Usually, we have seen this type of equipment, a thermistor. It is a type of resistor and easy to use. But we do not see it in normal use.
In fact, we can use all semiconductor devices as a temperature sensor.
Inside semiconductors have resistances. Its resistances will reduce when it to be hot.
In the opposite way, when it is cold its resistance rises.
In fact, when the diode is in forward biased. It has voltage across about 400mV to 500mV at current about 1mA. This voltage level depends on the current and the substance used to generate these diodes.
In silicon diode, the voltage across it will reduce about 2mV in each one degree Celsius temperature.
We so can use it. Also, you can use a transistor, diode bridge and more. They are also silicon semiconductors.
In the circuit above is a simple temperature sensor diode, D1.
First, get a steady voltage power supply. The Dual +15V/-15V DC Regulator is suitable for here.
Second, a constant current comes to D1 through R1. When the temperature of D1 changes, the voltage across it changes, too.
But this changing makes voltage changes very very low. We cannot measure it on a normal voltmeter. We need a helping of the comparator.
Third, use op-amp comparator IC, LM741. We set it into a inverting compator mode.
Why use this mode?
Since we need to measure the output voltage and Temperature is in the same way.
For example, Temperature is 20 degree Celsius, it makes output voltage is 2.00V.
Thus, we use this mode.
When Vin (pin 2) exceeds Vref(pin3), output switches from High to Low.
You do not worry if you do not understand it. You do not need to know all about it. Just you can use it is enough.
Fourth, we need to use VR1 and VR2 to adjust a right of this temperature sensor diode.
Last, when setting you should use the Digital Thermometer to compare real temperature. A digital voltmeter you may have it.
The importance, you take a piece of ice, hot water. but Diode cannot touch the water.
This circuit is not accurate full 100%. It just is good for learning in Science experiments.
Not only that. Look at other ideas!
Simple Differential temperature controller circuit diagram
If you need to control two things at the same temperature. You should look at a Simple Differential temperature controller circuit diagram. It will turn on the relay switch when the both temperature sensor is different.
Some people call that Balance temperature switch. You may use it in other ways up to you.
Why should you build it?
- It uses a few components
- It is an old circuit that uses general parts so easy to buy.
- You can learn basic electronics through it.
How it works
In the circuit, we use the main components, 741 op-amps, silicon diode, and relay.
Also, I use the temperature sensor Diode, 1N4148. You may read it first.
Then, back to see the circuit again.
- D1 and D2—both diodes are the temperature sensor.
- The IC1-741—is a comparator circuit. The gain of the IC1 is very high in open-loop mode.
- Q1—driver transistor to control a relay.
- RY1—turn on-off the loads.
First, The diode sensor gets a forward-biased current. Then, there are the current flows through VR1, R1, R2, R3.
We adjust VR1 to set the current, flows both diodes to be the same temperature.
When both diodes get a different temperature. The voltage across them also are different.
Which this different voltage will come to the input of IC1, voltage comparator.
Second, If the voltage across both inputs(pin 2, 3) is just slightly different. Then, a high voltage comes out of pin 6.
Third, the base of Q1 has a bias current, it is working to drive a small relay to be on.
In fact, if D1 is hotter D2, the relay will turn on.
Last, the contact of the relay is closed, the current can flow. To turns on the other external devices such as the solenoid, coils, lamps and more.
The setting of the circuit
This circuit request a steady voltage power supply, 12V.
In the process of setting. We must place two diodes at the same temperature.
Then, adjust VR1 to close the points of the balanced circuit. By the op-amp is not working.
If both diodes get a different temperature. The relay turns on.
Next, we test to place your fingers or other heat things to catch diode-D1. The Voltage drop across D1 and pin 2 is low. It makes the voltage at pin 3 is higher. The relay will work.
If you want to really use. Please read…
The diode sensor D1 is the main. If it is hotter than D2. The voltage across it is lower than D2. The output voltage of IC1 is LOW.
So, the base of Q1 is low, it is on. Because it is a PNP type. Thus it drives the relay to works, too.
But Experience, we cannot keep the temperature in the holding. It just makes the temperature of the coils are very similar.
Which preset temperature may be increased indefinitely. Up to the maximum coil temperature heat can produce up.
And also most electronics components, using the higher temperature, makes a shorter lifespan.
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This circuit requires enough power supply. Do you have this one? If you do not have it. Look:A lot of Power supply circuit
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