The circuit on a breadboard

When someone learns about the Schmitt trigger, i believe that a thermostat is the first application that comes into his mind. The Schmitt trigger works exactly as a common thermostat would do. If the temperature exceeds a specific point - the "High threshold", it will arm a relay until the temperature falls below the "Low threshold". The difference between high and low threshold is called "hysteresis". The only difficult thing is how to choose the right components as there are many parameters. This circuit is an implementation of a thermostat with a Schmitt trigger. I will show you how to make one, and how to choose the correct component values to change the settings of the thermostat: High threshold, Low threshold and Hysteresis.

First of all, i will show you a circuit that i made for my PC cabinet. The circuit can be adjusted from 22.5^oC up to 50^oC and has a hysteresis of around 2^oC. If for example the circuit is adjusted to 30^oC, the relay will be armed when the temperature exceeds this point (30^oC), and it will be disarmed when the temperature falls bellow 28^oC (30-2).

So, let's see the circuit:

Starting from R1 and R2, this is the temperature measuring twin. R1 is a 10K NTC thermistor. Along with R2, they make a voltage divider. The voltage output of this divider will change according to the temperature change. This voltage is then driven directly to the inverting input of the 741, a normal configuration for a single power supply Schmitt trigger using OP-Amp. The output of the 741 drives the PNP power transistor. The output is reversed and the load is directly driven from this transistor.

As i said before, the circuit can be adjusted form 22 to 50 ^oC. This means that when R5 is turned completely to the left (0 ohms), the load will be actuated above the 22^oC, and when it is completely right (max resistance) it will be actuated above 50^oC. I chosen these values because i plan to install this circuit in my PC cabinet. This range will fit my needs. Nevertheless, i will explain how you can select parts yourself, in order to change the range and hysteresis at your will.

The circuit was originally designed to control a fan. The transistor can handle up to 600mA load

Well luckily yes, you can. I used the 2N2907 that can handle up to 600mA. So, you can simple remove the relay and instead put a 12V buzzer. Or better, you can put a PC fan! Well OK, you got me! This is exactly what i had in mind from the beginning of this project. Usually, PC fans will not draw more than 350mA. This is about half of what the transistor can provide, so a PC fan can be easily controlled. Just remember to put the positive wire of the fan on the collector of the transistor, and the negative to the ground, like the following schematic:

The positive goes to A and the negative to B. Same is if you use a buzzer that has polarity! Positive to A and Negative to B

Resistors
R1	NTC 103 10 KOhm NTC Thermistor
R2	Resistor 4.7 KOhm 1/4 Watt 5% Carbon Film
R3	Resistor 100 KOhm 1/4 Watt 5% Carbon Film
R4	Resistor 5.6 KOhm 1/4 Watt 5% Carbon Film
R5	5 KOhms potentiometer
R6-7	Resistor 2.2 KOhm 1/4 Watt 5% Carbon Film
Integrated Circuits
IC1	LM741 Operational Amplifier
Transistors
Q1	2N2907 General purpose amplifier and switching transistor

I have to warn you that this is a little bit stiff if you do it for the first time. If you are not interested in changing the setting of the above circuit, just skip this step.

First of all, you need to know some more details for the thermistor. You need to know actually, how the resistance changes in relation to the temperature. Luckily, i had ran an experiment with this thermistor. You can find this experiment here. I discourage you though to run such a detailed experiment yourself. Instead, you need to make sure that the thermistor you have changes linear. If it is not, then you will probably fail to calculate the circuit. You can have this info from your thermistor vendor.

Then, you need to know the resistance of the thermistor in room temperature. This value, corresponds to the resistance of the component when the temperature is 25^oC. You can also learn this information from your vendor. Usually, this info is printed on the thermistor. A 10K thermistor for example (like the one that i use), will have 10K resistance in room temperature.

Finally, you need to know the slope of the change. That would be the hardest part if you are not in a laboratory... What you really need to know is the resistance of the thermistor in another temperature. So, either put it in the refrigerator, or put it somewhere hot. 10-20 degrees above or below room temperature is the place where you should put it. Put your imagination to work. When you do find the resistance in this specific temperature, then draw the characteristic of the thermistor as i explain in the thermistor experiment.

Now that you have the characteristic of the thermistor, you can easily calculate the various resistance values for various temperatures of the thermistor. I explain in the experiment how to use the line equation calculator to do this.

Now it is time to decide the upper and lower temperature limits. Suppose that you want to narrow the temperature settings (and thus increase the stability and accuracy). Suppose that you want to be able to set the circuit from 20 up to 40^oC, and the hysteresis about 4^oC. First of all, you need to calculate the thermistor resistance value for these temperatures. In my case:

Now you need to run some tests to find the best R2. The best R2 is the one that will generate the biggest voltage difference for these values, while it will not be too big or too small to cause problems. It should be minimum 1/10 of the thermistor nominal resistance and not bigger than 10 times the thermistor's nominal resistance. In my case, it should be between 500 ohms and 100 KOhms. Use the Voltage Divider Calculator to solve for V2, and speed up the process. I will use a resistor 2.2 KOhm to have a reasonable voltage output. The voltage outputs will be:

I want a hysteresis of 4^oC. I have to express this value in voltage difference. According to our previous calculations, from 20 to 40 degrees, the voltage difference is 1.65 volts. Thus, for 4 degrees it is 0.33 volts.

Now you have everything to start playing with the Schmitt trigger. You have a very handy tool for this, the Schmitt trigger calculator for single power supply OP-Amp. Each resistor value change will have a particular effect on some values, while on the others they will have only a slight affect. R_FB for example (R3 in my circuit) usually changes the hysteresis. Thus, i will use a rather big resistor to have so small hysteresis (0.33V). Thats a good point to start with.

First to calculate R4 R5 and R6. I will start with the minimum voltage level. In that case, R5 is 0 as the potentiometer is fully turned to the left. Use the calculator and fool around until you find some values for those resistors, that - along with R3, will give a V High near 1.8 volts and a V Low about 0.33 volts lower... you need time and experience. The more you play with, the more you understand what is going on. I suggest you use resistor values that do exist. If you do not know them by heart, then advice the Standard Resistor Calculator.

I came up with the following results: R4=10K, R6=6.2K, R3=220K. Possibly you may find other values. That is NOT wrong. They may also be correct. There are many ways to solve an electronic problem, and sometimes this is the biggest problem in electronics. Now for the potentiometer. During this step, you may come to the conclusion that the values you found for the 3 resistors may be wrong. Unfortunately, you will need to calculate resistor pairs... Now, there are very certain potentiometer values. I suggest you try the 1K, 2.2K, 4.7K, 10K, 22K, 47K and 100K. What you have to do is, to add every time the value of a potentiometer to the value of R6. If the result of V High is close to 3.45 volts and the result of V Low close to 3.15, then you are finished. Now let's see my resistor pair... Using a 10K potentiometer i get the following values:

Are these values OK? let's see. According to my thermistor experiment, 3.53V corresponds to around 41^oC and 2.91 corresponds to 35^oC. The High value is ok, the hysteresis is changed too much though. So i have to recalculate the resistors with a larger R3 in order to have less hysteresis. This is the point where you need experience and patience... And some luck.

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