Zero degree Celsius alarm

Description: This circuit is designed to trigger an alarm when the temperature drops below zero degrees Celsius. A thermistor is utilized to measure the temperature. The operational amplifier LM7215 is employed to compare the reference voltage with the voltage from the thermistor network. The reference voltage is supplied to the non-inverting input (pin 3) of the IC, while the voltage from the thermistor network is connected to the inverting input (pin 4). When the temperature falls below zero degrees, the voltage at the non-inverting input exceeds that at the inverting input, resulting in a high output from the op-amp. This output turns on transistor Q1, which activates a piezo buzzer to sound the alarm. The power supply section features the IC 7805, which converts a 9V battery to a stable 5V output.

The circuit operates on the principle of temperature sensing and voltage comparison. The thermistor, which is a temperature-dependent resistor, changes its resistance based on the ambient temperature. At temperatures above zero degrees Celsius, the resistance of the thermistor is relatively low, leading to a lower voltage at the inverting input of the op-amp. Conversely, as the temperature drops below zero degrees, the resistance increases, resulting in a higher voltage at the inverting input.

The LM7215 op-amp is configured in a comparator setup, where it continuously compares the two input voltages. The reference voltage can be set using a voltage divider or a fixed reference source, ensuring that it remains constant. When the temperature threshold is crossed, the resulting output from the op-amp transitions from low to high, which is sufficient to saturate the transistor Q1. This transistor acts as a switch, allowing current to flow from the power supply to the piezo buzzer, thereby generating an audible alarm signal.

The power supply section, which utilizes the 7805 voltage regulator, is crucial for ensuring that the circuit operates reliably. The 7805 takes the 9V input from the battery and regulates it down to a stable 5V, which is essential for the proper functioning of the op-amp and other components in the circuit. This design enhances the circuit’s robustness and ensures that it can operate efficiently over a wide range of input voltages, maintaining consistent performance in various environmental conditions.

Overall, this simple yet effective temperature alarm circuit is ideal for applications requiring temperature monitoring and alerting, such as in refrigeration systems, outdoor environments, or any other situation where freezing conditions may pose a risk.This simple circuit will produce an alarm whenever the temperature falls below zero degree. A thermistor is used here to sense temperature. The op-amp LM7215 is used to compare the reference voltage and voltage from the thermistor network. Reference voltage is given to the non inverting input (pin3) of the IC and voltage from thermistor network is given to the inverting input (pin4). When temperature becomes less than zero degree the voltage at the non inverting input becomes larger than the voltage at the inverting input and the output of the op-amp becomes high. This makes the transistor Q1 ON and drives the piezo buzzer to make the alarm. In the power supply section, IC 7805 is used to derive 5V from the 9V battery.