Energy Leak Detector

This sensitive circuit functions primarily as a comparator, capable of detecting minimal temperature changes relative to ambient temperature. It was originally designed to identify drafts around doors and windows that lead to energy loss, but it can be utilized in various applications where a sensitive temperature change detector is required. Two LEDs indicate whether the temperature change is above (Red LED) or below (Green LED) the ambient temperature. IC1 serves as a bridge detector and amplifier, with its output voltage increasing as temperature rises and decreasing as temperature falls. This behavior occurs because the negative temperature coefficient (NTC) resistor R2 reduces its resistance with increasing temperature, thereby unbalancing the bridge formed by R1, R2, R4, and R5. IC2A and IC2B together create a window comparator, with R8 functioning as the sensitivity control. Prior to initiating a measurement, the circuit must be balanced using R1 to ensure that both LEDs remain off. Setting R8 to zero resistance maximizes circuit sensitivity, causing one of the LEDs to illuminate upon detecting even a slight temperature difference. Increasing the value of R8 decreases the circuit's sensitivity. If only one LED is required to indicate a temperature change, regardless of whether it is an increase or decrease, D2 can be removed, and pin #1 of IC2B should be connected to pin #7 of IC2A.

This circuit employs a precision temperature detection mechanism, leveraging the characteristics of an NTC thermistor to monitor ambient temperature changes. The primary operational amplifier (IC1) is configured as a Wheatstone bridge, where the resistive elements R1, R2, R4, and R5 are arranged to provide a differential voltage output based on the imbalance caused by temperature variations. The NTC thermistor R2 is critical in this setup, as its resistance decreases with rising temperatures, directly influencing the voltage across the bridge.

The window comparator formed by IC2A and IC2B is responsible for interpreting the output from the bridge and driving the LEDs accordingly. The configuration allows for a dual indication system, providing visual feedback for both temperature increases and decreases. The sensitivity control resistor R8 can be adjusted to fine-tune the response of the circuit, allowing for flexibility in various applications. By carefully balancing the circuit with R1, the LEDs can be calibrated to remain off at the desired ambient temperature, ensuring that false readings do not occur due to environmental noise.

For applications requiring a single LED indication, the circuit can be simplified by omitting D2 and bridging the appropriate pins of the comparators, thus streamlining the design while maintaining functionality. This adaptability makes the circuit suitable for various temperature monitoring applications beyond its original intent, including HVAC systems, energy management, and environmental monitoring.This sensitive circuit is basically a comparator, detecting very slight temperature changes in respect to the ambient temperature. It was primarily intended to detect draughts around doors and windows that cause energy leaks but can be used in many other ways, when a sensitive temperature change detector is needed.

Two LEDs are used to signal if t he temperature change is pointing above (Red LED) or below (Green LED) the ambient temperature. IC1 acts as a bridge detector and amplifier: its output voltage raises when temperature increases and vice-versa. This happens because the n. t. c. resistor R2 reduces its resistance value as temperature increases and vice-versa, therefore unbalancing the bridge formed by R1, R2, R4, R5.

IC2A and IC2B form a window comparator and R8 is the sensitivity control. Before starting a measurement the circuit must be balanced by means of R1 in order to obtain that both LEDs are off. If R8 is set to zero resistance the circuit sensitivity will be at maximum and one of the LEDs will illuminate when a very slight difference in temperature will be detected.

As R8 value is increased the circuit sensitivity will decrease. If only one LED is needed to show a change in temperature, no matter if raising or decreasing, D2 can be omitted and pin #1 of IC2B must be connected to pin #7 of IC2A. 🔗 External reference