JPS649581B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a humidity detection circuit, and more particularly to a humidity detection circuit that detects humidity using a humidity sensing element whose resistance value changes exponentially depending on humidity.

In general, resistance-type humidity-sensing elements whose resistance value changes depending on humidity change exponentially with humidity, so when detecting humidity using such a humidity-sensing element, temperature compensation and aging are required. It is desirable that the humidity detection circuit is capable of obtaining an output that changes linearly with respect to humidity as an output after performing change compensation. Additionally, there is a high resistance measuring device that uses a bridge, but this outputs an analog signal, whereas a humidity detection circuit that is directly connected to a control circuit using a microcomputer outputs a digital signal. It is preferable that

Note that there is a device proposed in Japanese Patent Laid-Open No. 54-21789 that outputs temperature as a digital signal using a temperature detection element whose capacitance changes depending on temperature.

The present invention has been made in view of the above, and its purpose is to provide an output linearly with respect to relative humidity even if a humidity sensing element whose resistance value changes exponentially depending on relative humidity is used. An object of the present invention is to provide a humidity detection circuit that can change and generate a digital signal.

The present invention is characterized by a differentiation circuit that differentiates a pulse signal from a pulse generator, a humidity sensing element whose resistance value changes exponentially depending on the differential wave signal obtained by this differentiation circuit, and relative humidity, and a comparator. , the ratio of the amplitude of the differential wave input to this comparator and the DC signal is made to be proportional or inversely proportional to the resistance value of the humidity sensing element, and the change in relative humidity is determined from the comparison of the level of the differential wave and the level of the DC signal. The humidity detection circuit is comprised of a logarithmic conversion circuit that can obtain a pulse whose time width changes linearly, and a time width measurement circuit that measures the time width of the pulse.

The present invention will be explained in detail below with reference to the embodiments shown in FIGS. 1, 3, and 4, and FIG.

FIG. 1 is a circuit diagram showing an embodiment of the humidity detection circuit of the present invention, and FIG. 2 is a time chart of signals of various parts in FIG. In FIG. 1, 1 is a pulse generator, and a pulse signal A [a in FIG. 2] generated by the pulse generator 1 is sent to a differentiating circuit constituted by a capacitor 2 and a resistor 3. Therefore, it is differentiated, resulting in a differential wave signal B shown in FIG. 2b. This differential wave signal B is passed through a resistor 4 to an amplifier 6.
is input to the inverting input terminal of Reference numeral 5 denotes a resistance type humidity sensing element whose resistance value changes exponentially depending on humidity, and the humidity sensing element 5 is connected in a feedback circuit from the output side of the amplifier 6 to the inverting input terminal. Therefore, a differential wave signal having a waveform symmetrical with respect to the time axis is applied to the humidity sensing element 5, thereby eliminating deterioration that occurs as a result of polarization due to DC voltage application, which tends to occur in the humidity sensing element 5. I can do it.

Now, assuming that _the voltage amplitude of the pulse signal generated from the pulse generator 1 is V ₀ , the positive voltage portion V _B of the differential wave signal A _shown in FIG. /RC) ...(1) Here, C: the capacitance of the capacitor 2, and R: the resistance value of the resistor 3. If the resistance value of resistor 4 is R ₀ and the resistance value of moisture sensing element 5 is _R
The differential wave signal expressed by equation (1) is amplified as follows.

_{V C = ( R _} A comparator 8 compares it with a constant voltage V _T . Letting T be the _{time from} when the output V _C rises to attenuates to V _T expressed by equation (2), _{V T} = (R becomes. Therefore, _T = _{RCln ( R} The relationship _{with X} is R Then, T=-αRCH+D...(7) Here, D=RClnB+A...(8) In equation (7), αRC and D are constants, so
It can be seen that T changes linearly with respect to changes in relative humidity H.

The comparator 8 generates a pulse with a time width T shown in FIG.
Gate 9 is opened only during
A pulse signal E of a constant period from the crystal oscillator 10 shown in FIG. Reference numeral 12 denotes a control circuit that controls the counting circuit 11 to count the number of pulses included in the time width T.

According to the embodiment of the present invention described above, the differential wave signal A is amplified in proportion to the resistance value of the humidity sensing element 5,
Obtain a pulse with a time width T in which this amplified signal C is at a predetermined level or higher, and convert this time width T into a time T
A pulse signal E with a constant period passes through the gate 9 within
Since the measurement is made from the number of pulses of It can be obtained as a digital signal. Therefore, it is suitable for direct connection to a control circuit using a microcomputer. Moreover, the circuit configuration is simple. Further, since no DC voltage is applied to the humidity sensing element 5, there is no problem of deterioration due to polarization.

