Plant Watering

The first subsystem designed for this project was an indoor plant watering system. The purpose for this system was to alleviate the strain on my wife’s back that she experiences while carrying a watering can around to her various plants.

The implementation for this system is straight forward and relatively simple as can be seen in the block diagram given in the next image.

The activation signal comes from either the DTMF decoder circuit or from a pushbutton switch. This activates a timer which will drive a relay controlling a solenoid valve. The valve will remain open for the duration of the timers’ impulse. With the valve open water can be either drained from a gravity fed tank or connected to the house water line. The timer is also used to make sure that the plants do not get over-watered. This is done by keeping the valve open for a known quantity of time which will control the water flow to the plant.

A popular timer chip is the 555 timer made by a variety of companies. 555 timers are used in many different systems because they can be easily configured for either astable or for monostable operation. They also require a low component count and are very straight forward to work with.

While in astable mode, the timer acts like a common clock circuit providing a constantly changing pulse level that can be controlled with a great degree of precision. The duty cycle of the timer can be easily changed making this circuit popular for use with servo motors. A servo motor would fulfill the requirement for precision angular control as well as keeping with the modular theme of each subsystem. So a 555 timer is a good choice to keep in mind for use in future subsystems as well.

The monostable operation of a 555 timer is the mode of operation that is of interest for this particular application. While in monostable or ‘one shot’ mode the 555 timer will drive its output signal high for a predetermined period of time. The time that the pulse remains high is determined by the value of an RC network attached to the timer. A capacitor is charged and upon activation, is allowed to drain through a resistor to ground. The RC time constant of this setup determines the length of time for this drain to occur and in turn sets up the time frame for the duration of the output pulse.

The equation that determines the timer pulse duration is as follows:

(1)
$$t=1.1*R*C$$

Where t is time in seconds, R is resistance in Ohms, and C is capacitance in Farads. To vary the time within known limits, the resistor can be replaced with a potentiometer acting as a rheostat. This allows for simple changes to the pulse length to be made without having to take the system apart. This resistor and capacitor are labeled below in the system schematic.

The 555 timer circuit requires a low activation signal to turn on. The software for the remote access system outputs a high pulse as an activation signal. To reverse the logic fed from the microprocessor to work with the 555 timer, a 3904 NPN transistor was used to short the 555 timer trigger line to ground.

In future versions of the remote access system, a schematic similar to the one in the next figure will be used to provide high and low logic output for each individual line. A low signal will turn off one LED and light the other, while a high on the base will switch their output. Substituting a subsystem for the LEDs can be done to solve logic compatibility issues.

The schematic for the 555 timer operating in monostable mode can be seen in the next figure, which contains the system schematic for the plant watering system.

Several relays were tested in the system and none had any specific advantage over another. Eventually, I tried and settled on a Motorola TIP 110 NPN Darlington Pair to drive the solenoid valve instead of the relay. The TIP 110 works exactly like a typical NPN transistor with a higher operational voltage and current range. The TIP 110 can drive a load at 60 Volts and several amps. This was the same for the relays, but they cost several dollars whereas the TIP 110 cost fifty cents. The TIP 110 also only required 3 connections to be made instead of the four from the solid state relays.

This system was meant to be connected to either a water tank or to a house’s main water line. Depending on where the user lives the water pressure they receive can vary greatly from place to place. A typical house has about 60 psi on a water line, but this value can vary as high as 120 psi. The solenoid valve that was chosen would need to be able to withstand each of these pressures without leaking and would need to be able to be activated with one of the systems operational voltages. Many valves that match this description can be purchased online from mcmaster.com.

Most commercial solenoid valves operate from either 12 or 24 Volts direct current. With a 12 Volt power supply running a 7805 voltage regulator for the 555 timer circuit it was an easy choice to go for the 12 Volt valves. A valve from ECI valves was chosen for its high pressure rating (300 psi) and high Cv factor. A high Cv favor determines the flow rates capable for a valve, the higher the rate the higher the flow.
The ECI valve will be used when the system is installed in a residence, until then a secondary valve made by EHCOTECH was chosen for similar operational qualities. The EHCOTECH valve is also actuated by 12 VDC and is a normally closed valve to prevent water from flowing when not desired. This secondary valve can only withstand 150 psi so although it can be hooked up to a houseline it isn’t quite as secure as the ECI valve.

This valve is also rated for air systems, and the systems functionality will be demonstrated by blowing through a hose when the system is activated.

# Plant Watering System Implementation Details

The plant watering subsystem was also assembled with point-to-point soldering on a 2.75 mm proto-board. The LM7805 voltage regulator and 555 timer circuits were first tested using a LED and piezo buzzer for output. The TIP 110 Darlington transistor was then added and tested for adequate power delivery to drive the solenoid valve. Screw terminals were added to connect the driving circuitry to the outside world, and to accept a trigger signal.

Like the remote access system, this subsystem was encased within a RadioShack project box measuring 50 x 75 x 30 mm. Wires to the outside world were fed through holes drilled in the side of the project box. The wires were secured in place internally and tested externally for strain relief. A size K coaxial DC power jack was added to the side of the box and connected to the internal circuitry. This configuration is shown above in the previous image.

The test solenoid described above was attached to the system’s driving line and the circuit is actuated by applying an external voltage to the input lines. The system was tested with various driving times by changing the value of an internal potentiometer. The system is set for approximately a 5 second run time for the demonstration.

The solenoid valve was tested by connecting it to a submersible water pump. The valve would successfully only water the plants for the predefined period of time, and would remain off even in the absence of power to the system. The next image shows the completed system.

The test results met the required design parameters, and with a slight change in the external mechanical hardware connected to the solenoid valve, the system can be driven from a home line with no trouble. The subsystem is considered a success on all levels by the end user and designer both in terms of compactness and usability. This subsystem is capable of being used as a final product as designed and a PCB for easy assembly will be considered for future application.

Potential improvements include the design and use of a water level meter or soil moisture transducer to provide feedback that would prevent the plant from being over watered. This could also keep the water from overflowing the reservoir tank at the base of the planter box.

page revision: 7, last edited: 28 Apr 2008 01:54