Alarm Indicator for High Temperature Reading

The circuit has been designed to provide an alarm with the use of a buzzer when sensing high temperature by using a thermistor.

  • Thermistor – one type of resistor used as temperature-sensing element which demonstrates a large change in resistance proportional to a small change in temperature because of the composition of its semiconductor material.
  • BC550 – an NPN general purpose transistor with low current and low voltage used for low noise stages in audio frequency equipment.
The temperature can be regulated by the design of this circuit and it can provide a warning scheme in the case of excessive increase in the temperature. The control of the temperature starts from the thermistor TH1 which contains a negative factor. Since this component has resistance proportional to the temperature that it detects, the resistance can be modified wherein a 10K ohm resistance is produced ate 25° C while 1K ohm resistance will be produced at 94° C. to define a specific value of temperature, a trimmer TR1 having a value of 2.2K ohms is being used where transistor Q1 and Q2 are used.

These bipolar transistors form a Darlington pair to produce a very high current gain that is much higher then each transistor taken separately. This is made possible when the current amplified by the first transistor is further amplified by the second transistor. The overall gain is equal to the two individual gains multiplied together.A Darlington pair is designed in such a way that the collectors of both transistors are attached together and the emitter of the first transistor is directly coupled to the base of the second or output transistor. The base current of the output transistor is equal to the emitter current of the input transistor. When several transistors are used, it can form a cascade connection and the total current amplification factor will be very high since it is a product of current amplification factor of respective transistors.


Alarm Indicator for High Temperature Reading  Schematic



When the Darlington pair conducts in this circuit, it causes the relay K1 to close thereby triggering the buzzer to produce sound due to the increase in temperature that is more than the predetermined value. The location of the thermistor should be far enough from other components of the circuit to prevent inhibiting their temperature. The circuit is powered by a 9 V battery for ease of transportation but a dedicated power supply can be provided is the circuit would be in a permanent location. A load, like a lamp or a LED, can also be connected to the contacts of the relay to produce a light indication or notification of the occurrence of excess temperature. Since the circuit is well insulated, the thermistor, connected to the circuit with a plated cable, can be submerged to water to cool down or reset the sensitivity and will not cause any short circuit. Doing this will regulate the trimmer.

This circuit can be designed not only by setting a detection of high temperature but can also be modified to be set on detection of low temperature in some industries. It can be used for refrigeration, walk-in freezer, or walk-in cooler systems and other environments that are sensitive in temperature. Some circuits incorporating high temperature alarm modules are being used in motor vehicles to detect the occupancy wherein the temperature element senses a highly dangerous temperature in the passenger compartment of a motor vehicle and energizes the occupancy sensor to determine the presence or absence of an occupant. In the absence of the occupant, the sensor de-energizes after a period of time while an audible alarm is triggered to create attention to the motor vehicle in the presence of an occupant. This type of alarm can be reset with the use of a key.

Having a temperature alarm can provide benefits like protecting valuable equipment due to high temperature, low temperature, or sometimes high humidity. Other high temperature program can be used to protect against air conditioning failure or against the heating system to turn off. On the contrary, low temperature program can be used against heating system failure by aiding to prevent frozen pipes. The alarm can also reduce downtime by receiving a telephone call and being notified of a possible power loss or failure before the damage occurs in an equipment.

Alarm Indicator for High Temperature Reading

The Darlington pair used in the circuit can also be used as an important component in a switching device while high power Darlington transistors can be used in emergency power supply and in control circuits for AC and DC motor control. These applications make the Darlington suitable for switching large currents in high power circuits.
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