The article explains how to build a cool circuit that indicates temperature using two LEDs.
It is used in various kind purposes such as:
Checking the water showers temperature to prevent damage
Ensuring that the computer parts, amplifiers, heaters and even the water tank used in the car do not overheat.
What is a Two LED Temperature Indicator Circuit:
Simple electronic circuits that shows temperature changes visually is a two LED temperature indicator circuit.
Using two LEDs to show the temperature status is often used to monitor the temperature of a particular object or environment.
The LEDs acts as binary indicators such as one for low temperature and another for high temperature or they can indicate various temperature thresholds.
This article gives a complete tutorial with simple methods for building a color temperature indicator circuit with two LEDs
Circuit Working:
Parts List:
| Component | Description | Quantity |
|---|---|---|
| Resistors | All resistors are 1/4 W CFR | |
| 1k | 1 | |
| 10k | 2 | |
| Preset | 330k | 1 |
| Thermistor | NTC 10k | 1 |
| Semiconductors | ||
| Transistors | BC547 | 2 |
| LEDs | Green 5mm 20mA | 1 |
| Red 5mm 20mA | 1 |
The above temperature indicator circuit with two LED the green LED1 flashes at lower temperatures while the red LED 2 is activated at higher temperatures.
The working principle involves the use of a thermistor Th1 whose resistance varies with temperature.
At lower temperatures the increased resistance of thermistor Th1 raises the voltage at the base of transistor T2 causing it to open.
This in turn leads to the closure of transistor T1 allowing current to flow through LED1.
On the other hand as the temperature rises the thermistors resistance decreases lowering the voltage at the base of T2.
This results in the closure of T2 and the opening of T1 causing LED2 to illuminate.
To ensure LED1 functions properly a series diode e.g.1N4007 is added if the LEDs voltage drop is insufficient.
The common resistor R1 for both LEDs combined with the lower voltage drop of the red LED2 ensures that the green LED1 remains OFF when the red LED is active.
The threshold temperature is adjusted using potentiometer P1.
If the required temperature is not achieved adjust the values of P1 or R3.
Formulas and Calculations:
Below are the formulas with calculations for Two LED Temperature Indicator Circuit:
The following formulas provides a accurate way to calculate temperature based on resistance:
1/T = A * ln(R) + B * ln(R)2 + C * ln(R)3 + D
where:
- T is the temperature in kelvin K
- R is the thermistors resistance in ohms Ω
- A, B, C and D are constants specific to the thermistor model
The thermistors datasheet include these A, B, C and D constants.
After that one needs to solve the formula for T using a calculator that can calculate natural logarithms (ln).
How to Build:
To build a Two LED Temperature Indicator Circuit follow the below steps:
- As shown in the circuit diagram connect LEDs 1 and 2 to thermistor Th1 transistors T1 and T2.
- Based on the necessary temperature range and LED details set the resistor values for R1, R2 and R3.
- Adjust the threshold temperature setting with potentiometer P1.
- If necessary add a series diode 1N4007 to make sure that LED1 has enough voltage drop.
- Once the circuit is powered ON see how the LEDs behave at various temperatures.
- To ensure the correct temperature indication adjust P1 and resistor values as needed.
Conclusion:
Keep in mind that this is only a rough idea as final design may differ depending on the users particular requirements and choices.
Also advanced temperature monitoring applications makes use of temperature sensors with digital output.