How to Build a Simple Temperature Indicator Circuit Using NTC and PTC Thermistors

A thermometer may be needed to measure temperature precisely. However, in many cases, absolute values ​​are not needed and relative estimates are sufficient. For example, a consumer might be alerted that a drill or vacuum cleaner is getting hot by basic LED lighting or by changing the color of the LED.

It would be even better if a green light on these devices indicated that everything was fine temperature-wise. As the temperature rises, the light must gradually change color to signal to the user that the device is getting too hot.

In this post, we will learn how to build a simple temperature indication circuit using NTC and PTC thermistors.

As we all know, NTC thermistor is a temperature-dependent resistor whose resistance decreases as the temperature increases. Therefore, it is called negative temperature coefficient thermistor (NTC) because it responds to the increase in temperature with negative resistance.

A PTC thermistor is also a temperature-dependent resistor, but it exhibits the opposite function.

The resistance of a PTC thermistor increases as the surrounding temperature increases. Hence, it is called a Positive Temperature Coefficient Thermistor (PTC) because of its positive resistance response to an increase in temperature.

In the following simple temperature indicator circuit, we utilize the above mentioned properties of NTC and PTC thermostats to quickly indicate the temperature by illuminating an LED.

Circuit Description

The following simple thermistor-based temperature indicator does just that.

The value of R3 is low and the value of R4 is high at relatively low temperatures. During the positive half cycle of the supply voltage, the voltage across R3-D3 will be high enough to illuminate the green LED.

The value of R3 is set to ensure that the voltage across it is very small so that the red LED lights up during the negative half cycle of the supply voltage.

As the temperature increases, the value of the NTC thermistor R4 decreases, while the value of the PTC thermistor R3 increases. Eventually, the green LED will light up at a lower and lower brightness, while the red LED will begin to light up at a higher and higher brightness, until only the red LED is fully illuminated.

The current drawn by the LED is controlled by resistor R2 and capacitor C3. This approach minimizes dissipation.

R3 and R4 should be of moderate size, at least 6 mm in diameter and no smaller. The value of the NTC thermistor must be 22 to 25 kΩ and the value of the PTC thermistor must be 25 to 33 ohms at 25°C. The circuit should be handled with care since it conducts the entire supply voltage.