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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Automatic ventilation control in the kitchen. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Home, household, hobby The automatic machine brought to the attention of readers maintains a comfortable temperature regime in the kitchen, turning the fan on and off. However, this is not a heat stabilizer in the usual sense of the word. His work is based on a slightly different principle ... The starting point for the creation of the machine was the fact that when the stove is running, hot air is distributed throughout the room far from evenly. The heated one rises, and the cold incoming from the outside remains below. Thus, the difference between the readings of thermometers installed near the floor and near the ceiling of the room reaches 8 ° C even during the operation of the electric stove at a quarter of the power. The average air temperature, depending on the season and time of day, was in the range of 16...32°C. The boundary between the layers of air is expressed quite sharply and is clearly felt by a person. In this situation, the presence of a fan in the kitchen that mixes the air favorably affects. With its inclusion, the temperature at the bottom rises, and at the top - decreases. It is desirable to provide such a fan with a timer that automatically turns it off after a certain time. This will protect against the consequences of forgetfulness. Even better is to make a device that reacts to uneven temperature distribution and turns on the fan only when it is really needed. In the machine, the scheme of which is shown in the figure, both functions are combined. The main components of the timer are the DD4.1 RS flip-flop, the clock pulse generator on the DD1 chip and the DD3 binary counter. In the initial state of the timer, which is set by pressing the SB1 button, the output of the trigger DD4.1 (pin 2) and the input 1 of the DD1.1 element connected to it is a low logic level. As a result, the operation of the clock generator on the elements DD1.1 and DD1.2 is prohibited. A high level at the input R of the counter DD3 in all of its digits set log. 0. Transistors VT2 and VT3 are closed (it is assumed that the SA2 switch is open), the HL2 LED is off, the M1 fan motor is disconnected from the network by the open contacts of relay K1.
By pressing the SB2 button, the fan is turned on and the timer is started. As a result of a change in the state of the trigger DD4.1, a high logic level voltage from its output enters the base circuits of transistors VT2 and VT3. The HL2 LED lights up, and the triggered relay K1 supplies mains voltage to the fan. At the same time, the operation of the clock generator DD1.1, DD1.2 and the counter DD3 is allowed. After a certain number of oscillation periods of the clock generator, depending on the position of the switch SA1, the low logic level at the input 9 of the DD2.2 element will change to a high one, which will return the trigger DD4.1 and the entire timer to its original state and turn off the fan. Even before the expiration of the shutter speed, the fan can be turned off with the SB1 button and turned on again with the SB2 button, and the countdown will start from the beginning. Prolong the operation of the fan and a simple press of the button SB2. The temperature difference sensor is assembled on the DA1 comparator. Its sensitive elements are two thermistors. The first of them (RK1) is placed at a height of 2,2 m and at a distance of no more than 0,8 m from the stove horizontally. The second thermistor (RK2) is installed below the first at a height of approximately 0,6 m. If the temperature of the thermistors is the same, their resistances are equal. However, thanks to the resistor R2, the voltage at the inverting input (pin 4) of the DA1 comparator is higher than at the non-inverting one (pin 3), as a result, its output (pin 9) is a low logic level. Transistor VT1 is closed, LED HL1 is off. The fan, if it is not turned on using the SA2 button, does not work. Suppose the temperature of both thermistors increases or decreases in the same way. Together with it, their resistances change, remaining equal. Therefore, the state of the comparator remains the same. However, if the thermistor RK1 is heated more than RK2, the voltage at the inverting input of the DA1 comparator will become lower than at the non-inverting one, which will cause the comparator to switch. The voltage of the high logic level from its output will open the transistor VT1, and if the switch SA2 is closed, then VT3. The HL1 LED will light up, the K1 relay will work, the fan will be turned on regardless of the timer status. After equalizing the temperature of the thermistors, the DA1 comparator will return to its original state, turning off the fan. Capacitors C2 - C4 are used to suppress interference and pickup on long wires connecting thermistors to the device. The value of the capacitor C4 is deliberately chosen to be smaller than C3. This made it possible to eliminate the short-term switching on of the fan at the moment the supply voltage was applied to the machine. Voltage 12 V to power the machine is taken from any stabilized source. Current consumption (excluding relay K1) does not exceed 30 mA. The author used the KUTs-1 relay (passport RA3629000). Others are also suitable, for example, RES22 (passport RF4.523.023-05.01). Any type of fixed resistors can be installed in the device. Capacitor C1 - film series K73, C6 - ceramic, the rest - oxide K50-6 or K50-35. LEDs HL1 and HL2 - any corresponding glow color, for example, KIPD05A (red) and KIPD05B (green). It is possible to replace both with one bi-color common cathode, such as Kingbright's L-117EOW. Transistors VT1 - VT3 - with any letter index. The comparator K554SAZ is replaced by 521SAZ, taking into account the differences in the pin numbering. In the absence of the K561TP2 chip, the RS flip-flop (DD4.1) is assembled according to a well-known scheme from two elements of the K561LE5 chip or other OR-NOT. By lowering the supply voltage to 9 V, instead of the K561 series microcircuits, you can install their functional counterparts from the K176 series. Thermistors RK1 and RK2 - MMT-4. Their value (resistance at a temperature of +25 °C) is not critical and can reach 82 kOhm, however, the thermistors must be the same, best of all - "from one box". If there is any doubt about the identity of the characteristics of the thermistors, it is useful to check the equality of their resistance at different temperatures. When mounted in the machine, the leads of thermistors connected to their metal cases are connected to a common wire. Turning on the power of the machine, opening the SA2 switch and pressing the SB1 "Start" button, you need to make sure that the clock generator on the elements DD1.1, DD1.2 is working, the HL2 LED is lit, and the K1 relay has worked, starting the fan. Otherwise, you will have to check the correct installation, serviceability of microcircuits, transistors and other elements. If the SA1 switch is in the position indicated in the diagram, after 15 ... 20 minutes the fan should be automatically turned off, and the HL2 LED should go out. Moving switch SA1 to another position will double this time. In this case, it is not required to set the duration of the fan operation with high accuracy, but if necessary, it can be "adjusted" by selecting the values of the capacitor C1 and resistor R5. After making sure that the timer is working, they begin to establish a temperature difference sensor. Thermistors RK1 and RK2 are placed in advance so that they warm up to the same temperature. In this state, make sure that the logic level at pin 9 of the DA1 comparator is low, and the HL1 LED is off. If you heat the thermistor RK1 by a few degrees by bringing a hot object to it, the LED should light up, and after some time after removing the object, it will go out. The required sensitivity of the sensor is achieved by selecting the value of the resistor R2. It must be taken into account that during soldering, the elements of the machine are heated to a high temperature, which changes their characteristics. Therefore, after each intervention in the device with a soldering iron, it is necessary to wait a few minutes, allowing the elements to cool down. In conclusion, the best location for the thermistors RK1 and RK2 is selected empirically. Author: N. Latchenkov, Moscow
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