Simple Dual LED Temperature Indicator Circuit

The Simple Dual LED Temperature Indicator Circuit is designed to light up one of two LEDs depending on the temperature.

It features an LM35 IC temperature sensor, a CA3130 operational amplifier and two NPN BC547 transistors.

To ensure a stable 5V DC output the IC 7805 converts a 9V DC input into 5V.

If you already have a 5V DC source, you can connect it directly without any modifications.

In this circuit the LM35 serves as the temperature sensor.

Its output increases by 10mV for every degree the temperature rises.

This output is linked to the non-inverting input of the CA3130 op-amp.

This arrangement is particularly handy for projects that need to display temperature readings without using complex display systems.

Circuit Working:

Parts List:

The 9V direct current power supply is converted into a stable 5V output using a IC1 7805 voltage regulator

The IC2 LM35 temperature sensor generates an output voltage that corresponds directly to the temperature it measures.

The output from the IC2 LM35 is connected to the non-inverting input of the operational amplifier CA3130 IC3.

The inverting input of this amplifier can receive a reference voltage from preset VR1.

For instance, if the reference voltage is adjusted to 0.8V the voltage at the non-inverting input from the IC2 LM35 will reach 0.8V when the temperature hits 80 degrees Celsius.

At this point the output of IC3 goes into positive saturation activating transistor Q1 and illuminating red LED1.

Since the base of transistor Q2 is linked to the collector of Q1, Q2 will turn off keeping LED2 green in dark.

Conversely, if the temperature falls below 80 degrees Celsius the opposite occurs.

Formulas with Calculations:

Below are the formulas with calculations for Simple Dual LED Temperature Indicator Circuit:

Comparator Threshold Calculation:

The reference voltage at the non-inverting input of IC3 is set using R1 and VR1.

Vref = (VR1 / (R1 + VR1)) × Vcc

where,

• R1 is 10k in the circuit
• VR1 preset is 2.2k in the circuit and
• Vcc is 5V

Vref = (2.2k / (10k + 2.2k)) × 5V

= (2.2 / 12.2) × 5 = 0.9V.

If the LM35 output exceeds 0.9V i.e around 90°C the comparator changes the state.

How to Build:

To build a Simple Dual LED Temperature Indicator Circuit follow the below mentioned steps for connections:

• Gather all the components as mentioned in the above circuit diagram
• Connect pin INPUT pin of IC1 7805 to 9V DC
• Connect GND pin 2 of IC1 to ground o the circuit
• Connect the OUTPUT pin 3 of IC1 to PIN 1 of IC2 LM35 INPUT pin
• Connect capacitor C1 positive side to INPUT of pin 1 of IC1 and negative of capacitor connect to GND
• Connect the GND pin of IC2 to ground of the circuit
• Connect the OUTPUT pin of IC2 to pin 3 of IC3 CA3130
• Connect capacitor C2 positive side to the junction of OUTPUT pin IC1 and INPUT pin IC2 and negative of capacitor C2 connect to GND
• Connect resistor R1 one terminal from OUTPUT pin of IC1 and other terminal of R1 connect to one terminal of VR1 preset.
• Connect the center pin of VR1 preset to pin 2 of IC2 and third leg of VR1 preset to GND of the circuit
• Connect pin 4 of IC3 to GND of the circuit
• Connect pin 6 of IC3 to base of transistor Q1 through resistor R2
• Connect pin 7 of IC3 to positive supply of the circuit
• Connect collector of transistor Q1 to the cathode of red LED1 and anode of LED1 connect to one terminal of resistor R3 and other terminal of R3 resistor connect to positive supply
• Connect emitter of transistor Q1 to GND of the circuit
• Connect one terminal of resistor R4 between collector of transistor Q1 and cathode of LED1 and other terminal connect to base of transistor Q2
• Connect the emitter of transistor Q2 to GND.
• Connect the collector of transistor Q2 to cathode of LED2 Green and anode of LED2 connect to one terminal of resistor R5 and other terminal of R5 resistor connect to positive supply.

Conclusion:

Simple Dual LED Temperature Indicator Circuit which controls temperature offers an easy and efficient method for keeping track of temperature changes.

By using an IC LM35 sensor along with a comparator IC CA3130 it guarantees precise switching.

The circuit is simple to build and is great for situations where temperature is important.

References:

Temperature sensor circuit with 3 LEDs