Author: Wan Hua Wangchuan
The display uses an integrated temperature sensor LM35DZ to perform a temperature measurement probe, select low-power ICL7136 as A / D conversion and liquid crystal drive, use the liquid crystal block that is easy to purchase on the market to display, use the battery board on the toy to modify the power supply, This makes the entire production process simple, low cost, and realize green environmental protection, and advocates low-carbon life philosophy.
The display has a wide range of ranges, in addition to availability of places where it is often required to test temperature, it is also particularly suitable for places with sunless and no power, such as field vegetable greenhouses, marine fish farms, tobacco natural fermentation library , Wild maternity, etc.
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I. System Circuit and Working Principle 1. The system circuit is shown in the working schematic of the temperature display, and the above is mainly a temperature measurement circuit and display circuit.
2. The working principle The temperature measurement portion uses the LM35DZ to induce the temperature because the output voltage value is linearly proportional to the Celsius temperature, and the voltage can be directly sent to the ICL 7136 for A / D conversion, and then the 7136 drives the LCD to display the real-time temperature value.
The system selects multiple low power devices, using comparative simple circuits, and uses optical control switch technology, stopping powering on the system in the dark, ensuring that the display works under very low power, saving energy, and uses solar cell power supply To make full use of new renewable energy.
3. The temperature measurement principle analysis Precision integrated temperature sensor LM35DZ is a three-terminal device, the output voltage is linear, the proportional coefficient is + 10.0mV / ° C, that is, when the temperature is 25 ° C, the output voltage is + 250mV: When the temperature is OOC When the output voltage is 0.0mV.
This can directly connect its 2 feet to the positive signal input terminal IN +.
With the voltage-adjusted RP1 as a reference voltage, the RP1 is preferably used to use a precision multillar potentiometer to adjust it to make the reference voltage UREF + = L.00V. The In-end and the UREF-end are short of the COM in the actual circuit.
For the use of ICL7136, there is not much explanation here, and its usage and the familiar ICL7107, ICL7106 is basically the same.
Second, the system low power consumption and energy saving analysis How to achieve energy saving effect by reducing power consumption.
1. Use low power devices to reduce the power consumption of the entire system by selecting some of their own power consumption.
The single piece of three-bit A / D converter uses ICL7136, which is produced in INTERSIL, which is currently widely used in a single-piece three-semi-A / D converter. The voltage range is wider: 6.8 ~ 15V, the typical operating current is about 1 mV, and its own power consumption is less than 1mV, which is equivalent to 1/20 of the ICL7106, with a 9V multimeter battery, can work for more than 3,000 hours.
Temperature detection The LM35DZ is the temperature sensor produced by the State, which is linear with the temperature standard of the Celsius. At normal temperature, there is no need for additional calibration processing to reach the accuracy of ± 1/4 ° C. Its power supply mode has two types of single power supply and positive and negative dual power. The operating current is about 50μA in single power mode, and the operating voltage is wider, and it can work normally within the supply voltage range of 3.8 ~ 20V, very power saving.
2. Simplify circuits, improve circuitry reduction power to simplify circuits and optimize improvements to some una-reasonable circuits to achieve the purpose of consumption.
Low voltage detection and decimal point display circuits are shown in Figure 1, and conventional circuits are generally built with a CD4070 four-oriented or door integrated circuit, and the ICL7136 itself is fully utilized, and the characteristics of capacitive charge, discharge are utilized. The device implements the display and low voltage detection and prompt of the decimal point. Reduce power consumption by simplifying and improving vintage circuits.
The clock oscillator of the ICL7136 is composed of an inner inverter F1, F2, and an external blocking element R0, C0. By increasing the value of RO, CO, the FO is reduced to 20 kHz, which reduces the working frequency of ICL7136, so that the measurement frequency is reduced, and the measurement rate is not about 1 / second, so it does not affect the test speed and does not hinder People look at the real time of temperature. With this reduction of operating frequencies to achieve depletion, energy saving purposes.
