A temperature change is what this circuit is designed to detect to as it performs the function of a comparator.
TL061 – a high speed JFET input single operational amplifier incorporating well matched, high voltage JFET and bipolar transistors in a monolithic integrated circuit with features such as low offset voltage temperature coefficient, low input bias & offset currents, high slew rates, very low power consumption, wide common mode & differential voltage ranges, internal frequency compensation, latch up free operation, and output short circuit protectionLM393 – low power dual voltage comparator with features such as TTL, DTL, ECL, MOS, CMOS compatible outputs, differential input voltage range equal to the supply voltage, low output saturation voltage, input common-mode voltage range includes ground, low input offset voltage and current, very low supply current independent of supply voltage, and wide single supply voltage range or dual supplies Circuit Explanation
The operation of the circuit is basically a comparator which compares two voltages or currents and switches its output to indicate which is larger. It is sensitive enough to detect a very slight change in temperature with respect to the ambient temperature. The ambient temperature is the air temperature of any environment that ranges between 60 and 75 degrees Fahrenheit and is often used to denote the prevailing room temperature.
The circuit was intended to detect primarily the draughts around the windows and doors that would be the cause for energy to leak out. The circuit is not limited to this function and can also be utilized in applications or cases when a sensitive temperature change detector is required. To provide a visual indication, the two LEDS are used to point out if the change in temperature will be above or below the ambient temperature. The above change will be denoted by Red LED while the below change is denoted by Green LED.
The output voltage and temperature are directly proportional, such that the increase in temperature also increases the voltage and vice versa while IC1 will serve as an amplifier and a bridge detector. The proportionality was made possible due to the reduction of resistance of resistor R2 as the temperature increases which causes unbalancing of the bridge that consists of R1, R2, R4, & R5. The control of sensitivity is handled by resistor R8 while the function of a window comparator is undertaken by IC2A & IC2B.
In order to achieve the status of both LEDs to be OFF, the circuit must be balanced by means of R1 before performing a measurement. The sensitivity of the circuit will be at maximum if R8 is set to zero resistance. As soon as a very slight difference in temperature is detected, this circuit sensitivity will cause one of the LEDs to eliminate. On the other hand, the increase in value of R8 will decrease the sensitivity of the circuit.
Before the operation, the value of the thermistor can be set to 10K – 22K Ohms at 20ºC range while the value of R1 should be approximately twice the thermistor value. This will only be possible if a 47K Ohms potentiometer is used with a 22K Ohms thermistor. The casing of the thermistor determines the speed of detection of temperature changes where a smaller casing corresponds to a faster detection.
The temperature detector may be used in refrigerators to indicate if the door was sealed properly. Other important applications lie in some industrial needs where gas handling system is being used which are maintained at room temperature. This is true to most nuclear companies to ensure safety.