Thermometer Circuits

Temperature measurement using two cables

Temperature measurement using two cables

In those cases we want to measure the temperature in an area far from where it is. The imaging unit installed, you should find some way to connect the latter with the necessary sensor. The cables we use most often will have three two conductors for power sensor and one more to carry the signal representing the temperature. But it may have only two, assuming that one will carry the potential of land and the second supply voltage with the signal temperature. This is possible if used as sensors or the LM334 or the LM335. The disadvantage arises but with them is that the temperature display is provided for reference the absolute zero (Kelvin scale), which most often is not as convenient as we would like. If so the issue is a signal proportional to the temperature scale Celsius which in everyday life, you should use a different sensor, such as the LM45. ..

Thermocouple Amplifier circuit (CA3193)

Thermocouple Amplifier circuit (CA3193)

The circuit needs 15 volts and uses a precision op amp CA3193 BiMOS to amplify the signal generated more than 500 times. Three 22-megohm resistors provide large-scale output if the thermocouple opens. ..

Digital Thermometer

Digital Thermometer

A device designed for measuring time and temperature used in chemistry laboratory. The circuit employs a 89C4051, 20-pin CMOS Microcontroller with built-in 4kB code memory. Temperature was measured by LM35D, National Semiconductor Temperature sensor producing 10mV/°C. The CA3162, 3-digit DVM converts dc output provided by LM35D and sends BCD output to port1 (P1.0-P1.3). The program resided in code memory of 89C4051 was written in ‘C’ language, thermo.c. The program read BCD output from the A/D converter, performs digital filtering,10-point moving average, and sends the output reading to a 16x1 line LCD display. A 10ms cputick was used as a timebase producing 1 s for time counting. The LCD displays time in 1 s and temperature in 0.1°C resolutions...

LED Digital Thermometer schematic

LED Digital Thermometer schematic

This digital thermometer with 7 LEDs on the temperature measured with a NTC. The circuit works by using an opamp, the known 741 that the voltage difference between its positive and negative input strengthened. This gain (sensitivity) can be set P2. With P1, the bridge of P1 and R1 be adjusted...

12V Electronic Thermostat with Relay

12V Electronic Thermostat with Relay

Here is a simple thermostat circuit that can be used to control a relay and supply power to a small space heater through the relay contacts. The relay contacts should be rated above the current requirements for the heater. Temperature changes are detected by a (1.7K @ 70F) thermistor placed in series with a 5K potentiometer which produces about 50 millivolts per degree F at the input of the LM339 voltage comparator. The two 1K resistors connected to pin 7 set the reference voltage at half the supply voltage and the hysteresis range to about 3 degrees or 150 millivolts. The hysteresis range (temperature range where the relay engages and disengages) can be adjusted with the 10K resistor between pins 1 and 7. A higher value will narrow the range..

1000 Watt Thermostat controller

1000 Watt Thermostat controller

Below is a thermostat circuit I recently built to control a 1300 watt space heater. The heater element (not shown) is connected in series with two back to back 16 amp SCRs (not shown) which are controlled with a small pulse transformer. The pulse transformer has 3 identical windings, two of which are used to supply trigger pulses to the SCRs, and the third winding is connected to a PNP transistor pair that alternately supply pulses to the transformer at the beginning of each AC half cycle. The trigger pulses are applied to both SCRs near the beginning of each AC half cycle but only one conducts depending on the AC polarity. ..

Thermometer with 10 LED scale

Thermometer with 10 LED scale

This LED thermometer is designed for in home use, to read temperatures between about 60 and 78 degrees Fahrenheit. It is based around a precision temperature sensor IC, the LM34DZ. This sensor require no calibration and can measure temperatures of between -50F and +300F. While the circuit shown here does not use the full range of that sensor, it can be modified to do so by simply changing the voltage reference to U2 at the sacrifice of precision...

Simple Thermocouple Amplifier circuit

Simple Thermocouple Amplifier circuit

Continuing with the thermocouple interface concept, the next step is to amplify the TC's millivolt signal into a more readable analog voltage, on the order of 0 to 5VDC. This simple circuit fits the bill. The LM358N is a dual op-amp IC. I'm quite sure any op-amp IC would do fine, just be sure it can accept a Vcc which is compatible with your battery pack. The IC draws ~ 1.0 mA. The resistors form a feedback loop into the op-amp, with a gain described in the schematic, and based upon the resistances R1 and R2. I used 100K and 1K respectively for the breadboarding, delivering a gain of roughly 100. This works well with the TC's range of up to 40 mV, with the output then being from 0 to 4VDC...

