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Like most modern gadgets,

freezers offer excellent reliability

and problems with them are ex-

tremely rare. It is easy to be lulled

into a false sense of security by

this reliability, because like every

other gadget freezers can and do

go wrong occasionally.

If the problem is not spotted

in time, the likely result is a great

deal of wasted food. Unfortu-

nately, unless smoke starts to

pour out the back of the freezer, it

is unlikely that the problem will be

noticed until the food has de-

frosted and you are confronted

with a soggy mess.

This very simple alarm pro-

ject provides an early warning of

problems by sounding an audible

alarm if the temperature inside

the freezer rises above a preset

threshold level. The user is

alerted to the fault long before the

food has a chance to defrost, and

hopefully in time to get the prob-

lem fixed before the food is ru-

ined.

The circuit is battery powered

and is therefore immune to failure

of the mains supply. Although the

unit must be left running continu-

ously, the current consumption

has been kept to a very low level

that ensures each set of batteries

has virtually its “shelf” life.

E\ 52%(57 3(1)2/'

conditions the resistance of R3

is very similar to that of R4

(about 680 kilohms).

This gives approximately

half the supply potential from

this section of the bridge. If the

thermistor is made colder its

resistance rises, and the output

voltage from that side of the

bridge reduces. Conversely, if

its temperature is increased, its

resistance falls, causing the out-

put voltage to increase.

The output potential from

the other section of the bridge is

dependent on the setting of po-

tentiometer VR1. The wiper

(moving contact) voltage of

VR1 can be set to anything from

one third of the supply potential

to two thirds of the supply volt-

age. In practice this is adjusted

for an output voltage that is

fractionally higher than the out-

put voltage from the other arm

of the bridge.

For a small outlay you could save yourself from an

expensive “thaw-out”!

ject” for the newcomer to elec-

tronics. The temperature sensor

is a negative temperature coef-

ficient thermistor (R3), which is

effectively a resistor whose

value changes with variations in

temperature. The higher the

temperature of the thermistor,

the lower its resistance be-

comes.

A form of bridge circuit is

used, with thermistor R3 and

resistor R4 forming one arm of

the bridge. Resistors R1 and R2

together with potentiometer VR1

form the other section of the

bridge circuit.

Each arm of the bridge gen-

erates an output voltage that is

some fraction of the supply volt-

age. The output voltage from

thermistor R3 and R4 depends

on the resistance of the thermis-

tor, and the circuit is designed

so that under normal operating

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CIRCUIT OPERATION

The full circuit diagram for

the Freezer Alarm is shown in

Fig.1 and is an ideal “starter pro-

Maxfield & Montrose Interactive Inc

[

Fig.1. Complete circuit diagram for the Freezer Alarm. Compo-

nent designated R3 is the thermistor temperature sensor.

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VOLTAGE COM-

PARATOR

In this circuit an operational

amplifier (opamp), IC1, is used

as a voltage comparator. An

operational amplifier amplifies

the voltage difference across its

two inputs, and at DC it pro-

vides an extremely high voltage

gain. A theoretical operational

amplifier has infinite voltage

gain, but a typical “real world”

device exhibits a voltage gain of

about 100,000.

Consequently, only a

minute voltage is needed across

the inputs in order to send the

output fully positive or negative.

The output goes positive if the

non-inverting input (pin 3) is at

the higher potential, or negative

if this input is at the lower volt-

age.

In this case VR1 is adjusted

so that the inverting input (pin

2) is at the higher voltage under

normal conditions, which sends

the output of IC1 (pin 6) to a

very low voltage. This results in

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Fig.2. Multi-project printed circuit board component layout, in-

terwiring, and (approximately) full-size copper foil master.

Double-check the layout as not all holes are used.

tor R3 rises

slightly, its resis-

tance falls and

the voltage sup-

plied to IC1’s non-

inverting input

(pin 3) increases.

This takes the

non-inverting in-

put to a higher

voltage than the

inverting input,

and the output of

IC1 then goes high. This

switches on transistor TR1,

which in turn activates warning

device WD1. The circuit there-

fore provides the desired effect,

with a warning being provided if

thermistor R3 is taken above

the threshold temperature set

using control VR1.

It is possible that buzzer WD1

will provide a highly inductive

load for transistor TR1, and pro-

tection diode D1 has been in-

cluded to protect TR1 from any

high reverse voltages that are

generated. Capacitor C2 and C3

help to prevent electrical noise

from giving erratic operation

when R3 is very close to the

threshold temperature.

