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Thermostat Schematic Explanation



Construct a Precision Thermostat Project.

The schematic is around a MC4558 dual operational amplifier operating as a comparator which operates on a single 5 volt supply with a LM34DZ temperature sensor.

The LM34DZ operates on 5 volts and one end is positive (+) and the other end is negative (−). The center lead is the output lead and feeds through an 180,000 ohm 1/2 watt resistor to pin 3 of the sensor. I have built two of these sensors alone, but with different digital panel meters. The first sensor reads accurately with the 180,000 ohm resistor and the second sensor reads accurately without the 180,000 ohm resistor so apparently the digital panel meter you use makes a difference. Also, if you mount the sensor inside of a plastic, metal, wood or any kind of an enclosure, YOU WILL BE MEASURING THE INSIDE TEMPERATURE OF THE ENCLOSURE AND NOT THE TEMPERATURE OF THE ROOM. Mount the sensor outside the enclosure.

Pin 2 of the MC4558 sets the "HI" setting of the thermostat and pin 6 sets the "LO" setting of the thermostat. When a digital panel meter is connected to pin 3 of the MC4558, but without a decimal point, because we selected it that way on the back of the digital panel meter, the reading on the digital panel meter will have numbers only and actually read room temperature, the first 2 or 3 digits is degrees F, and the last digit on the right is tenths of a degree F. The actual measurement put out by the sensor is in millivolts (i.e. .758 volts, being 75.8oF and showing on the digital panel meter as 758).

A voltage is put on the pin 2 which sets the "HI" limit and a voltage is put on the pin 3 which sets the "LO" limit. It also is indicated by a digital panel meter with a switch indicating the "HI" reading and then switched indicating the "LO" reading.

On my schematic drawing, in the lower right hand corner, you will find a drawing of a 2N2222 transistor. Actually, there will be 2 of them. One transistor will be for the "HI" setting and the other transistor will be for the "LO" setting. With regard to this circuit, be sure when you connect the voltage to it, the variable resistor in each circuit is set on the end closest to ground, otherwise it will apply bias to the transistor and probably go into current saturation. You set up both circuits the same. With a voltmeter applied to the emitter, shown connected to the top of the variable control, slowly change the resistance while observing your reading on the voltmeter, until it reaches .8 volts, then stop. Not all 200 ohm controls will be exactly 200 ohm, but you have set both controls where they will read .8 volts (which would be a setting for 80°F), but we are not setting anything yet. We are just enabling what it is capable of. Both transistors will be drawing microamperes and not more than 2 milliamps, so there is no necessity for a heat sink on either of these 2 transistors. We have finished the capabilities of both. They are now capable of settings of 80°F to 70°F, for both heating and air conditioning. THIS THERMOSTAT IS FOR GAS HEATING AND AIR CONDITIONING ONLY.

There is another drawing on the schematic of another 2N2222, located down from the top right of the drawing. This makes a total of three 2N2222s. This one is connected to a 5 volt relay, but because a specific relay with a specific resistance has not been specified, the transistor will be drawing a different amount of current, depending on what you select. Likewise the resistor shown hooked up to the base, will also vary. Basically, the 2N2222 has a current gain of 200, so if you have a relay requiring 10 milliamps, you would need to feed into the base 50 microamps (200 X .000050 amps=10 milliamps). Since this transistor draws more current than the other 2N2222s, IT REQUIRES A HEAT SINK.

The digital panel meter MUST be of the 2 V range and on DC. The ones I have used have an accuracy of .8% and vary in physical size. On the back side is an "IN" and "GND" for input and a "POWER" with the drawing of a battery, indicating by the drawing what is positive and what is negative. Different digital panel meters have different power requirements and most in the microamps range. None, so far, have required sensor amplification, but if you are going to hook the sensor to an analog meter, that's different. On the schematic, one digital panel meter is hooked up to the LM34DZ sensor and the other digital panel meter, by switch, is hooked up to the "HI" or "LO" input to the comparator.

Most thermostats have a switch on the faceplate, which selects HEAT, OFF and COOL. There is another switch that selects whether the FAN (blower) is on AUTOMATIC or ON. This one has one also. However, there are 2 sections on the HEAT, OFF and COOL connect you either to Q or the complement of Q (in effect HEAT or COOL), through the switching characteristics of the 7400.

Now let's set our switch for HEAT and then set our other switch for "HI" and then let's set the reading on the second digital panel meter where it will indicate 765. We have now set the heater so that when the room temperature reaches 76.5°F, the burner will turn off, but the blower will continue to run a little longer until all the metal surrounding the heater has cooled off and then the blower will discontinue. This will actually make the room warmer and if it is too warm, change that setting to a lower and comfortable setting, but not too much.

Now let's set our switch for "LO" and then let's set the reading on the second digital panel meter where it will indicate 736. We have now set the heater so that when the room temperature reaches 73.5°F or lower, the burner will turn ON, but the blower will not turn ON immediately until just a little later. Blowers are not designed to come ON or OFF right away until they either cool off to a certain temperature or heat up to a certain temperature, both out of your selective control.

How does a comparator work? When the positive (+) input is higher in value than the negative (−) input, a fixed positive (+) voltage is sent out on the output pin of that particular comparator. Translated, if a voltage of .766 or more appears on pin 3, pin 1 of that comparator will output a positive voltage of about +4 volts, because the value of .766 volts and higher on input positive (+) pin 3 is higher than .765 volts on the negative (−) pin 2.

The same rule applies to the 2nd comparator. Now we will have positive (−) output voltages to turn "ON" a circuit and to turn "OFF" a circuit. Ultimately, we intend to use the positive (+) output from one comparator to drive a transistor and turn a relay "ON". In the HEAT cycle, when the room temperature goes below 73.6°F, we want the heater to turn ON, and we want to LATCH it (stay-on) while the room is heating and even when the temperature of the room later is now above the "LO" setting and no longer has the positive (+) input exceeding in value the negative (−) input value, normally required to keep the comparator output voltage present. So, in the HEAT cycle we need to LATCH the circuit when it turns on. However, the opposite is true in the COOL CYCLE. When the room temperature exceeds a certain value, we need to turn the air conditioner ON and LATCH it, in the higher setting.

We do this with the 7400 circuit. Next to the drawing is a table, which shows the output of the 7400 according to what the input is of the 7400. By selecting Q or the complement of Q, we will have the result we want to the 2N2222 and the intended consequences.

I had a circuit using an SCR until later and deeper thinking, I came up with this circuit. Later thinking reduced my previous components of 3 times what I had to what I now have. Before I make my printed circuit board, I am testing the use of a thyristor in place of the output 2N2222 and relay. I have to be careful. With a lot more thinking, I could even become dangerous!