High temperature controller based on K-type thermocouple. PIC16F676 - Thermometers - Constructions for home and garden High temperature thermometer on pic and thermocouple

PIC16F676 Application: soldering station, control of high-temperature processes, etc. with PID control function of the heating element

I decided to insert a thermometer into my laminator, a K-type thermocouple thermometer. To make it more informative for me, I believe that a hobby radio amateur cannot be content when only two LEDs “POWER” and “READY” are lit on such a device. I arrange the scarf for my details. Just in case, with the ability to cut it in half (this is some versatility). Right away with a place for the power part on the thyristor, but for now I’m not using this part, this will be my circuit for a soldering iron (when I figure out how to attach a thermocouple to the tip)

There is not enough space in the laminator (the mechanisms are located very tightly, you know in China), I use a small seven segment indicator, but that’s not all, the whole board doesn’t fit either, this is where the versatility of the board comes in handy, I cut it in two (if you use the connector, the upper part fits many of the designs on the pins from ur5kby.)

I set it up, first I do as stated in the forum, I don’t solder in the thermocouple, I set 400 (although if this parameter is in memory, this item will disappear), I set the variables to approximately room temperature and exactly to boiling point,

Such a controller theoretically operates up to 999°C, but at home such a temperature is unlikely to be found, at most it is an open fire, but this heat source has strong nonlinearity and sensitivity to external conditions.

here is a sample table.
and also for clarity

So there is little choice in choosing a source for adjusting the controller readings.

There is no more playing with buttons, everything can be collected,
I used a thermocouple from a Chinese tester. And a post in the forum advised me that this thermocouple can be multiplied, its length is almost half a meter, I cut off 2 cm.

I make a transformer by twisting it with charcoal, a ball is obtained, and to the two ends it is exactly the same, along a copper wire, for good soldering to my wires.

The reason for assembling this circuit was the breakdown of the thermostat in the electric oven in the kitchen. Having searched on the Internet, I didn’t find a particular abundance of options on microcontrollers, of course there are some, but all are mainly designed to work with a temperature sensor like DS18B20, and it is very limited in the temperature range of upper values ​​and is not suitable for the oven. The task was to measure temperatures up to 300°C, so the choice fell on K-type thermocouples. Analysis of circuit solutions led to a couple of options.

Thermostat circuit - first option

The thermostat assembled according to this scheme has a declared upper limit of 999°C. This is what happened after assembling it:

Tests have shown that the thermostat itself works quite reliably, but I didn’t like it this option lack of flexible memory. Sewing the microcontroller for both options is in the archive.

Thermostat circuit - second option

After some thought, I came to the conclusion that it is possible to connect here the same controller as on the soldering station, but with a little modification. During the operation of the soldering station, minor inconveniences were identified: the need to set the timers to 0, and sometimes an interference occurs that switches the station to the SLEEP . Considering that women do not need to remember the algorithm for switching the timer to mode 0 or 1, the circuit of the same station was repeated, but only the hair dryer channel. And minor improvements led to stable and “interference-free” operation of the thermostat in terms of control. When flashing AtMega8 firmware, you should pay attention to the new fuses. The following photo shows a K-type thermocouple, which is convenient to mount in the oven.

I liked the work of the temperature controller on the breadboard - I started the final assembly on printed circuit board.

I finished the assembly, the operation is also stable, the readings in comparison with the laboratory thermometer differ by about 1.5°C, which is basically excellent. When setting up, there is an output resistor on the printed circuit board; I have not yet found an SMD of this value in stock.

The LED models the heating elements of the oven. The only caveat: the need to create reliable common land, which in turn affects final result measurements. The circuit requires a multi-turn tuning resistor, and secondly, pay attention to R16, it may also need to be selected, in my case it is 18 kOhm. So, here's what we have:

In the process of experimenting with the latest thermostat, more minor improvements appeared that qualitatively affected the final result, look at the photo with the inscription 543 - this means the sensor is disconnected or broken.

And finally we move from experiments to the finished design of the thermostat. I implemented the circuit into the electric stove and invited an authoritative commission to accept the work :) The only thing that my wife rejected were the small buttons on the convection control, general power supply and airflow, but this can be solved over time, but for now it looks like this.

The regulator maintains the set temperature with an accuracy of 2 degrees. This happens at the moment of heating, due to the inertia of the entire structure (the heating elements cool down, the internal frame is temperature equalized), in general, I really liked the scheme in the work, and therefore it is recommended for independent repetition. Author - GOVERNOR.

