This guide will show you how to build a cool circuit that uses two LEDs to show temperature.
It can be used for all sorts of things, like:
Checking the water temperature in your shower to avoid getting scalded
Making sure your electronics are not getting too hot, like amplifiers, heaters, computer parts or even your car engine coolant.
What is a Two LED Temperature Indicator Circuit:
A Two LED Temperature Indicator Circuit is a simple electronic circuit that provides a visual indication of temperature changes.
This type of circuit is often used to monitor the temperature of a specific environment or an object and display the temperature status using two LEDs.
The LEDs can represent different temperature thresholds or serve as a binary indicator e.g one LED for “low” and another for “high” temperature.
This article presents a detailed guide on the construction of a color temperature indicator featuring two LEDs, providing construction steps.
Circuit Diagram:
Parts List:
| Component | Description | Quantity |
|---|---|---|
| Resistors | 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 |
Color Temperature Indicator with 2 LEDs:
In the above temperature indicator circuit with two LEDs, the green LED 1 is illuminated at lower temperatures, while the red LED 2 is activated at higher temperatures.
The operational 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 LED 1.
Conversely, 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 LED 2 to illuminate.
To ensure LED 1 functions properly a series diode e.g.1N4007 can be added if the LEDs voltage drop is insufficient.
The common resistor R1 for both LEDs combined with the lower voltage drop of the red LED 2 ensures that the green LED 1 remains off when the red LED is active.
The threshold temperature can be adjusted using potentiometer P1.
If the desired temperature cannot be achieved modifications to the values of P1 or R3 may be necessary.
Formulas and Calculations:
The equation provides a more precise way to calculate temperature based on resistance, the equation is:
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
Your thermistors datasheet may include these A, B, C and D constants.
After that, you will need to solve the equation for T using a calculator or software that can calculate natural logarithms (ln).
Construction Steps:
- Connect thermistor Th1, transistors T1 and T2 and LEDs 1 and 2 according to the provided schematic.
- Set resistor values for R1, R2, and R3 based on the desired temperature range and LED specifications.
- Utilize potentiometer P1 to fine-tune the threshold temperature setting.
- If needed, add a series diode e.g.1N4007 to ensure sufficient voltage drop for LED 1.
- Power up the circuit and observe LED behavior at different temperatures.
- Adjust P1 and resistor values as necessary for accurate temperature indication.
Conclusion:
It is important to note that this is a basic outline, and the actual design may vary based on the specific requirements and preferences of the designer or user.
Additionally, temperature sensors with digital output can be used for more advanced temperature monitoring applications.
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