Humidity Control System

Humidity and moisture are related concepts but refer to different aspects of the presence of water in the air and soil, respectively. Humidity refers to the amount of water vapour present in the air, while moisture refers to the amount of water present in soil or other materials.

System Overview

Humidifier

According to the mechanism of mist generation, humidifiers can be classified into two categories: ultrasonic and evaporative. Ultrasonic mist generators are suitable for this greenhouse project because they create a fine mist that can be evenly spread in the air, supplying plants with necessary moisture.

In the project kit, you should have received a set of ultrasonic mist generator module including a circuit board and piezoelectric disc with micro-holes.

This circuit board functions as an ultrasonic signal generator. To conduct a quick test, power this board with 5V using micro-USB cable, which therefore enables its internal power amplifier circuitry and generate a 108kHz output signal capable to source a decent amount of current that resonant the piezoelectric disc.

Design a Timer

If the humidifier is set to be activated once in a while, a timer circuit is needed to turn on the device after a specific interval, therefore control timing using a circuit is the topic of discussion in this session.

There are several methods to control timing in a circuit, one of the simplest and low-cost way is to use an RC circuit. The capacitor voltage in an RC circuit has a well-defined exponential relationship, which gives timing information by monitoring capacitor voltage.

However, the time constant of an RC circuit is typically measured in milliseconds or shorter. For applications that require timing control in the range of hours, it is recommended to use specialized timer integrated circuits.

In this project we will introduce the CD4020 counter that is often used to control something in hours. This is a 14-bit binary counter first introduced by Texas Instrument in 1970s and is still a reliable and cost-effective solution for many simple timing control tasks.

Interfacing a CD4020 is simple. By feeding a clock signal to the INPUT (pin 10), the 12 buffered output pins may divide clock frequency by different power of 2. The term “buffered”, as you have just learned, means it can be directly connected to other devices (unless it loads very large amount of current).

The outputs Q1, Q2, ..., Q14 represent the count of the input clock frequency divided by a power of 2. For example, if the input is fed to a clock signal of frequency 1Hz, the output frequencies of each pin is calculated in the table.

Mono-stable Circuit and 555 Timer

CD4020 is able to generate long period square wave while the issue is the greenhouse only need a few seconds for the pulse within each cycle to activate the mist generator.

According to the previous lookup table, to generate a 1-hour counter using a 1Hz clock signal, it indicates that we can simply take the Q12 pin from the chip and use it to control the mist generator. But there is an issue. Given that the Q12 frequency is 1/4096Hz, it implies that the duration of Q12 being in a HIGH state, or the pulse width of Q12 would be 2048 seconds, or 34 minutes. If we were to adhere to the previous idea, the mist generator would operate for 34 minutes every time it is activated.

Adding a mono-stable circuit, also known as one-shot circuit, is a solution. Mono-stable circuit produces a single short output pulse when triggered by an input signal. The output pulse duration is usually determined by the circuit's timing components, such as resistors and capacitors. A commonly used approach for creating a mono-stable circuit is to utilize a 555 timer.

The 555 timer is an integrated circuit first introduced in 1970s by Signetics Corp., which was designed for timer and oscillator applications.

Many useful circuits can be realized by simply knowing basic configurations of the 555-based circuits, which allow you make use of its practical applications.

In this mono-stable circuit, a pair of RC circuit (resistor–capacitor circuit) is connected to the discharge and threshold pin, which determines the pulse duration of the output being activated by the input signals. The pulse duration, t, is calculated by:

If we need to activate the mist generator for a duration of 3 seconds upon each triggering, we can select C2 = 47uF and R2 = 57kΩ.

Keep in mind to avoid small resistance (in ohms) and large capacitance (in Farads) in design unless they are required to meet specific specifications. Otherwise, very low resistance may result in the resistor burning due to high current, while very high capacitance leads to increased size and higher cost.

The last part of the circuit involves generating a clock signal that consistently feeds a 1Hz square wave to the timer, which is achieved through a 555 timer-based square signal generator provided in this project.

By combining the previous circuits in this section together, we have built a timer that serves the control signal for the mist generator.

Complete Circuit

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