Discuss the article THERMOREGULATOR DIAGRAM

Recently, due to the frequent use of various step-down, step-up, charging and control modules, a need has arisen for a thermometer with a wide measurement range. Since the available multimeter did not have a temperature measurement function, I thought about purchasing a separate device. I immediately rejected immersion thermometers - they are too inertial. Pyrometers, although they allow you to measure temperature remotely, are deterred by their price and do not shine with quality. At least those that came into my hands were not impressive.
As a result of the search, an electronic thermometer TM 902C was ordered for $3.99


There are a great many similar devices on Aliexpress, but I settled on this one for the following reasons:
- highly specialized device without additional functions;
- wide measurement range;
- the device is equipped with a TR-02 thermocouple with an upper measurement limit of 750 degrees Celsius.

There is another modification of the thermometer - powered by two AAA elements, but complete with a TP01 thermocouple with a measurement limit of 350 (400 according to some sources) degrees. I didn’t see any point in buying a TP02 thermocouple separately and turned a blind eye to power supply from Krona.
What do the manufacturer and the seller declare to us according to the instructions in a language that we all understand)?

Although few of us actually understand the language, at least a few technically literate people will understand that the device:
- with its dimensions 24 * 72 * 108
- powered by 9 Volts (Krona, 9F22);
- relative humidity ≤ 75%;
- capable of measuring temperatures from -50 to 1300 degrees Celsius (1370 according to the instructions);
- works with type K thermocouples of the corresponding range.

Judging by the information in the instructions, the device errors range as follows (in Celsius):
From – 40 to – 20: -± 3 degrees;
From -20 to – 0: -± 2 degrees;
From 0 to 500: -± 0.75-1 degree;
From 500 to 750: -± 1%;
From 750 to 1000 and from 1000 to 1370: could not interpret accurately.
The most common thermocouples are TP01 and TP02 with ranges from -50 to 350 (400) and from -50 to 750 degrees Celsius, respectively.
When purchasing, the seller was asked a question about what kind of thermocouple would be included in the kit.
Assurances were received that the thermometer would measure temperatures from -50 to 750 degrees, i.e. The kit will include a TP02 probe, which is confirmed by further tests.
Externally, the device is made very carefully, the casting is of high quality.

Weight with battery and thermocouple

The back cover is secured with two screws. The board is also secured with the same screws - simply, reliably and economically.
The display is secured to the board with two screws and two latches.

The viewing angles are wide.
Inside, the casting of the case is less careful, which is not critical.

The board is made of getinax.
The quality of processing of one of the four ends of the board (do not forget about the price of the device)

The 1.9-inch display is connected to the board via a conductive rubber band, so I didn’t remove the screen - it’s unlikely to be possible, and then put it back correctly.

There are eyelets on the edge of the screen for fastening with screws to the case - in this case, such a fastening scheme is not used.
There are light traces of flux, but I think this will not affect the performance in any way.

As you can see, there are almost no elements on the board - there is probably a blob microcircuit hidden under the screen, which is responsible for processing the signal from the probe, performing calculations and displaying information on the screen.
Having studied the inner world of the device, I moved on to field tests.
At first, I used an immersion kitchen thermometer and a room thermometer to compare readings. The indoor one did not inspire confidence for a long time and was subsequently excluded from the competition program.
Refrigerator freezer

Immediately after removal from the freezer, the submersible one showed 0.2 degrees lower, but it is not possible to photograph at the same time due to the rapid reaction to changes in the temperature of the object being monitored and the inertia of the submersible thermometer.
Outdoors

Veranda

Room

Hot water


Boiling point of water


Next, soldering irons were used as a heat source. The immersion thermometer is no longer used as it is difficult to adapt to point source heat, and it’s difficult to warm up the entire body.






On last photo it can be seen that the temperature of the heating element is above 400 degrees, which indicates that the kit actually contains a TP02 thermocouple.
During the tests, the fiberglass braiding of the thermocouple cord was slightly damaged - it fell into the flame of a gas stove. However, this can also be considered a test - it was not burned, but only slightly changed color.


The advantages include:
- narrow specialization of the device;
- decent appearance and quality of execution;
- complete with thermocouple TP02;
- it seems to me that there is sufficient measurement accuracy and, thanks to this, a wide range of measurements;
I didn’t find any downsides, other than the 9 Volt power supply and the lack of a thermocouple protective cap.