4 Universal Electronic Thermometer Circuits

Here we learn four best electronic thermometer circuits which can be universally used for measuring body temperatures or atmospheric room temperatures ranging from zero degrees to 50 degrees Celsius.

In the previous post we learned some of the features of the outstanding temperature sensor chip LM35, which gives outputs in varying voltages that's directly equivalent to ambient temperature changes, in Celsius.

This feature in particular makes the construction of the proposed room temperature thermometer circuit very simple.

1) Electronic Thermometer using a Single IC LM35

It just requires a single  IC to be connected with a suitable moving coil type of meter, and you start getting the readings almost immediately.

The IC LM35 will show you a 10mv rise in its output volts in response to every degree rise in the temperature of the atmosphere surrounding it.

The circuit diagram shown below explains it all, no need of any complicated circuitry, just connect a 0-1 V FSD moving coil meter across the relevant pins of the IC, set the pot appropriately, and you are ready with your room temperature sensor circuit.

Setting up the unit

After you have assembled the circuit and finished doing the shown connections, you may proceed with the setting of the thermometer as explained below:

1. Put the preset in the midway range.
2. Switch ON the power to the circuit.
3. Take a bowl of melting ice and immerse the IC inside the ice.
4. Now carefully start adjusting the preset, such that the meter reads a zero volts.
5. The setting up procedure of this electronic thermometer is done.

Once you remove the sensor from the ice, within seconds it will start displaying the present room temperature over the meter directly in Celsius.

2) Room Temperature Monitor Circuit

The second electronic thermometer design below is another very simple yet highly precise air temperature sensor gauge circuit has been presented here.

The use of the highly versatile and accurate IC LM 308 makes the circuit respond and react superbly to the smallest temperature changes happening over its surrounding atmosphere.

Using the Garden Diode 1N4148 as the Temperature Sensor

Diode 1N4148 (D1) is used as an active ambient temperature sensor here. The unique drawback of a semiconductor diode such as a 1N4148 which shows forward voltage characteristic change with the influence of ambient temperature change has been effectively exploited here, and this device is used as a efficient, cheap temperature sensor.

The electronic air temperature sensor gauge circuit presented here is very accurate in its function, categorically due to its minimum level of hysteresis.

Complete circuit description and construction clues included herein.

Circuit Operation

The present circuit of an electronic air temperature sensor gauge circuit is outstandingly accurate and can be very effectively used to monitor the atmospheric temperature variations. Let’s briefly study its circuit functioning:

Here as usual we use the very versatile “garden diode” 1N4148 as the sensor due to its typical drawback (or rather an advantage for the present case) of changing its conduction characteristic in the influence of a varying ambient temperature.

The diode 1N4148 is comfortably able to produce a linear and an exponential voltage drop across itself in response to a corresponding increase in the ambient temperature.

This voltage drop is around 2mV for every degree rise in temperature.

This particular feature of 1N4148 is extensively exploited in many low range temperature sensor circuits.

Referring to the proposed room temperature monitor with indicator circuit diagram given below, we see that, IC1 is wired as an inverting amplifier and forms the heart of the circuit.

Its non inverting pin # 3 is held at a particular fixed reference voltage with the help of Z1, R4, P1 and R6.

Transistor T1 and T2 are used as a constant current source and helps in maintaining higher accuracy of the circuit.

The inverting input of the IC is connected to the sensor and monitors even the slightest change in the voltage variation across the sensor diode D1. These voltage variations as explained, is directly proportional to the changes in the ambient temperature.

The sensed temperature variation is instantly amplified into a corresponding voltage level by the IC and is received at its output pin #6.

The relevant readings are directly translated into degree Celsius through a 0-1V FSD moving coil type meter.

Parts List

• R1, R4 = 12K,
• R2 = 100E,
• R3 = 1M,
• R5 = 91K,
• R6 = 510K,
• P1 = 10K PRESET,
• P2 = 100K PRESET,
• C1 = 33pF,
• C2, C3 = 0.0033uF,
• T1, T2 = BC 557,
• Z1= 4.7V, 400mW,
• D1 =1N4148,
• IC1 = LM308,
• General Purpose Board as per size.
• B1 and B2 = 9V PP3 battery.
• M1 = 0 – 1 V, FSD moving coil type voltmeter

Setting Up the Circuit

The procedure is a bit critical and requires special attention. To complete the procedure you will need two accurately known temperature sources (hot and cold) and an accurate mercury-in-glass thermometer.

The calibration may be completed through the following points:

Initially keep the presets set at their midways. Connect a voltmeter (1 V FSD) at the output of the circuit.

For the cold temperature source, water at about room temperature is used here.

Dip the sensor and the glass thermometer into the water and record the temperature in the glass thermometer and the equivalent voltage outcome in the voltmeter.

Take a bowl of oil, heat it to about 100 degrees Celsius and wait until its temperature stabilizes down to about 80 degrees Celsius.

