Capable of measuring ambient temperature of -50 ° to +150 ° C, it is not only suitable for home or office, but also for air-conditioned rooms, freezers, ovens, etc.. Simple and compact, it uses the circuit as a sensor KTY10, and gives the temperature on an LCD screen of 3 + 1 / 2 digits.
A thermometer is always useful, whether in summer or winter, indoors or outdoors, at home or office, in a greenhouse or a laboratory. Whatever its shape and dimensions, it is found everywhere. It can be incorporated into any device placed on a worktop or suspended anywhere.
It is used both in our daily lives, to make very precise measurement operations in the laboratory. Whatever its size and precision, we still need.
That's why we have compiled this nifty project: a digital electronic thermometer with display on LCD screen of 3 + 1 / 2 digits, a device very useful, well finished, reliable and modern, can be used in many applications if we consider that the probe may take into account the temperature between -50 ° and +150 ° C. One can even imagine switching different probes for measurements and cyclic successive two or more different devices.
The LCD is very convenient since it allows to read the temperature at a distance and without fatigue. The compact assembly enables the device to be mounted on switchboards. Its low power consumption can be fed by a single 9-volt battery. Mounted in a special case, it constitutes a laboratory notebook, albeit very accurate.
The realization is simple and you will be satisfied. After you have explained what it is in the lines that follow, we analyze the wiring diagram.
Short Description
As you can see from the pictures, the assembly is quite small (90 x 55 mm). It consists of an LCD display, a voltmeter / driver type ICL7106, and a temperature sensor KTY10 semiconductor which outwardly resembles a transistor in TO-92 but with only two legs . The thickness, voluntarily reduced its easy installation almost anywhere, so it can be mounted on handheld devices.
To understand how the whole, we must first understand that this is only a digital voltmeter whose input voltage is determined by a Wheatstone bridge of which the sensor labeled "VAR" the diagram. The ambient temperature determines the voltage applied to the input of ICL7106, which brings up the corresponding value on the screen 3 +1 / 2 digit. Thus, four digits, the first of which can only be a 1, can be displayed, giving an accuracy of a tenth of a degree. The digit (DIGIT) rightmost (tenth of ° C) is separated from others by a decimal point.
The heart of the circuit is the integrated circuit U3, which is an Intersil ICL7106, Maxim, etc.. This is a very good electronic voltmeter with a driver for controlling the LCD display. It contains an analog / digital clarity, a control unit and a decoder which divides the output data of the A / D into as many groups of bits, each counting as one digit.
An oscillator connected to pins 38, 39 and 40 (OSC3, OSC2, OSC1) generates the clock signal for the converter, not only for logic control of the display (clock frequency: 200 Hz) but also to refresh the screen.
This cooling is essentially the polarization of the LCD, which is to function properly, requires the use of an AC voltage or variable on the electrode placed on the bottom (backplane) to create the necessary electric field the LCD display as well as segments of each item placed on the digital back. These digital numbers are invisible because they are performed with the transparent conductive resin.
Regarding the entry, integrated ICL7106 takes the tension with a dif ferential circuit not referenced to ground supply. He is in fact referenced to pin 30. Then amplifies it by using a comparator circuit (automatic reset network which includes pins 27, 28 and 29) and an integrator which minimizes the offset of the action, thus, never exceeds 10 microvolts. A very satisfactory result.
The ICL7106 also includes a current generator internal reference provides 2.8 V below the voltage applied to the positive supply pin (V +, pin 1).
The voltage thus obtained is available on pin COMMON (pin 32) which is precisely the point of reference voltage applied to the inputs 30 and 31.
The measurement is done in three phases which are cyclically repeated: 1) Auto-Zero: The differential input is disconnected internal points ICL7106 IN (pins 30 and 31) and connected to the common (pin 32) while capacitor C2 is charged by the reference voltage.
Then, still internal capacitor connected to pin 29 is connected mode feedback to the circuit composed of the circuit differential input of the comparator and the integrator.
2) Integration of the signal: The internal connection is restored and the feedback loop auto-zero is eliminated. The differential input is connected to pins (connections) Input ICL7106 (the usual 30 and 31), and the output of the differential stage provides a voltage which is integrated by integrator internally. This produces a sawtooth pulse which stops after a short period.
