The cellar thermostat is used to ensure the most comfortable conditions for storing vegetables and fruits. There are many types of structures - some are designed for use in the basement, others - for installation on balconies. The design of balcony boxes is very simple - a box with high-quality thermal insulation, heating elements are installed inside, with the help of which the optimum temperature is maintained. Such a mini-cellar will be very useful if the balcony is unheated, and it is very cold outside in winter.
General principle of thermostat operation
The simplest homemade cellar thermostat allows you to maintain the set temperature using heating elements. Depending on the temperature difference, you can use the following heating elements:
- Powerful wirewound resistors.
- Nichrome spiral.
- Incandescent lamps.
- TENY.
If you need to equalize the temperature in a small range, it is enough to install several incandescent lamps - this is exactly what is done inhomemade incubators for growing poultry. But if you need to equalize the temperature in a wide range, you will need to use more effective means - heating elements or wire resistors.
The easiest way to maintain the desired temperature is to install a bimetal temperature sensor. It will turn off the heating elements when the set temperature is reached. But you can use a temperature sensor and a simple electronic device on the same chip. The advantage of this design is that it is more reliable - it does not have mechanical breakers. All switching work is performed by the microcircuit.
Sensor calibration
For novice radio amateurs, the most difficult thing will be setting up the device, namely the calibration of the sensor and microcircuit. To do this, several actions are performed - the reader is first immersed in water, the temperature of which is 0 degrees, then in boiling water. To calibrate the regulator, you need to measure intermediate values and put the appropriate marks.
It is possible that before connecting the thermostat, you will need to carry out the setup procedure several times. But in order not to bother with the independent manufacture and configuration of the device, you can buy a finished device and put it in the cellar without any problems. Most of the sensors are designed to work with microcontrollers, they have a digital signal at the output, which is transmitted via a bidirectional single-wireinterface type 1-WIRE. This allows you to design quite complex devices, for example, multi-point thermometers - these are devices that allow you to measure the temperature in several rooms at once.
Temperature controller LM335
Among all thermostats, one can single out the cheapest and simplest - LM335. It has several modifications - with the designations 235, 135. In the marking, the very first digit indicates the scope:
- The number "1" means that the device is designed to work in devices for the military-industrial complex.
- The number "2" - the element is used in industry.
- "3" - for installation in household appliances.
The appearance of the thermostat is the TO-92 case. The internal circuit contains 16 semiconductor transistors. Sometimes sensors can be found in the SO-8 package, but the differences are observed only in appearance - the internal circuit remains unchanged.
The principle of operation is somewhat similar to a zener diode. The stabilization voltage directly depends on the temperature. With an increase in temperature by 10 Kelvin, the stabilization voltage increases by 10 mV. In this case, the operating current of the device is 0.45-5.0 mA. In the event that the maximum current value is exceeded, the sensor will overheat and it will measure the temperature of its case.
What does the gauge show?
But how will this device behave in the cellar thermostat circuit? We need to figure this out. Let's say absolute zero indoors is 273 degrees belowzero Celsius. This is 0K, so you'll have to do a little conversion. It is in conditions where the temperature is 0 K that the sensor will not generate a signal.
As soon as the temperature rises by 10 K, the voltage will increase by 0.01 volts. And this will happen with each increase in temperature. But you need to take into account that such temperatures do not exist, and 0 degrees Celsius is 273 K. Normal conditions, according to all textbooks, are 25 degrees Celsius, or 298 K. After a few simple steps, you can determine that at a temperature of 25 degrees Celsius, the signal output of the sensor will have a voltage of 2.9815 volts.
The range of temperatures in which the device operates lies in the range of -40..+100 degrees Celsius. Moreover, its main characteristic is linear in this range - this facilitates the calculation of stresses and temperatures. And do not forget that absolute zero is 273.15 K. In precise calculations, you should not neglect even hundredths of values.
Temperature controller circuit
The device has a kind of instruction. The thermostat can be made according to the schemes given in the datasheet. This is a detailed manual that describes all possible schemes for switching on the device, its main characteristics, and features of operation. And there is no need to invent anything extra - all designs have been tested for years and work stably.
All industrial samples of finished devices of temperature controllers are madeexactly according to the schemes indicated in the datasheet. One thing to consider is that you cannot directly control the heating element, since the signal is very weak. You will need to use a FET amplifier or assembly. An amplified signal can be applied to a magnetic starter or relay.
Comparator operation
A comparator is a device that compares the temperature with a set value. Without knowing the basics of how it works, it will not be possible to make a thermostat for a cellar on a balcony. At the heart of the circuit is a comparator from LM311 - it has two inputs and the same number of outputs. Inputs:
- Straight - marked with a "+" sign.
- Inverse - has the designation "-".
The work algorithm is very simple:
- If the direct input receives more voltage than the inverse one, then the output is set to a high level. The transistor opens and the heating is turned on.
- If the voltage at the inverted input is higher, then a low level is set.
- When the response temperature is reached (it is set by a variable resistor), a transition to a low level occurs - the transistor closes and the heater is de-energized.
How to connect the device to the heater
The heating element is the load of the entire device. It is desirable that the switching elements have a margin of safety - install relays with a degree of protection corresponding to wet rooms, or magnetic starters. The signal from the microcontroller must be givento a field effect transistor and amplified. Only after that it can be used to control the coils of a relay or starter, which are included in the break in the power circuit. Since the cellar thermostat will be in a humid environment, take care of safety - install circuit breakers and RCDs.