Front panel layout showing

the mounting of the warning

buzzer.

switching transistor TR1 being

turned off, and no power is sup-

plied to warning device (buzzer)

WD1.

However, if the freezer fails

and the temperature of thermis-

Maxfield & Montrose Interactive Inc

PRACTICAL APPROACH

A bridge circuit and a ther-

mistor may seem to be a slightly

old fashioned solution to temper-

ature sensing, but this arrange-

ment often represents the most

practical approach in applications

that only require a certain temper-

ature to be sensed rather than

precise temperature

measurement.

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COMPONENTS

Resistors

R1, R2 100k (2 off)

R3 47k bead N.T.C. thermistor

R4 680k

R5 2k2

R6 1k5

All 0.25W 1% metal film

(except R3, see text)

Potentiometer

VR1 100k carbon rotary, linear

Capacitors

C1 100u radial electrolytic, 10V

C2, C3 2u2 radial elect. 50V (2 off)

Semiconductors

D1 1N4148 signal diode

TR1 BC549

npn

silicon transistor

IC1 LF441CN low power opamp

Miscellaneous

B1 6V battery pack (4xAA cells

in holder)

S1 s.p.s.t. miniature toggle switch

WD1 6V miniature DC buzzer

Printed circuit board available

from the

EPE Online Store

, code

7000932 (www.epemag.com);

medium size plastic case; PP3

battery connector; control knob;

approx 36s.w.g. (0.19mm) enameled

copper wire; multistrand connecting

wire, solder pins; solder, etc.

Positioning of components inside the two halves of the case.

Note the space for the battery pack.

sonably well on a 6V supply. The

LF441CN works well in this circuit

and the use of alternative

opamps is not recommended.

The use of a high value ther-

mistor also helps to minimize the

battery drain. Although R3 has a

nominal resistance of 47 kilohms,

this is its resistance at +25

C. In

this application it will operate at a

much lower temperature of

around

C where its resis-

tance is over ten times higher.

The high resistance through

R1, VR1, and R2 also helps to

minimize the current consump-

tion. The total current consump-

tion of the circuit is typically un-

der 200mA, which should provide

many months of continuous oper-

ation from even the cheapest of

AA batteries.

See also the

SHOP TALK Page!

CONSTRUCTION

The

Freezer Alarm

project

utilizes the

EPE

multi-project

printed circuit board (PCB). The

component layout and wiring,

together with the

(approximately) actual size cop-

per foil master pattern, are

shown in Fig.2. This board is

available from the

EPE Online

Store

(code 7000932) at

www.epemag.com

The usual words of caution

about using this particular board

have to be given. The majority

of the holes in the board are left

unused, making it relatively

easy to get a component in the

wrong place. It is therefore es-

sential to take a little more care

than usual when fitting the com-

ponents onto the board.

Approx. Cost

Guidance Only

(Excluding Batteries)

$19

A major advantage of this

type of circuit is that it is inher-

ently stable. The temperature at

which the bridge balances and

the output voltages are equal is

not affected by changes in the

supply potential. The inevitable

changes in the battery voltage

due to aging consequently have

no affect on the accuracy of

the unit.

For battery operation to be

a practical proposition it is es-

sential for the circuit to have a

very low current consumption.

For this reason IC1 must be a

low power opamp, and it must

also be capable of working rea-

Maxfield & Montrose Interactive Inc

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modern buzzers invariably re-

quire the supply to have the cor-

rect polarity, and that the red

and black leads must be con-

nected in the manner shown in

Fig.2.

THERMISTOR SITING

Obviously, the thermistor

R3 must be mounted

inside

the

freezer and not in the alarm

unit. It must be connected to the

alarm circuit by way of very fine

wires that will enable the lid or

door of the freezer to shut prop-

erly. A thin gauge of enameled

copper wire is probably the best

choice and something like

34s.w.g. to 38s.w.g. (0 236 to

0 15mm diameter) wire is a

good choice. The connecting

wires can be a few meters long

if necessary.

The completed PCB. Note the small link wire

at the top left corner, next to the transistor.

In all other respects, con-

struction of the board is mainly

straightforward. The LF441CN

used for IC1 has a JFET input

stage that does not require anti-

static handling precautions, but

it is still advisable to use an IC

holder for this component. Be

careful to fit the three capacitors

and diode D1 the right way

round, and leave D1 until last.