3) Derivation: During this last phase, the ramp produced by the integrator is compared with a comparator, this after the IN LO (pin 30) has been connected internally to the COMMON (pin 32) and IN HI capacitor Reference C2. The connection is thus made to force the load of the integrator capacitor (C4, on the circuit), in order to determine the value of the input voltage. The circuit Intersil account the time required for the outgoing part of the same integrator and fell to zero.
Note that to ensure that the measuring bridge (made on an arm of R2/VAR and on the other R7/R9) is the outgoing part applied to the differential inputs (pin 30 and 31) while being isolated everything else we had to disconnect the pins 32 and 35 of the spindle 30.
It is true that to allow proper operation of the voltmeter, pin 32 (COMMON) of the ICL7106 should be connected to IN LO, or to pin 30. However, in doing so, the Wheatstone bridge would be improperly connected and be referenced to ground.
Laboratory tests show that it is necessary to apply the negative terminal of the voltmeter on the reference voltage produced by the pins 32 and 35 and, of course, the positive terminal on the + BAT (9 volts DC). We have, on pins 30 and 31, a differential voltage suitable for controlling the entry of the circuit.
Obviously, the module must be powered by a single potential.
COMMON pin (32) that normally goes with the IN LO (according to the arrangement recommended by the manufacturer ...) is actually separated from the latter. It is connected to the REF pin SO (35), in order to fulfill the conditions described above.
The mechanism of action is: the temperature change varies the current through the sensor VAR. It follows a decrease of recorded voltage on its terminals, which also leads to lower power on pin 31 of ICL7106. In contrast, the state of the pin 32 remains unchanged and is a constant tension imposed at the outset by the adjustable R9, which is used to set the zero of the temperature scale when the calibration phase. Therefore, the thermal variation produces a differential voltage between pins 30 and 31, which allows the LCD to give an accurate indication. For example, if the differential voltage is 100 millivolts, the display will show 100.0 as the decimal point is placed on the last Chif eng right (through JFET Q1 controlled via pins 21 and 37 of U1).
Note that direct connection with single input (pin 30 joined to 32 and 35), can not only get a maximum precision of measurement, but also simplifies a lot the first floor of the thermometer.
It also does not reflect the typical variation of C / mA. Since, balancing as it should be the resting potential, thanks to the trimmer R9 and resistor R8, the sensor fits properly. So much for the measurement of temperature.
Analysis of the wiring diagram
Wiring diagram.We will follow the wiring diagram of which few details are worth to be clarified. Let's adjustable R3 used to record the state of the input differential stage and, therefore, perfectly calibrate our instrument acting on pin REF HI (36) and applying a voltage, which varies in some limits, between 2.8 V and 2.68 V below the battery voltage. The voltage supplied to pin 36 allows fine adjustment of the differential voltage obtained with the input stage of the ICL7106, in order to correct any offset (offset) internally.
The network C3/R5 used to the sunsetting of the oscillator clock. For its part, the network C4/R6/C5 is dedicated to the input section to achieve autozero we already know, and the integration or disintegration (this term applies here in its sense of inverse of integration, not destroy!) of the differential voltage. Resistor R1, connected in series to pin 31, allows to adapt the sensor KTY10 the ICL7106 and also acts as a filter with capacitor C1. Thus connected, the sensitivity of the voltmeter is increased to 199.9 mV full scale. With regard to the decimal point, use the pin 16, which is the first and most right and which provides an accurate reading to the tenth of a degree C activated with a trick, well demonstrated on the wiring diagram.
The JFET T1 has its source connected to pin 37 (TEC) and the gate (the gate) connected through the capacitor C7 to Pin 21 (backplane) of U1.
Same for the drain which is mounted with a pull-up resistor (maintaining the high level) to the most food and connected with pin 16 of the LCD. Between the FET conduction through rectangular signal that passes through the electrolytic C7 which polarizes the backplane of the display. It feeds the impulse segment on the decimal point, therefore, seems constantly lit, even though in reality the pulses are fast. The point display appears more distinct than the numbers.
