Humidity meter

A humidity meter comprising;

• a humidity sensor (1) which provides impedance depending upon humidity to be measured,
• a humidity-frequency converter (2) for providing an alternating signal of frequency which depends upon the impedance of said humidity sensor (1),
• a differentiation circuit (3) coupled with output of said humidity-frequency converter (2) for providing a differentiated signal by differentiating of output of said humidity-frequency converter (2).
• a wave-form shaping circuit (4) coupled with output of said differentiation circuit (3) for providing an output pulse when an output of said differentiation circuit exceeds a predetermined threshold level,
• said differentiation circuit (3) and said wave-form shaping circuit (4) comprising a pulse width modulator for controlling pulse width of said alternating signal,
• a feed back means (5) for providing a control potential at an output of said wave-form shaping circuit (4) to said differentiation circuit (3) to adjust the time constant of said differentiation circuit (3),
• an integrator (6) coupled with the output of said wave-form shaping circuit (4) for providing a potential proportional to frequency and pulse width of the output of said wave-form shaping circuit (4),
• an output terminal coupled with the output of said integrator (6) providing measured humidity,
• said differentiation circuit (3) comprising an capacitor (31) with one end coupled with the output of said humidity-frequency converter (2), and a series circuit of a variable impedance element (32) having a control input (323) coupled with said fee back means (5), and a non-linear impedance element (33) having larger impedance when the potential applied thereto is low as compared to when the potential applied thereto is high, said series circuit being connected between the other end of said capacitor (31) and ground.