Humidity Indicator and Controller

Sani Theo


Presented here is a simple humidity indicator and controller. In industries such as textile, the change in moisture content has a direct impact on the properties of fabric, such as tensile strength, elasticity, fibre diameter and friction. Therefore the process is executed within a permissible humidity environment only. Depending on the type of fabric and the process being undertaken, the requirement of particular humidity level varies. Cotton and linen have to be processed at very high relative humidity (RH) levels of around 70-80 per cent, since they are very brittle. Wool requires RH levels of around 65 per cent. Silk needs to be processed between 65 and 70 per cent. With this circuit, you can not only monitor humidity levels between 30 and 90 per cent RH but also control it.

Fig. 1: Circuit diagram of humidity indicator and controller

Circuit and working
The circuit diagram of humidity indicator and controller is shown in Fig. 1. Humidity sensor SY-HS-220 is used for sensing humidity levels and LM3914 dot/bar display driver to drive the LEDs (LED1 through LED7) corresponding to sensed humidity levels. The LEDs indicate the humidity level from 30 per cent RH to 90 per cent RH, as shown in the circuit. Transistors T1 and T2 are relay drivers through which electrical devices can be switched on or off.

Fig. 2: Humidity sensor module
Fig. 3: Relative humidity (RH) versus output voltage

Humidity sensor (SY-HS-220). Humidity sensors are gaining popularity in the field of measurement and control technology. The sensors for measuring water vapours as RH can be classified by the transduction scheme that they use to convert water vapour concentration into an electrical signal, viz, capacitive, resistive (DC resistance or AC impedance) and piezo-resistive.


Fig. 4: Actual-size, single-side PCB layout for the humidity indicator and controller circuit
Fig. 5: Component layout for the PCB

Download PCB and component layout PDFs: click here

The SY-HS-220 series humidity sensor is based on capacitive technology. This sensor module converts the RH directly into equivalent millivolts analogue output. The driver circuit is in-built in the sensor assembly (see Fig. 2). The rated voltage is 5V DC. Its operating temperature range is 0-60°C and humidity is 30-90 per cent RH. Its standard output at 25°C and 60 per cent RH is 1980mV DC.




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