battery
1 9V battery clip lead
1 LDR with greater than 1MΩ dark
resistance (Jaycar RD3485,
Altronics Z1619 or similar)
2 10mm M3 tapped spacers
2 M3 x 6mm countersunk screws
2 M3 x 6mm pan head screws
4 PC stakes
Semiconductors
1 MM74C14, CD40106BC (-40°C
to 85°C) hex Schmitt trigger (IC1)
4 1N914, 1N4148 diodes (D1-D4)
Capacitors
2 100µF 16V low leakage
electrolytics
1 220nF MKT polyester (code 224
or 220n or 0.22µF)
Resistors (0.25W, 1%)
1 10MΩ (10%)
1 1MΩ
1 150kΩ 1 100kΩ 2 2.2kΩ
2 1MΩ horizontal trimpots (VR1,
VR2)
board while the edge that have the pillar cutouts is held within the integral
side supports on the case. Place the PC
board in the case with its edge pressed
into the side supports and mark out
the hole positions for the outer edge
mounting holes. Drill out these holes
in the base of the case and countersink
them from the underside of the box
suitable for countersunk screws.
The side supports on the other side
of the case need to be removed to
provide space for the battery to mount
between the box side and PC board.
Full-size etching
pattern for the
fridge door-open
alarm PC board.
These are removed with a pair of pliers twisting them sideways until they
break out. Alternatively side cutters
could be used or a chisel. Use safety
goggles when doing this as pieces can
fly out as they break.
Secure the 10mm tapped spacers to
the base of the case with the countersunk screws. The PC board is secured
to the top of the spacers using M3 pan
head screws. Solder the battery leads
to the supply PC stakes as shown on
the overlay diagram.
Place the lid onto the case and mark
out the centre position of the piezo
siren. The siren will have a label attached that says, “remove after washing”. This label can be removed now.
The hole in the lid needs to be about
6mm in diameter to ensure the full
sound intensity can be emitted from
the siren.
Testing
The alarm is now ready to be tested.
Adjust VR1 to centre position and
VR2 fully anticlockwise. Connect up
the battery. The alarm should sound
after about 10 seconds giving short
bursts of sound. If this does not happen, Make sure you are not working
in the dark. Also check that the parts
have been correctly placed on the PC
board. Also measure the voltage at pin
2 of IC1. This should be close to 9V.
Pin 4 of IC1b should be at 0V. Voltage
between pin 7 and pin 14 of IC1 should
be about 9V.
Adjust VR2 for the desired timeout before the alarm sounds. Fully
clockwise will provide a nominal 100
seconds before the alarm will sound.
The alarm needs to be placed in
complete darkness before the siren can
be silenced. Simply placing a finger
over the LDR is not sufficient. Note
also that the alarm may take some 10
to 20 seconds to switch off in darkness
as the LDR slowly increases its dark
resistance. In a freezer, this time might
increase to several minutes!
You can test the alarm by placing it
inside a drawer instead of the refrigerator. Adjust VR1 so that the alarm
sounds if the drawer is opened slightly.
Now place the alarm unit inside the
fridge or freezer and check that it operates correctly after its temperature
has stabilised.
You will need to readjust VR1 if the
alarm is placed inside the freezer. This
is because the threshold voltages for
IC1a change with temperature. Also
the dark resistance of the LDR does not
rise to the same value found at room
temperatures.
Variations
If you want a longer delay time,
increase the value of capacitor C1. A
220µF capacitor will double the delay
time. If you want to increase the alarm
burst rate, decrease C2 in value.
The Refrigerator Alarm could also
be used as a locker or drawer alarm.
In this case, a shorter delay time may
be better. Reducing C1 will reduce the
time. Also an on and off switch could
be placed in the supply to the battery
SC
to disable the alarm.
Resistor Colour Codes
o
o
o
o
o
o
siliconchip.com.au
No.
1
1
1
2
3
Value
10MΩ (10%)
1MΩ
150kΩ
100kΩ
2.2kΩ
4-Band Code (1%)
brown black blue silver
brown black green brown
brown green yellow brown
brown black yellow brown
red red red brown
5-Band Code (1%)
brown black black green silver
brown black black yellow brown
brown green black orange brown
brown black black orange brown
red red black brown brown
June 2004 63
|