3 and 4 are circuits showing other embodiments of the present invention, and the same parts as in FIG. 1 are denoted by the same reference numerals.
The explanation will be omitted here. In FIGS. 3 and 4, the level of the differential wave signal is fixed, and the level of the pulsed voltage to be compared with this is changed according to the resistance value of the resistive moisture sensing element 5. There is.

In FIG. 3, reference numeral 13 indicates a constant current source, and reference numeral 14 indicates a gate that supplies a pulse signal having the same waveform to the resistive moisture sensing element 5 in accordance with the signal from the pulse generator 1. The reason why a pulse signal is applied to the humidity sensing element 5 is to minimize deterioration of the humidity sensing element 5 due to polarization. Now, assuming that the magnitude of the constant current is i, the potential of the humidity sensing element 5 on the constant current source 13 side, that is, the comparison voltage V' _T is V' _T = iR _X (9). On the other hand, the waveform of the differential wave signal is expressed by equation (1). The time width when V′ _T and the differential wave signal intersect is
When T′, iR _X =V ₀ exp(−T′/RC)…(10). From this, T'=RCln( _{V 0} /RC)...(11) If the resistance value R T' changes linearly with humidity H. The following operations are the same as in FIG. Even if this is done, the effect is the same.

In FIG. 4, the comparison voltage V _T is provided by dividing the voltage between the resistance type moisture sensing element 5 and the constant resistor 15. In FIG. The resistance value of the humidity sensing element 5 is R _x , the resistance value of the resistor 15 is R ₀ , the voltage amplitude of the pulse signal generated from the pulse generator 1 is V ₀ , and the amplifier 16
When _the amplification _factor of is _β , _{the comparison voltage V ″ T when R} Become. If the time at which this comparison voltage V″ _T intersects with the differential wave signal expressed by equation (1) is T″, then βV ₀ R ₀ =V ₀ exp(−T″/RC) (13). From this, T''=RCln(V ₀ R _X /βV ₀ R ₀ ) = _RClnR If the resistance _R is the same as .The effect is the same even if it is done this way.

As explained above, according to the present invention, even if a humidity sensing element whose resistance value changes exponentially depending on the relative humidity is used, the output changes linearly with the relative humidity, and the digital signal can be,
Moreover, there is an effect that the deterioration of the moisture sensitive element over time can be reduced.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing one embodiment of the humidity detection circuit of the present invention, Fig. 2 is a waveform diagram of various signals of Fig. 1, and Figs. 3 and 4 show other embodiments of the present invention. It is a block diagram. 1...Pulse generator, 2...Capacitor, 3...
...Resistor, 5...Moisture sensing element, 6...Amplifier, 8...
... Comparator, 9 ... Gate, 10 ... Crystal oscillator, 11 ... Counting circuit, 13 ... Constant current source, 1
4...Gate.

Claims (1)

[Claims] 1. In a device that detects humidity using a humidity sensing element whose resistance value changes exponentially depending on relative humidity, there is provided a differentiating circuit that differentiates a pulse signal from a pulse generator, and a differentiating circuit that differentiates a pulse signal from a pulse generator. The obtained differential wave signal, the humidity sensing element, a comparator, and the ratio of the amplitude of the differential wave input to the comparator and the DC signal are proportional or inversely proportional to the resistance value of the humidity sensing element, and the differential wave a logarithmic conversion circuit that obtains a pulse whose time width changes linearly with a change in relative humidity by comparing the level of the DC signal with the level of the DC signal;
A humidity detection circuit comprising a time width measurement circuit that measures the time width of the pulse. 2. An amplifier circuit in which the logarithmic conversion circuit amplifies the differential wave signal in proportion to the resistance value of the humidity sensing element, and a pulse having a time width such that the output signal of the amplifier circuit is inputted and the output signal is at a predetermined level or higher. 2. The humidity detection circuit according to claim 1, further comprising a comparator that outputs the humidity. 3. A time proportional to the time during which the differential wave signal obtained by differentiating the pulse signal from the pulse generator by the logarithmic conversion circuit becomes equal to or higher than the voltage level obtained by changing the voltage level of the pulse signal according to the resistance value of the moisture sensing element. 2. The humidity detection circuit according to claim 1, comprising a comparator that outputs a pulse having a certain width.