Keywords: diode thermometer
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Author: Wang Renli
This vegetable greenhouse temperature meter does not have a special temperature sensor, no single-chip microcomputer, showing intuitive, setting over temperature alarm, low temperature alarm is simple and convenient, cost does not exceed 20 yuan. It is also possible to simultaneously display the temperature / humidity after appropriate changes. This temperature meter of this vegetable greenhouse is very practical. First, the temperature of the voltage conversion principle silicon diode has a positive conduction voltage with temperature rise and decreases, in the case of a constant current bias, the rate of change is about -2mv / ° C, which means a negative temperature coefficient. Since the constant current source is complex, it is often connected to a fixed voltage with a silicon diode in series to form a voltage dividing circuit. The temperature rises, the voltage of the silicon diode is lowered due to temperature, and the fixed voltage di-pole tube voltage = resistance voltage is increased, and the current is increased. The diode is rising as the bias current increases, and the temperature coefficient is less than the constant current bias under this reaction. Increase the bias current, diode dynamic resistance decrease, this effect will decrease. Over-increasing bias current, diode fever, measured temperature is higher than the ambient temperature. Both should be taken into account. Select R5 = 3.9k bias current about 0.8mA. Using a 33/4 digit table, the following circuit measured voltage was 0 ° C: 2145 mV, 100 ° C: 1630 mV, estimated 50 ° C: (2145 + 1630) / 2 = 1888 mV. A1: LM358 is a double op amp, and half A1_1 is used as a follower, and the other half A1_2 is used as an inverted amplifier, and its multiple is -11 times. The output was 4.97V when the output was 2.15 V, 50 ° C. The same gear measurement is used in the measurement, such as DC20V.
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Second, the segmentation comparison principle of LM3914 uses resistors to divide 2145 ~ 4972mv segment 10 equivalents, compared to the comparator and input voltage, respectively. 11 results can be obtained. Less than or equal to 2145mV, greater than 2145mV (corresponding 0 ° C), greater than 2145 + 283mV (corresponding 5 ° C) ... 10 indicators are in turn, 1 light is on, and 2 lights are bright ... form a light column. For more eye-catching, lower than 15 ° C is displayed with green lights above 35 ° C, and the red light is displayed in the middle. The RHI of the LM3914 is the upper limit, and the RLO is the adjustment limit. The interior contains 9 1kΩ voltage resistance. As shown on the right. 2145MV, 4972mV must be stable, cannot be affected by power supply voltage fluctuations or ambient temperature changes. There is a stabilizing circuit of 1.25V within the LM3914, located between 7 (UOUT), 8 (ADJ). UOUT is adjusted by RL, R2, and the calculation form is UOUT = (1 + rl / r2), given nearly 5V output of 50 ° C operational amplifier, design UOUT is 5.3V. Use WL to adjust RHI. W2 adjusts RLO, it is best to perform fine adjustment with a multi-ring potentiometer. R2 is used as a lamp current control, R2 is 1.2kΩ, each lamp current is 10 × 1.25V / 1.2kΩ = 10.4 mA. The 9th foot Dot / Bar of the LM3914 is high, and the shaped display, DOT / BAR is low, point-shaped display.
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Third, the upper and lower temperature alarm settings use double row gold fingers 01 to 010. If the upper limit temperature is 45 ° C, 09 low level passes through JP2, R9 reaches the first foot of U2, 4001 is or non-door, 1 pin input low, door 1 is equivalent to inverters, RL0 hangs 2 foots and 3 feet A negative feedback, 2 foot voltage is about 1/2 power supply voltage is located in linear amplification work points, 2 foot voltage slight change After 2-3, 5-4 two levels, passing by the capacitor C2 is forwarded to 2 feet, causing Self-excitation. The frequency is determined by RL0 and C2. 4 foots and high and low control of 4001 doors 3, 4 oscillation, producing intermittent "嘀" sound. Piezoelectric ceramic partial alarm. At this time, the base of VT1 is biased due to a low level of 02, and the VT1 is turned off. Does not affect the 09 loop. ~ When the temperature is less than 5 ° C, only 01 light is not lit, 02 ~ 010 does not light, output high level, 02 passes the base of the VT1 by JP2, R7, R8, VT1, VT1, VT1 C Extremely low, 1 pin of 4001 low, producing alarm sound oscillation. At 5 ° C ~ 40 ° C, the base of VT1 is turned off due to a low level of 02-jP1. 09-JP2 high level, 4001 1 foot high level → 3 foot low → 5 foot low → 4 foot low → 12 foot low → 11 foot high → 9 pin low → 10 pin low. All feet are fixed at a fixed level without shaking. Not alarm. JP1 and JP2 can be set, especially convenient.