When the display 3 + 1 / 2 digit value appears, it is none other than reading the voltmeter. It is obvious that 100 ° C correspond to 100 mV, 10 mV at 10 ° C, etc.. Naturally, the zero after the tenth millivolt is eliminated. So, the previous values correspond to the data 100.0 and 10.0. Consider this for calibration.
Note that above 199.9 mV, exceeding 200 mV, the ICL7106 switches overflow (overflow) and controls the display of the pin 2 (pin 20 of U1), which shows, in left, the minus sign (-). This indication (hardly possible anyway unless completely disrupt R3) states that the limits of measurement were outdated and that the measure is off the scale. Pin 20 of U1 is required to view the sign polarity of the applied voltage between + and - input. This is because our unit is also a measure of negative magnitudes displaying the minus sign to the left of the display and display no sign if the measured quantity is positive. The polarity is referenced at points of entry and, in practice, is whether the temperature is audessous or above zero. With the existing wiring, the minus (-) corresponds to negative values. If the measure is positive, no sign appears excluding significant figures of the measure. All that being said, we can now conclude the theoretical phase indicating that the module operates with a voltage of between 5 and 9 volts, and absorbs only 1 milliamp.
Specifications
The main features of the voltmeter proposed in these pages: - Temperature measured ....................................... -50 + 150 ° C - Tolerance of reading ....................................... + / - 0.1 ° C - Operating Temperature ...................................... 0 / 50 ° C - Supply voltage ............................................. 9 VDC - Consumption (average) ...................................... 1 mA
By operating temperature means the temperature at which the circuit must be integrated to function properly.
In contrast, the sensor KTY10 be subjected to a range between -50 ° and +150 ° C without damage or alteration of operation.
The temperature sensor
To raise the temperature of air or liquid, we used a PTC, the KTY10. This thermistor having a positive temperature coefficient, when heated its resistivity increases, while the latter drops when cooled.
Exception of tolerance and lack of linearity, we can say that thanks to the Wheatstone bridge, any change in the circuit operates at a rate of about 1 mV / ° C.
Thus, for each variation of one degree up or down, it follows a rise or a drop of 1 mV between pins 31 (connected to KTY10) and 30 (referenced to a fixed voltage through trimmer R9 which fixes the zero).
At -50 ° C, there was a decrease of -50 mV. At 0 ° C, there is no voltage drop, whereas at +150 ° C, the increase is at least 150 millivolts (150.0 on the screen).
The temperature coefficient is about 1.25 Ω / ° C. For example, an increase of 80 ° C causes an increase of 100 Ω on the thermistor. -50 ° C to +150 ° C, the amplitude is, therefore, 250 Ω cons resistance of 1.9 kΩ at 25 ° C.
Pinout ICL7106.
Our thermometer uses the sensor KTY10 is actually a thermistor (thermistor) PTC. This means that its temperature coefficient is positive: in the heat, it increases its own resistance value, and what goes down when cooled. The left graph shows the development of sensor resistance as a function of temperature: Rt = Kt x R25 = f (Ta), with Ib = 1 mA and a resistance of 2 kilohms R25. The right graph shows the relationship between the temperature factor (Kt) and the temperature itself.Practical realization
To produce the device, it must first make the film for photogravure by a photocopy of the circuit airside copper layer or transparent (the circuit board is supplied with the kit).
Having cut and pierced the map, you can start to assemble the components by following the step by step implementation plan, and taking care to respect the usual rules.
Start with the resistance and supports the integrated circuit ICL7106 (20 + 20 pin DIL) and display (LCD). For the latter, there are special connectors made of strips of contact SIL at 2.54 mm. It is also possible to solder on the circuit two female connectors with 2.54 mm, 20 pins each. One can also use two pieces of 20 pin extracts support as that used for the ICL7106.
Continue with the capacitors in ensuring the polarity of the electrolyte, and remember the two adjustable potentiometers multi-turn type vertical thumbnails that to limit the height, you should have folded after lying their legs to 90 degrees.
