Make a Simple Refrigerator Thermostat

Want to make an accurate electronic refrigerator thermostat for your refrigerator? The circuit of a solid state thermostat described in this article will surprise you with its “cool” performance. The unit once built and integrated with any relevant appliance will instantly start exhibiting an improved control of the system saving electricity and also increasing the life of the appliance.

Conventional refrigerator thermostats are expensive and not very accurate. Moreover these are prone to wear and tear and therefore not permanent. A simple and much efficient electronic refrigerator thermostat device is discussed here.

A thermostat as we all know is a device which is able to sense a particular set temperature level and trip or switch an external load. Such devices can be electromechanical types or more sophisticated electronic types. Thermostats typically are associated with air conditioning, refrigeration and water heating appliances. For such applications the device becomes a critical part of the system without which the appliance may reach and start operating under extreme conditions and ultimately get damaged. Adjusting the control switch provided in the above appliances ensures that the thermostat cuts off power to the appliance once the temperature crosses the desired limit and switches back as soon as the temperature returns to the lower threshold. Thus the temperature inside refrigerators or a room temperature through an Air conditioner is maintained to favorable ranges.

The circuit idea of a refrigerator thermostat presented here can be used externally over a refrigerator or any similar appliance to control its operation. Controlling their operation cab be done by attaching the sensing element of the thermostat to the external heat dissipating grid normally situated behind most cooling devices that use Freon. The design is more flexible and wide ranged compared to the built-in thermostats and is able to exhibit better efficiency. The circuit can easily replace the conventional low tech designs and moreover it’s much cheaper compared to them.

Let’s understand how the circuit functions:

Parts List

R1=10k NTC,

R2=Preset 10K

R3,R4=10K

R5=100K

R6=510E

R7=1K

R8=1M

R9=56 OHM/1watt

C1=105/400V

C2=100uF/25V

D1=1N4007

Circuit Description

  The diagram alongside shows a simple circuit built around the IC 741, which is basically configured as a voltage comparator.

A transformer less power supply is incorporated here to make the circuit compact and solid-state.

A bridge configuration comprising R3, R2, P1 and the NTC R1 at the input forms the main sensing elements of the circuit.

The inverting input of the IC is clamped at half the supply voltage using a voltage divider network of R3 and R4.

This eliminates the need of providing a dual supply to the IC and the circuit is able to produce optimum results even through single pole voltage supply.

The reference voltage to the non-inverting input of the IC is fixed through the preset P1 with respect to the NTC (Negative Temperature Coefficient.)

In case the temperature under check tends to drift above the desired levels, the NTC resistance drops and the potential at non-inverting input of the IC crosses the set reference. This instantly toggles the output of the IC, which in turn switches the output stage comprising transistor, triac network, switching off the load (heating or the cooling system) until the temperature reaches the lower threshold.

The feedback resistor R5 to some extent helps to induce hysteresis into the circuit, an important parameter without which the circuit may keep flip-flopping quite rapidly in response to the sudden temperature changes.

Once the assembly is completed, setting up the circuit is very simple and is done with the following points:

REMEMBER THE ENTIRE CIRCUIT IS AT AC MAINS POTENTIAL, SO EXTREME CAUTION IS ADVISED WHILE GOING THE TESTING AND THE SETTING PROCEDURES. USE OF A WOODEN PLANCK OR ANY OTHER INSULATING MATERIAL UNDER YOUR FEET IS STRICTLY RECOMEMDED; ALSO USE ELECTRICAL TOOLS WHICH ARE THOROUGHLY INSULATED NEAR AND AROUND THE GRIPPING AREA.

You will need a sample heat source accurately adjusted to the desired cut-off threshold level of the thermostat circuit.

Switch on the circuit and introduce and attach the above heat source with the NTC.

Now adjust the preset so that the output just toggles (the output LED comes on.)

Remove the heat source away from the NTC, depending upon the hysteresis of the circuit the output should switch off within few seconds.

Repeat the procedure many times to confirm its correct functioning.

This concludes the setting up of this refrigerator thermostat and is ready to be integrated with any refrigerator or similar gadget for an accurate and permanent regulation of its operation.

