SOME modern freezers contain alarms which sound if you leave the door
open and allow the internal space to warm up. However, they do not work
if the freezer suffers a power failure, which is a bit of a drawback.
Making a temperature-sensitive circuit which can sound an alarm is not
too difficult but what is required here is a lowcost circuit which can
run on batteries for a very long time. This design uses a circuit based
on a PIC, using a feature about which little has been written, namely
the ability to send it to sleep! The circuit is extremely simple, and
the software uses several techniques which could be useful in other
projects.
Circuit Description
If
you are the sort of person who enjoys the challenge of constructing
complex circuits, you will be disappointed! The complete circuit
contains only five components, as shown in Fig.2. The clever stuff, of
course, is provided by the PIC. The temperature sensor used is a lowcost
disc thermistor, R1, which can be attached via a length of 2-core
cable. A small preset variable resistor, VR1 is used to set the
operating point, the temperature threshold at which the alarm sounds.
Capacitor C1 is used to make the input circuit time-dependant, as
described in the next section. For the alarm, a piezo sounder (WD1) is
used because it can make a relatively large amount of noise whilst using
a very small amount of electrical power. The whole circuit will
conveniently run off a 6V battery.
Construction
Construction
is very simple. The suggested stripboard component layout and
track cut details are shown in Fig.10. The thermistor can be soldered to
a short length of wire such as thin audio coax. An improvement would be
to waterproof the thermistor connections by dunking it in polyurethane
varnish. The wire can be fed into the freezer via the door seal. It is
important to resist the temptation to add a light emitting diode as a
battery indicator – the l.e.d. would take about a thousand times more
power than the rest of the circuit! The PIC should be plugged into the
board via an 8-pin d.i.l. socket. The circuit and batteries can be
housed in a plastic box to sit outside the freezer, a small hole being
provided to glue the piezo sounder behind. You should not need to
replace batteries very often.
Testing
The
circuit will work quite happily at room temperature. Once the batteries
are connected (it seems to work well on 6V although this is higher than
the maximum recommended). Gently rotate preset VR1 until the threshold
is found between the alarm bleating or not. Set it so that the alarm is
just off. Then hold the thermistor in your fingers to warm it up, and
the alarm should sound; let go to allow the thermistor to cool again to
room temperature, and the alarm should stop. Once you are convinced all
is well, put the thermistor in the freezer, and after allowing time for
the temperature to stabilise, increase the resistance on the preset so
that the alarm threshold is set where you would like it. In fact, the
best way to find out if the batteries are OK is to let the thermistor
warm up a bit when you open the freezer – if it is working and the alarm
sounds, the batteries are fine!
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