The FET must be placed upright in the holes provided for this purpose, placing the flat portion to the outside of the PCB. Remember to solder interconnection straps (7 total), which serve to complete the connections between components.
The circuit is now ready. KTY10 temperature sensor can be mounted outside on the deck. It should be connected to it by two pieces of insulated copper wire to a length of 10 meters or better, a small two conductor shielded cable with braided shield, the latter being connected to ground and two conductors at the terminal connecting to pins 30 and 31.
The device is complemented by the introduction of the integrated circuit ICL7106 and the display. For these two components, it is essential to follow the direction indicated on the drawing assembly, otherwise the circuit would not work.
Component List
R1: 1 MΩ
R2: 5.6 kΩ
R3: 50 kΩ multiturn trimmer
R4: 47 kilohm
R5: 100 kΩ
R6: 4.7 kΩ
A7: 47 kilohm
R8: 1 MΩ
R9: 50 kohm trimmer multiturn
C1: 100 nF multilayer
C2: 100 nF multilayer
C3: 100 pF ceramic
C4: 220 nF polyester
C5: 470 nF 63 V polyester
C6: 100 uF 16 V electrolytic
C7: 1 uF 16 V electrolytic
C9 100 nF multilayer
U1: ICL7106 driver for LCD
SAS1: LCD 3 1 / 2 digit
BAT: Battery 9 V
T1: FET BF245
VAR: sensor KTY10
Miscellaneous:
- Terminal 2 poles (2)
- Support 20 + 20 pins shall
- Strip 20 poles at 2.54 mm (2)
- PCB ref. S268.
(Unless otherwise specified, all resistors are 1 / 4 watt 5%).
Calibration
Once completed and checked the thermometer needs adjustment that is vested with two adjustable, so as to ensure proper operation and also obtain the expected accuracy.
To calibrate the device, you can proceed in various ways, although the most convenient and quickest way is to buy a digital multimeter with high input impedance. In this case, we must adjust the scale of 200 to 500 millivolts DC voltage, then connect the + and - respectively to pins 31 and 30 of the circuit ICL7106.
Then we read the value and adjusted the adjustable R3 to have the same value on the display. Thus, if the tester indicates a value of 40 mV, turn the adjustment screw R3 in one way or another, to get 40.0 on the LCD. The number displayed will always be provided with a point.
This fact must resolve the other adjustable.
For this it is essential to have a thermometer reliable witness that we have near the sensor KTY10. When reading the value, it acts on the screw of R9 to obtain exactly the same value. Consider an example.
Assuming we have 30 ° C on the thermometer indicator, and a value of 20.0 on the LCD display of our circuit. Turn the screw in one direction or another to raise the value of reading to 30.0.
Now he must make another try at zero degrees, for example, and if necessary modify the calibration of R3.
Naturally, the whole process we have just described is valid only if the circuit is powered by 9 volts DC to the + / - BAT. Power can be provided, for example, by a 9 volt battery whatsoever.
Consumption will be slightly greater than 1 milliamp.
Polarity is critical, and therefore it must be remembered that the positive and negative going to the + and - BAT. If using a battery, we recommend using a proper outlet, ensuring proper connection of the red (or black / white) on the BAT + and the black wire to the BAT.
A series of three zeros or random numbers must appear immediately after the powered device.
On the left, it is possible that the minus sign is flashing.
After adjustment, the thermometer is ready, and you can mount it in the cabinet of your choice.
The device has no usage limit, or special requirements, except that it must be kept dry and that the PCB should not be put in contact with metal objects. You can put it in one of those little boxes used for electronic assemblies or electrical installations, painting on a larger unit, behind a decorative object or as a frame, etc..
For the implementation of the sensor, remember that the maximum length of connecting wire is 10 meters. Also make sure that the legs of the sensor are never in contact with metal objects conduct electricity, let alone with any liquid. If necessary, insulate them with shrink tubing or silicone sealant.
To raise the temperature in a tank or other container, insert the KTY10 the bottom of a sealed tube or in a thin glass test tube and immerse the whole after it is attached properly.
It is obvious that in this case the reading will be done with a certain delay, due to the slowness with which the temperature of the liquid is transmitted by the glass and the glass sensor.
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