This circuit monitors the temperature level in Air-conditioned rooms, Labs, Cold storage etc. and intimate you whether the low temperature is maintained in the room or not. It gives audio visual indications when the Air-conditioning system or freezing system fails and temperature rises. This prevents the spoilage of stored materials. If the temperature is above the preset level, it gives LED indications first and after Four minutes it gives loud beeps inviting immediate attention.
The circuit uses NTC Thermister as temperature sensor. Its resistance decreases when the temperature in its vicinity increases above one degree or more.IC1 is wired as a voltage comparator with its inverting and non inverting inputs tied to two potential dividers. The non inverting input (pin3) receives a variable voltage from the potential divider comprising the NTC Thermister and VR1. Similarly, the inverting input (pin 2) also receives an adjustable voltage from the potential divider R1 and VR2. Preset VR1 adjusts the voltage level at pin 3 slightly lower than that of pin 2 in the idle state in which the room temperature is below 26 degree. VR2 adjusts the voltage level at pin2 of IC1 slightly higher than that of pin 3.
Errata : Pin 8 of IC CD4060 is ground. It should be connected to negative rail.
Normally IC1 will give Zero volt output since its pin3 gets lower voltage than pin2.This is due to high resistance of Thermister in cold. When the Refrigeration system fails, room temperature rises above 30 degree and the resistance of Thermister decreases. This allows more current to flow into pin 3 and output of IC1 becomes high. This triggers T1 and Yellow LED turns on indicating that the room temperature is rising. If this condition persists, T1 remains conducting and the reset pin 12 of IC2 reach the ground potential and the IC2 starts oscillating. With the given values of C1 and R5, Q8 output becomes high after 2 minutes. This makes the Red LED flashing. If the condition continues, Q9 output becomes high after 4 minutes and buzzer starts beeping. In short, if the refrigeration system is not restarted within 4 minutes buzzer beeps to invite the attention.