Temperature Monitor circuit

A thermistor positioned as indicated creates a heat-activated sensor. Variations in temperature will modify the output of the operational amplifier, triggering the relay and illuminating the LED. Reversing the placement of the thermistor and the 47k resistor converts the setup into a cold or frost alarm.

The described circuit utilizes a thermistor, which is a type of temperature-sensitive resistor, to detect changes in ambient temperature. The thermistor is connected to an operational amplifier (op-amp) configured as a comparator. When the temperature increases, the resistance of the thermistor decreases, resulting in a change in voltage at the op-amp input. This change in voltage is compared to a reference voltage set by a voltage divider, which includes the 47k resistor.

When the temperature exceeds a predefined threshold, the op-amp output transitions from low to high, activating a relay. The relay serves as a switch that can control a larger load, such as a lamp or an alarm system. In this case, the activation of the relay also completes the circuit for an LED, causing it to light up, thereby providing a visual indication of the temperature increase.

Alternatively, by swapping the positions of the thermistor and the 47k resistor, the circuit can be configured to detect lower temperatures, effectively transforming it into a frost alarm. In this configuration, as the temperature drops, the resistance of the thermistor increases, which will again alter the voltage at the op-amp input. If the temperature falls below a certain threshold, the op-amp output will change state, energizing the relay and illuminating the LED to signal the presence of frost conditions.

This circuit design is versatile and can be applied in various temperature monitoring applications, including HVAC systems, refrigeration units, and environmental monitoring setups. Proper selection of the thermistor type and the reference voltage is crucial for achieving the desired sensitivity and response time for specific applications.Using a thermistor in the position shown makes a heat activated sensor. A change in temperature will alter the output of the opamp and energize the relay and light the LED. Swapping the position of the thermistor and 47k resistor makes a cold or frost alarm.. 🔗 External reference