As above, immerse the two sensors and compare them with the above result. The voltage reading should be equal to the temperature change in the glass thermometer times 10 mill volt. Didn’t get it? Well, let’s read the following example.

Suppose, the cold temperature source water is at 25 degrees Celsius (room temperature), the hot source, as we know is at 80 degrees Celsius. Thus, the difference or the temperature change between them is equal to 55 degrees Celsius. Therefore the difference in the voltage readings should be 55 multiplied by 10 = 550 mill volts, or 0.55 volts.

If you don’t quite get the criterion satisfied, adjust P2 and continue to repeat the steps, until finally you achieve it.
Once the above rate of change (10 mV per 1 degree Celsius) is set, just adjust P1 so that the meter shows 0.25 volts at 25 degrees (sensor held in water at room temperature).

This concludes the setting of the circuit.
This air temperature sensor gauge circuit can also be effectively used as an room electronic thermometer unit.

3) Room Thermometer Circuit using LM324 IC

The 3rd design is probably the best one as far as cost, ease of construction and accuracy is concerned.

A single LM324 IC, a 78L05 5V regular IC and a few passive components are all that is needed to make this easiest room Celsius indicator circuit.

Only 3 op amps are used from the 4 op amps of the LM324.

Op amp A1 is wired to create a virtual ground for the circuit, for its effective working. A2 is configured as a non-inverting amplifier where the feedback resistor is replaced with a 1N4148 diode.

This diode also acts as the temperature sensor, and drops around 2 mV from every single degree rise in the ambient temperature.

This 2 mV drop is detected by the A2 circuit and is converted into a correspondingly varying potential at pin#1.

This potential is further amplified and buffered by A3 inverting amplifier for feeding the attached 0 to 1V volmeter unit.

The voltmeter translates the temperature dependent varying output into a calibrated temperature scale to produce the room temperature data quickly through the relevant deflections.

The entire circuit is powered by a single 9 V PP3.

So folks, these were 3 cool, easy to build room temperature indicator circuits, that any hobbyist can build for monitoring the ambient temperature variations of a premise quickly and cheaply using standard electronic components, and without involving complex Arduino devices.

4) Electronic Thermometer Using IC 723

Just as the above design here too a silicon diode is employed like a temperature sensor. The junction potential of a silicon diode goes down by approximately 1 millivolt for each degree centigrade, which allows temperature of the diode to be determined by calculating the voltage over it. When configured as a temperature sensor, a diode offers the benefits of high linearity with a low time constant.

It could additionally be implemented over a broad temperature range, from -50 up to 200 C. As the diode voltage needs to be assessed quite accurately, a reliable reference supply is necessary.

A decent option is the IC 723 voltage stabilizer. Even though absolute ti value of the zener voltage within this IC can be different from IC to another, the temperature coefficient is extremely small (typically 0.003% per degree C).

In addition, the 723 is known to stabilize the 12 volt supply throughout the circuit. Observe that the pin numbers in the circuit diagram are only suitable for the dual -in - line (DIL) variant of the IC 723.

The other IC, the 3900, includes quad amplifiers where just a couple of are utilized. These op amps are designed to work a little differently; these are configured as current driven units instead of as voltage driven. An input could best be considered to be the transistor base in a common-emitter configuration.

As a result, the input voltage is often around 0.6 volt. R1 is coupled to the reference voltage and a constant current hence moves through this resistor. Due to its large open loop gain, the op amp is able to adapt its very own output in order that the exact same current runs into its inverting input, and the current through the temperature -sensing diode (D1) thus stays constant.

This set up is important due to the fact the diode is, essentially, a voltage source having a specific internal resistance, and any kind of deviation in the current moving via it might as a result create a variation in the voltage which could end up being erroneously translated as a variation in temperature. The output voltage at pin 4 is hence the same as the voltage at the inverting input as well as the voltage around the diode (the latter changing with temperature).

C3 inhibits oscillation. Pin 1 of IC 2B is attached to the fixed reference potential and a constant current consequently moves into the non inverting input. The inverting input of IC 2B is hooked up by means of R2 to the output of IC 2A (pin 4), in order that it is operated by a temperature-dependent current. IC 2B amplifies the difference between its input currents to a value that the voltage deviation at its output (pin 5) could quickly be read with a 5 to 10 volt f.s.d. voltmeter.

In case a panel meter is employed, Ohm's law may need to be configured to determine the series resistance. If a 100-uA f.s.d. meter with an internal resistance of 1200 is employed, the total resistance for 10 V full-scale deflection has to be as per the calculation:

10/ 100uA = 100K

R5 must as a result be 100 k - 1k2 = 98k8. The closest common value (100 k) will work well. Calibration can be done as explained below: the zero point is initially fixed by P1 using the temperature sensor immersed in a bowl of melting ice. Full-scale deflection can after that be fixed with P2; for this the diode can be submerged inside hot water whose temperature is identified (let's say boiling water tested with any standard thermometer to be at 50°).

About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I'll be most happy to help!