How to build a simple "Thermometer"

How to build a Thermometer project at home and lab. 

Introduction

In this blog post, we are going to study how to build a simple, easy Thermometer circuit. A simple electronic instrument to measure temperature easily by observing a moving coil meter. This is a beginner-friendly circuit focused on students, hobbyists, and electronics enthusiasts. Now we shall go through the project for easy assembling. 

Often, measuring room temperatures or the temperature of an enclosed chamber or an item requires cost-effective instruments. The project described here is a low-cost assembly with expenses based on easily available discrete components from the electronics mart. 

Know your circuit

The circuit schematic of the so-called 'thermometer' circuit is given here. Please go through how the vital components, such as sensors and temperature reading scale instruments, are put together to measure temperature. The circuit is straightforward forward and a beginner in electronics can understand it easily the schematic design easily. However, give specific attention to the temperature sensors and their associated components bridged in the versatile circuit.  

Parts List

Schematic Reference	Parts Name	Type	Quantity
D1	Rectifier diode	1N4001	1
D2	LED	RED LED indicator	1
D3	ZENER Diode	5.1v	1
D4 & D5	Rectifier Diode	1N4148	2
C1	Electrolytic capacitor	220 µFD 16v	1
R1	Resistor	560 Ώ	1
R2	Resistor	1.5K Ώ	1
R3 & R4	Resistors	5.6K Ώ	2
R5	Temperature Sensor	KTY 10	1
P1	Preset	500 Ώ	1
P2	Preset	10K Ώ	1
M1	Moving coil meter	± 50 µA	1

Know your Design

R5 - KTY 10 is the temperature sensor used in the circuit to detect temperature variations. Actually, it is a temperature-dependent resistor. Its value varies according to the quantum of heat falling on the resistor. Its value varies according to the temperature it senses. More heat means fewer ohms, while its value is high when it is cold.   It is configured to the bridge circuit to pass on the change in its values to the circuit bridge. 

The moving coil meter is centered at '0'. The '0' is set at 0 degrees C temperature through the preset P1. The moving coil meter does not deflect at this temperature and stands still at '0'. Any temperature variation other than '0' causes the bridge to be destabilised. The variation produces a proportional indication on the meter. 25 degrees C is carried out on the meter for deflection using the preset P2. 

Typically, the bridge circuit is powered with a stable power of 5V supply. The supply voltage is regulated by the 5.1V zener for steady operation. D2 LED ensures that the supply is on. However, if one desires, the entire power supply circuit can be replaced with a 9V battery. But to meet the power consumption of the bridge circuit for long-time operation, one can go for an efficient IC-regulated power supply source, such as IC 7805.   

You can refer to this post to build a 5V IC-regulated power supply. 

Know your construction

The circuit can be assembled on a general-purpose PCB easily. The moving coil meter may be fixed outside the PCB for ease of construction. Or one can go fully digital using a digital display with additional cost. Before using the circuit, one should calibrate the moving coil meter with known temperature sources and values. 

Conclusion

This is a simple school project that the students love to assemble and show their potential assembling skills in electronics. A fascinating hobby electronics project for enthusiasts. Please give your valuable comments for further improvement and education.