Close tolerance metal film

resistors are specified in the

components list, and it is defi-

nitely advisable to use high

quality resistors if the unit will

be used in a garage or other

outbuilding where the ambient

temperature is likely to vary

over a wide range. Ordinary five

percent tolerance carbon film

resistors should suffice if the

alarm will only be used indoors.

Capacitor C3 must be a

good quality electrolytic or tan-

talum capacitor. Otherwise any

leakage through this component

could impair the performance of

the circuit.

A single link-wire is needed,

and this can be made from a

piece of wire trimmed from a

resistor leadout. Fit single-sided

solder pins at the points on the

board where lead-off connec-

tions will eventually be made to

thermistor R3, buzzer WD1, the

Maxfield & Montrose Interactive Inc

battery connector, switch S1

and temperature control VR1.

The tops of these pins should

be generously “tinned” with sol-

der.

FINAL ASSEMBLY

A small to medium size

plastic case is adequate for this

project. Very small boxes are

unlikely to be suitable as they

will not accommodate the bat-

tery pack which consists of four

AA-size cells in a plastic holder.

The connections to the holder

are made by way of an ordinary

PP3 battery connector.

From the mechanical point

of view construction offers little

out of the ordinary, but the

buzzer (WD1) has unusual

mounting requirements. The

easiest way to mount this de-

vice is to fit it on the front sur-

face of the front panel, see pho-

tographs. It is then only neces-

sary to make two small mount-

ing holes for the M2 5 or 8BA

mounting bolts, plus a third to

permit the two “flying” leads to

pass into the case.

Stripping the insulation from

the ends of the wires can be

awkward, because normal wire

strippers do not work well (if at

all) with thin wire of this type. It

is a matter of carefully scraping

away the insulation using a

modeling knife or a small file.

Then “tin” the ends of the wires

with solder.

The wires can be connected

to the circuit board via a plug

and socket or a connector

block, but direct connection to

the circuit board is cheaper and

easier. A small entrance hole

about one or two millimeters in

diameter must be drilled in the

rear panel of the case.

IN USE

It is advisable to position

the temperature sensor (R3)

well into the freezer where it will

not be subjected to large rises

in temperature every time the

freezer is opened. Leave the

sensor in place for a few min-

Alternatively, it can be

mounted on the rear surface of

the panel if a rectangular cutout

for the body of the component is

made in the panel. Note that

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utes before switching on the

alarm so that the sensor has

time to adjust to the tempera-

ture inside the freezer. After

switch-on it takes several sec-

onds for the voltages to settle

down to their normal operating

levels.

By adjusting Temperature

control VR1 it should be possi-

ble to switch the buzzer on or

off. Adjust it just far enough in a

clockwise direction to activate

the buzzer, and then back it off

very slowly and carefully in a

counter-clockwise direction to

switch the buzzer off again. The

alarm should now exhibit good

sensitivity, and removing the

sensor from the freezer should

result in the alarm sounding al-

most immediately.

If the unit seems to be mal-

functioning in any way, switch

off immediately and recheck all

the wiring. If the alarm is found

to be too sensitive in use, with

frequent false alarms, back off

control VR1 fractionally in a

counter-clockwise direction.

tion. To do this, simply adjust

VR1 in a clockwise direction to

activate the alarm.

If the buzzer operates at full

volume the batteries are in good

condition. Control VR1 is then

set back to its normal operating

position. If the volume is low, or

starts at the normal level but

noticeably falls away after a few

seconds, it is time to replace the

batteries.

life. Operation at temperatures

in the region of

20°C is possi-

ble simply by adjusting control

VR1 for the correct threshold

temperature.

OTHER

APPLICATIONS

It should be possible to

modify the unit for operation in

other applications that require a

totally different threshold tem-

perature. It is just a matter of

altering the value of resistor R4.

The value of resistor R4

should be approximately equal

to the resistance of the thermis-

tor (R3) at the required thresh-

old temperature (e.g. 47k at

C and 3k at 100

C). It is pos-

sible to obtain satisfactory re-

sults with threshold tempera-

tures from about

C to

+100

However, note that the cur-

rent through the sensor circuit

increases significantly when

high threshold temperatures are

used, giving reduced battery

NEXT STARTER

PROJECT - 4

POWER CHECK

Each set of batteries is

likely to last a year or more, but

it is advisable to check the unit

about once a month to ensure

that they are still in good condi-

LOW-BUDGET

SHORTWAVE

RECEIVER

Maxfield & Montrose Interactive Inc

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