Fourth, the production and adjustment of 3 glass packaged silicon diodes are welded in series, leading to two thin lines (preferably with high temperature wires), to do DLL ~ 13. It is then loaded into a thicker empty beaded pen, and the so-called "glass glue" is sealed with 704 silica gel or decoration, waiting for natural curing. It is frozen into ice, and the water is mixed in the thermos cup to ensure that both ice has water, put DLL to 13 in ice water mixture, and the digital table measures its end voltage, and waits for the number to stabilize the readout of 2145mV. Put the probe into the boiling water pot, and wait for the number to stabilize to read it is 1630mV. If it is not a plain area, the boiling point of the water is not 100 ° C, or it can be calibrated with a thermometer. First go to 2 free ice water and a boiling water, put it in the thermometer, monitor the water temperature with the thermometer, then add a spoon of water, until it is 40 ° C, put the DLL ~ 13 release, read it 1835mV; by calculation of 50qc voltage. Welding the diode, replaced with a LOKΩ multi-circle potentiometer. First, the voltage of the O ° C is adjusted, and the WL is adjusted, so that the RLO is equal to the voltage of OOC. 10 lights are all unlimited. In the second step, the voltage of 50 ° C is adjusted, and the adjusting W2 allows RHI to equal the voltage of 500c. The 9 lights are on, and the 10th lamp is just on the critical state of the bright and unlighted, so that the 10th light is on. The specific magnification of LM358 is not too important. Therefore, R3, R4 does not have to use precision resistors, and it is not necessary. There is no 33/4 digit table, you can use a 31/2 digit table + TLV431 to be replaced with the following graph. When a point to ground, such as 1.251V, when the diode is then measured, the black pen is always a point, and the reading is as 0.896 V, the actual voltage is 1.251 + 0.896 = 2.147V. This allows 3 effective numbers after the decimal point.
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Problems encountered in debug: The light-emitting lamp current is too large, which not only causes the chip to heat and the logic low level of the output end 01 N010 is high due to the internal limit of the internal limit, but U2 cannot be identified. The figure below is experimented with three methods. 1 One resistor in series, the problem is: a brightness and multi-light brightness is uneven, and the brightness between the multi-lamp and the lamp is uneven. 2 A SV glass seal regulator tube is connected in series, and the pipe is severely hot 3 access 8050 transistor buck. The effect is better, and it also heats. The VCC is changed from 12V to 9V, and the fever problem is improved. 5. Expansion to the three-handed temperature meter, the same temperature meter, the first set of temperature measuring the dry temperature, the second set of moisturizing temperature, the smaller the humidity, the larger the difference, the humidity saturation, the two indicates the same. Since the first set of thermometers is 5 ° C, the second set thermometer indicates that the temperature difference after the water is flooded, only 1 ° C is one interval. , The second 3914 R102 picks the first block 3914 of IN1. The second block 3914 RHI was raised by a diode, ie RHI2 = R102 + 0.66. Note that the selection of 6 diode characteristics is consistent and the forward voltage drop at room temperature is the same. If there is a fine difference, R253.9KΩ can be adjusted. Water evaporation is related to air flow velocity, and the probe package of the moisturizing temperature is placed in the ventilation yarn. In no strict calibration, experience can only be established. 10 lights are full of brightness to the smallest humidity, 1 or a few lights are on, indicating that the humidity is large.
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