Feb 28, 2013


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 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.
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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