In addition, there are many situations that allow instruments to sleep without having to work. For example, people have a rest at night, no one cares about the temperature, this time the monitor work is purely waste. For energy saving, a light-control switching circuit is designed, as long as some shine during the day, the photosensitive resistor RP2 varies, and the higher voltage is loaded on the base of the triode 9013, which can be saturated, which is equivalent to the power switch On the upper, the display can work properly; when a dark is dark, the photosensitive resistance resistance value varies greatly, and 9013 enters the cut-off state, the power switch is broken, and the entire system is almost no current, in sleep. This will also greatly save energy and play a good consumption. During the darkness of the day, the triode 9013 has a short transition phase by saturated to the cutoff, and the power supply supply ICL7136 is insufficient. At this time, the lower voltage display icon with low voltage in the left side of the liquid crystal block is prompted at the moment. The temperature value is not allowed, and it is inconvenient to record. In response to this phenomenon, in order to ensure that the reading is accurate, D2 is connected in series to raise the LM35 supply voltage, ensuring that the output voltage is linear before ICL7136 stops.
After the above modes, the power consumption of the display is greatly reduced, and the normal operating current of the system is about 110μA.
Third, system making and commissioning 1. Making (1) The method of making the LM35 temperature measurement probe is to directly weld it directly on the board, but in this way, it is generally only detected the temperature of the air around the display. Sometimes you want to test the temperature of other objects, it will be packaged. The packaging method is as follows: first use the two-core shield line to be welded to it, and the metal shield line is generally connected to the third foot, and the thermoplastic tube is heated in the welding.
Then put it in a stainless steel sleeve having a diameter of 5 mm and a length of about 200mm, and can be replaced by a corresponding thickness of waste antenna, and then seal both ends with 703 silica gel, and then dry the silica gel, and then use the thermoplastic tube package. I.e. The temperature of the gas and liquid substance can be detected long distances.
(2) Similar products in the production of solar cell power supply are generally powered by the mains or 9V batteries, so that the problem of power outages or batteries will be replaced, it is more troublesome. Now I use solar cells and buckle charging batteries to supply power and can be used for a long time.
Now there are a variety of solar LED small flashlights on the market, as a small gift of the toy class, the price is very cheap. You can buy 2, after removing the plastic housing, you can easily get a 9V DC power supply in tandem in the solar panels.
At sunlight, each solar cell can produce 4.5V left and right voltages. On the other hand, on the one hand, the display can be powered directly, and the excess electric charge can be charged 2 3.6V buckle lithium battery.
When the external light is weak, the solar cell voltage is insufficient, and the lithium battery is automatically switched to the display power supply. Due to the low power consumption of the whole machine, even if the rain is rainy weather or in a dark environment, it can work for half a year.
(3) The production system of the circuit board can be made on a multi-functional cave plate of 100 mm × 150 mm. If there is conditional, the PROTEL software can also be drawn, and the engraving machine is used on a single-sided copper foil plate.
Generally placing a three-bit LCD liquid crystal display on the upper end of the board, which installs the 40 pin IC socket, according to the definition of the foot, is carefully connected to the LCD display and the LC socket with the short wiring.
Plug the peripheral device and welded, and 2 solar panels are bonded to the board with the circuit board. After checking, the ICL7136 can be plugged in, and the test system is powered.
2. Bronching first, a hydrome temperature of a 100 ° C is inserted into the ice water mixture with the temperature probe, see if it is 00.0 ° C, if the deviation can be appropriately changed, so that the display is 00.0 ° C; then the mercury thermometer and The temperature measurement probe is taken out, and after the air is balanced for a while, it can avoid thermal impact loss of mercury thermometer and LM35, and then put it together into boiling water, adjust RP1, so that the display is 100.0 ° C. This makes it repeatedly adjusted, and the actual temperature can be displayed very accurately.
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