Precision thermocouple signal amplifier

Precision thermocouple signal amplifier

This circuit greatly expands upon the capabilities of circuit 02. A thermocouple signal is quite small, and the long thermocouple leads often induce quite a bit of noise into the system. This new circuit is far more accurate, stable, and less immune to noise. It makes use of a Linear Technologies LT1025 chip to provide CJC (Cold Junction Compensation), an LTC1098 8-bit 2-channel Analogue to Digital Converter, and an LTC1050 chopper amp. ..

Room thermometer with LEDs

Room thermometer with LEDs

This circuit uses a germanium diode (marked Ge) as a temperature sensor. I used an OA90 diode but others should do. The 'ring' of Tr1 and Tr2 bias each other. Tr1's collector current is stabilized by the 5v6 zener, causing Tr2 to operate at about 5mA: this collector current flows through the LED to generate about 1.5v bias for Tr1 so the current through the Ge diode is stabilized. The voltage across the diode will depend on its leakage current and you may need to alter the 1K5 to get a sensible range with a different diode. The voltage across the diode is inverted by the first 'op-amp' (in fact one section of an LM339 comparator used as an op-amp) to drive the LED line, shown in the pecked box. This LED line has been explained elsewhere on 4QD's pages, but it is biased at constant current by Tr3...

Electronic Thermostat

Electronic Thermostat

Here is a circuit that implements phase-control of heaters, adjusting their power smoothly from zero to around 95% of the rated power, keeping the room temperature to within 1 or 2 degrees, while using few components. Notice that this is entirely an AC design. The TRIAC, DIAC, capacitor, resistor, potentiometer and thermistor are all AC devices. I did not have a suitable MOV at hand, so I used two Zener diodes in back-to-back fashion. The poorest aspect is that I don't know of the existence of any AC transistors, so I had to pair two complementary ones, joining the collectors through diodes, thus creating my AC transistor from 4 parts!..

Advanced LCD Graphic thermometer

Advanced LCD Graphic thermometer

A unified thermometric controller that can be programmed with simple scripts, integrating the "classic" thermometer/controller pair. You can build a variety of simple machines with the same hardware and a different script : a charting thermometer, a vending machine that dials your number when empty, a leavening cell. thermometer with 1°F (0.5C) resolution. four switch inputs, four relay outputs. only an handful of cheap parts. graphic LCD display...

PC thermometer schematic

PC thermometer schematic

This incredibly simple thermometer plugs on any free serial port. Does not make use of any programmable components as microcontrollers. It gives temperature readings accurate to 0.5°C with no calibration. It's cheap, so I've put one on any PC I use. And it is so nice to have the temperature shown on the Windows taskbar, that a million friends asked me to build one! Since I have no time to build a million pcTHERMs, I give you the plans and the software to build one on your own. ..

Heat sensor relay switch

Heat sensor relay switch

The Thermistor, or NTC (Negative Temperature Coefficient) of 10K, is a standard type. Most types will work. The one in the diagram is a 10K model made by Fenwal (#197-103LAG-A01). The resistance lowers as the surrounding temperature increases which affects the output (pin 6) and energizes the small relay and Led1(optional, just cosmetic and can be left out). P1 is a regular Bourns trimmer potentiometer and adjusts a certain range of temperatures. I used a 50K, 10-turn type for a bit finer adjustment but any type will work. R2 is optional in case your relays tends to 'chatter' a bit. It provides a bit of hysteresis when the set temperature of the thermistor reaches its threshold point. ..

Temperature Fan Controller

Temperature Fan Controller

This circuit controls very accurately a fan of any size. Just adjust the associated resistors for a different type like the R6 resistor of 100 ohm, 2 watt type and you're all set. The above circuit diagram is for a small 12 volt fan, the size and type determined by the user. Temperature is sampled via the 1N4148 diodes and presented at pins 2 and 3 of the differential type 741 op-amp. R7 (10K) is used to create a voltage difference between the inverted and non-inverted input pins 2 and 3 of the 741. All signals presented at pin 2 will be inverted on the output pin 6. Obviously then, the input pins are very important...