Humidity Sensor – Humidity to Frequency Output

This sensor board provides frequency output as per relative humidity value. The project is based on a unique capacitive cell, this relative humidity sensor is designed for high volume, cost-sensitive applications such as office automation, automotive cabin air control, home appliances, and industrial process control systems. It is also useful in all applications where humidity compensation is needed. The circuit is built using a low-cost NE555 timer as an astable generator, humidity sensor MK1 acts as a variable capacitor and is connected to the TRIG and THRES pin. Pin 7 is used as a short circuit pin for resistor R3. The HS1101LF equivalent capacitor is charged through R2, R3, and PR1 is set the threshold voltage (approximately 0.67Vcc) and discharged through R2 only to the trigger level (approximately 0.33Vcc) since R3 is shortened to the ground by pin 7. Since the charge and discharge of the sensor run through two different resistors, R2 and R3, the duty cycle is determined by the following formulas.

Duty cycle Calculation:

• t_high = C@%RH*(R2+R3)*ln2
• t_low= C@%RH*R2*ln2
• F = 1/(t_high+t_low) = 1/(C@%RH*(R3+2*R2)*ln2)
• Output duty cycle = t_high*F = R2/(R3+2*R2)

Refer to the lookup table below for humidity RH (%) VS frequency output. PR1 Trimmer pot provided to calibrate the fine output.

Output Table

Features

• Supply 5V (Range 5 to 10V DC)
• Current 10mA
• Frequency Output 10% RH =7155 Hz to 95% RH =6210
• Duty Cycle Approx. 50%
• High reliability and long-term stability
• Fast response time and very low temperature coefficient
• Full interchangeability with no calibration required in standard conditions
• PCB Dimensions 38.89MM X 16.99MM

Schematic

Parts List

Connections

Gerber View

Photos

Video

HS1101LF Datasheet

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