US 4,282,480 AGranted

Apparatus for humidity detection

AssigneeMatsushita Electric Industrial Co., Ltd.

InventorsKatsuyuki Fujito, Seiro Hasegawa, Takehiko Unoguchi, Atsushi Nishino, Akihiko Yoshida

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Abstract

Apparatus for humidity detection comprises a humidity sensing element, the electrostatic capacitance of which varies in accordance with the ambient humidity, a fixed capacitor, first and second pulse generating circuits respectively responsive to the sensing element and the capacitor, and a computing circuit responsive to pulses from the first and second pulse generating circuit. The sensing element and the fixed capacitor are respectively charged via resistors to produce first and second pulses in accordance with voltage across the sensing element and the capacitor. The difference between the widths of the first and second pulses is detected in the computing circuit to produce an output pulse indicative of the relative humidity, thereby compensating for undesirable infuenece due to dielectric loss in the sensing element so as to accurately measure humidity irrespectively of the ambient temperature.

Claims27

Dependent claims

A humidity sensor comprising:

(a) a first pulse generating circuit including a humidity sensing element, an electrostatic capacitance of which varies in accordance with the ambient humidity, for producing a first pulse, the width of which indicates the capacitance of said sensing element, said first pulse generating circuit including a first charging-discharging circuit connected to said sensing element for charging said sensing element via a resistor, a reference voltage generating circuit, and a comparator responsive to the voltage across said sensing element and to said reference voltage;
(b) a second pulse generating circuit including a fixed capacitor for producing a second pulse, the width of which indicates the capacitance of said fixed capacitor, said second pulse generating circuit including a second charging-discharging circuit connected to said fixed capacitor for charging said fixed capacitor, a variable reference voltage generating circuit for producing a reference voltage variable in accordance with the ambient temperature, and a comparator responsive to the voltage across said fixed capacitor and to said variable reference voltage;
and
(c) a computing circuit responsive to said first and second pulses for producing a third pulse by detecting the difference between the widths of said first and second pulses.

1 more element

A humidity sensor comprising:

(a) a first series circuit of a humidity sensing element the electrostatic capacitance of which varies as a function of the ambient humidity, and a first resistor, said first series circuit being interposed between a power supply and ground;
(b) a first switching circuit connected in parallel with said humidity sensing element;
(c) a first voltage divider interposed between said power supply and ground for producing a fixed reference voltage;
(d) a first comparator responsive to the voltage at a junction between said humidity sensing element and said first resistor, and said fixed reference voltage;
(e) a second series circuit of a fixed capacitor and a second resistor, said second series circuit being interposed between said power supply and ground;
(f) a second switching circuit connected in parallel with said fixed capacitor;
(g) a second voltage divider interposed between said power supply and ground for producing a variable reference voltage, said second voltage divider including a thermister so that said variable reference voltage changes as a function of the ambient temperature;
(h) a second comparator responsive to the voltage at a junction between said fixed capacitor and said second resistor, and said variable reference voltage;
(i) a computing circuit responsive to the output signals of said first and second comparators to produce an output pulse the width of which indicates the relative humidity;
and
(j) a control circuit for controlling on/off operations of said first and second switching circuits.

8 more elements

Dependencies are inferred from the claim text.