The programmable thermostabilizer on the microcontroller. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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The programmable thermostabilizer on the microcontroller. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Microcontrollers

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The need for this device arose when solving the problem of temperature control and stabilization in technological processes at one of the enterprises. For heating and cooling, steam from the boiler house and cold water from an artesian well were used there, which were supplied through dampers to heat exchangers, boilers, cooling jackets, etc. The dampers on the pipelines were of two types: purely pneumatic membrane type with a chamber, compressed air pressure in which determined its position and electromechanical with a reversible engine.

The result of the work carried out was the proposed device, which allows you to control both types of dampers using two powerful keys on triacs. For a pneumatic damper, two normally closed electric valves are installed at the chamber inlet. One of them supplies compressed air of high pressure into the chamber, the second releases it from there with the first valve closed. The electric motor of the electromechanical damper is connected directly to the device; in this case, its direction of rotation is set with the keys. Below are the electrical diagram, principles of operation and parameters of the device. Delivery options for custom-configured device boards are possible. Waiting for your suggestions.

Fig.1. Scheme of a programmable stabilizer (click to enlarge)

Details: U1 - K561LN1, U18 - K1533ID7, U7 - PIC16F84-04I/P, U9 - DS1820, U10,U11 - AOU115V, U17,U14,U15,U16 - HD1077G.

The temperature sensor U9 measures the temperature and transmits it to the microcontroller U7, which displays it on a four-digit indicator on U14-U17, controls two powerful keys on X7,X8. The keys are galvanically isolated from the rest of the circuit using U10, U11. LEDs U12,U13 light up when the keys are open. The third task of the microcontroller is to poll the keyboard connected via the X9 connector.

The scheme allows using a standard telephone keypad 4*3 keys, but only one of its columns (STL0) is supported by software, i.e. 4 buttons. They are sufficient for the functions performed. To scan the keyboard, dynamic indication pulses are used, which, after passing through the pressed key, are amplified by the amplifier U1, and, this is a feature of this circuit, they are received on the port where the data of the indicator bits are issued (RB0 for STL0). During the time when the keyboard is not being polled, the outputs of the amplifier are transferred to the Z-state, for which the RB7 port is used. Resistors R1-R3 - protective. The mains voltage 220V is supplied to the FAZA output common for the keys, and to the valve or motor, which are connected to KLAPAN1 and KLAPAN2 by other outputs.

For the most accurate temperature stabilization, the following principles are incorporated in the software control of the keys:

1. Key X7 is for increasing the temperature, X8 is for decreasing. If the temperature does not match the required one, the desired key is selected and it opens for a programmable opening time, then, during the same programmable settling time, with the key closed, the temperature is measured and the key is selected again. Thus, in the stabilization mode, it is possible to single out periods consisting of the opening time and the settling time.

2. When deciding to open the corresponding key, the increment or decrease in temperature obtained during previous measurements is taken into account. To eliminate the overshoot effect, the time elapsed from the moment the required temperature is passed to the moment when the temperature is minimum or maximum is calculated and used to determine when to open or close the damper.

3. Stabilization mode can be disabled from the keyboard. When it is turned off, the damper is closed by turning on the reducing key for a few seconds.

4. From the keyboard, the required temperature, opening time and settling time are entered and stored in permanent memory.

The DS1820 temperature sensor transmits the already processed temperature to the microcontroller in a digital serial code via a single-wire interface. The length of the connecting wires between the sensor and the device can be up to 10 - 15 meters and does not affect the measurement accuracy. More information about this chip can be found on the Dallas Semiconductor website (dalsemi.com). And the text of the subroutine used in the temperature stabilizer to obtain the temperature (working with the sensor) can be viewed here.

Documentation (Datashit) on MK PIC16F84

I wish you creative success.

Author: Vladimir Shashin; Publication: cxem.net

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