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How to Make a Simple Piezo Buzzer Circuit Piezo Electric Buzzer Explained

In this article we will learn how to make a very simple piezo electric buzzer using hardly any electronic components. Just a single transistor, a coil, a piezo buzzer are enough to make it “buzz” or rather “twit” for you, with an output that may be quite ear piercing.

Circuit Description:

The buzzer circuit described here actually works in a quite unique way. Instead of the normal working concept employed by other forms of oscillators which require resistor and capacitor networks for generating the oscillations, this circuit use inductive feedback for the required operations.

Referring to the figure we find that the transistor T1 along with the inductor forms the heart of the circuit. Basically the coil which is specifically called the buzzer coil, is in fact positioned for amplifying the created oscillations while the actual feed back is provided by the center tap of the three terminal piezo element used for the present application.

When a voltage is introduced in the circuit, the transistor conducts, operating the piezo element across the buzzer coil, however this also leads to the grounding of the base of the transistor through the center tap of the piezo element, this instantly switches off the transistor and in turn the piezo also switches off, releasing the base of the transistor.

 The transistor reverts to its original state and the cycle repeats, generating oscillations or the required “buzzing” frequency.

The center tap from the piezo transducer plays an important role in sustaining the oscillations and therefore in this particular design we need a three terminal piezo rather than a two terminal one.

The oscillations produced at the collector of the transistor is dumped into the coil, saturating the coil with magnetic inductions. The coil kicks back the stored energy during the oscillations, magnifying the generated AC across it.

This stepped up AC is applied across the anode and the cathode of the piezo element, which starts vibrating sharply according the pitch of the frequency, generating a shrill, ear piercing sound in the air.

However to make the sound audible at maximum intensity, the piezo transducer needs to be stuck or installed in a special way inside its housing.

For this particular application the piezo element needs to be stuck at the base of its housing which must consist of a hole having a diameter of about 7 mm. 

The piezo element cannot be stuck directly over the base of the housing, rather it must stuck and positioned over a soft, pure rubber ring, having diameter 30 % less than that of the piezo transducer.

Only if the above fixing procedure is followed, the buzzer will sound, otherwise the sound may get choked and fail to reproduce.

 Parts List

R1 = 100K,
R2 = 4k7,
T1 = BC547,
L1 = Buzzer inductor,
PZ1 = Piezo element, 27mm, three terminal
Rubber ring = 22mm

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How to Make a Dual Tone Siren Circuit

This electronic siren gives out a continuously varying high amplitude sound. Since the supply voltage is not critical, it can be used in cars, motor cycles or at home. It can replace the ordinary call bell. - . The circuit consists of two separate free running multivibrator and an oscillator.

A free running or astable multivibrator is one which has two quasi-stable states and the output of one stag is connected to the input of the other through a coupling capacitor.

Since both the states are quasi-stable, the output attained is continuously varying in nature i.e. high, low high low-.

 The output is in the form low pulses, the frequency of which depends on the base biasing resistor and the coupling capacitor, When these resistances and condensers for both the stages are of different values, the output ` wave form is rectangular; this is because the time constant of the two quasi-stable states becomes different.

If this time constant of the two, states is made the same, the output obtained then is square wave. Two states of the multivibrator are made identical by the use of the same values of components.

The components used in the circuit (Fig). result in a square wave output and the time constant selected is so as to give a fairly good rise and fall of the siren.

However, one may change the value of coupling capacitors to get any other desired time constant. The second unit is an oscillator section. The condenser connected at the output is the feed back condenser. It determines the tone of the siren.

Higher the value of the condenser the lower is the pitch. for high pitch sound (generally used in siren) feed-back condenser ranging from 0.047 uf to 0.1 mfd should be selected. The speaker may be metallic case (horn type) or small planer cone. The metallic cone horn gives better results.

Parts for dual tone siren circuit

R1,R2,R5,R6= 22K
R3,R4 = 2K2
R7 = 10 Ohms, 1 watt
C1,C2,C4 = 0.1uF
C3 = 22uF/25V
T1,T2 = BC557
T3 = BC547
T4 = 2N2907 or 8550

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