3.6.4 LM 335Z (Precision temperature sensor) :
This circuit is a very accurate temperature sensor that will switch off a motor when a predetermined temperature level is reached.
Circuit diagram:
Components
- SW1 = Toggle switch
- R1 = 10k ohm
- R2 = 10k ohm
- R3 = 1200 ohm
- R4 = 1k ohm
- R5 = 1200 ohm
- VR1 = 10k ohm
- VR2 = 10k ohm
- Power FET Transistor (BUK100)
- 7805 the voltage regulator
- Temperature sensor (LM335Z)
- 741 operational amplifier
- Motor/Solenoid (12V)
Operation:
The operational amplifier is used in the comparator mode. When the voltage at pin 3 is less than the voltage at pin 2, then the output at pin 6 is low (1.8V) approximately. When the voltage at pin 3 is greater than the voltage at pin 2, then the output the voltage at pin 6 is high (7V). R1 and R2 and the variable resistor VR1 placed in series form a voltage divider network and as such, the voltage on the wiper connection can be set to any required value. This wiper connection is attached to pin 2 of the op amp. For ease of explanation a hypothetical voltage of 3.03V is set on pin 2.
The LM335.is a temperature sensor which produces an output of 10 mV / degree Kelvin.
To convert degrees Centigrade to degrees Kelvin, simply add 273 to the Centigrade scale, i.e. 30 degrees Centigrade = 273+30 = 303 degrees Kelvin.
The LM335 converts temperature levels to the voltage levels.
Thus the voltage drop across the sensor = 303 x 0.010 =3.03V.
The sub circuit R4, VR2, LM335 is arranged as shown. Note the junction of R4 and VR2 is connected to pin three of the op amp. VR2 is placed in the circuit to calibrate the sensor.
To carry out this task, a thermometer and voltmeter are required.
The temperature in which the sensor is placed must be recorded, add 273 to convert this temperature into degrees Kelvin, and then multiply the result by 0.010. The voltage at pin 3 must be set to this value; i.e. the circuit is calibrated.
Consider now the ambient temperature is 25 degrees Centigrade. Voltage output at pin3 = (273+25) x 0.010 = 2.98V. Thus, the voltage at pin 3 is less than the voltage at pin 2, so output at pin 6 is low.
Consider now a temperature of 32 °C, Voltage at pin 3 is now 3.05 V. Voltage at pin 3 is now greater than the voltage at pin 2, so output (pin 6) now is high.
The LM 335 sensor is so precise that even a very small voltage fluctuation can upset it. For this reason the rail voltage must be constant, as the output from a small battery is liable to change a voltage regulator is inserted. To clamp the voltage rigidly to 5 V i.e. a voltage regulator-7805 is used.
Output: In this case, a special transistor is used as a switching device (BUK 100).
With less than 2V on the GATE, the transistor is switched off. No current can flow through the motor. With more than 2V, the transistor switches on, current can now flow in the 12V circuit. Note the output of 741 Op-amp is connected to a voltage divider circuit R3, R4. Connection to the gate of the transistor is from the junction of R3, R4.
CIRCUIT DIAGRAM :
Working:
Our circuit is mainly based on the working of transistor. We know that, a transistor can work in two different ways; as an amplifier and as a switch.
Here we are using transistor as a switch. Working of transistor as a switch can be understood with the help of circuit diagram given in the figure. Positive terminal of 5V battery is connected to the collector of transistor while negative terminal is connected to emitter after a lamp.
When switch ‘S’ is open, there is no current through base terminal and hence collector-emitter is also isolated. Thus, the lamp will not glow. Now if we close switch S, 0.2V battery will be connected to base, which will give very small current to the base terminal. This small current will short collector to emitter and the lamp will glow. Here lamp will glow due to 5V battery but not due to 0.2V battery.
In short, if we provide very small current to the base terminal, collector and emitter will be shorted.
In this circuit, we are controlling opto-isolator’s L.E.D. through software using printer port also called parallel port. In an opto-isolator, if L.E.D. integrated inside I.C. is lighted, it generates small current for base terminal of the integrated transistor.
Thus, the transistor is switched on. We also use one external transistor for delivering proper & operating current to the Relay.
Now, when L.E.D. is lighted by software, it switches on the internal and thus the external transistor, which further switches the relay or supply potential difference to the relay coils. Now if there is a potential difference across the relay coils, the iron core of relay coil converts into Electro-magnet.
This Electro-magnet attracts the iron connected to the moving hand of relay thus switches on the mains supply.
3.9 FLOW CHART :
probably time to remove this article
as parallel ports are very rare
It will be good to provide the circuit diagram.
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