Implementation Project

When I showed my idea to my friend Kevin he immediately thought of an application in his home for the MiniSBC – control his sprinkler system.  His current controller has the following characteristics:

1. Controls up to 12 zones
2. Has one minute resolution
3. Only one zone at a time may be activated
4. No communication method

Using an embedded device to control a sprinkler.

His home currently only uses eight of the 12 available zones so the MiniSBC is able to handle his requirements.  The programming is completely under our control so we can activate as many zones in parallel as we want – limited only by our power supply.  We can choose the RCM we want based on the communication technology we want to employ.  In this case he wanted to use Wi-Fi so we used the RCM5400W.

In order to allow for local control of the system an LCD is interfaced via one of the two RS232 ports.  The 4 x 20 character LCD we selected is from Matrix Orbital and is connected to the MiniSBC via one of its two RS232 ports.  In addition, a small printed circuit board was developed to allow for operator input.  This is a very simple board (see schematic 4) which has three LEDs (driven by LCD outputs), a five position joy stick switch and three push button switches.  A basic menu system was developed which enables the operator to enter and modify a watering schedule.  Some of the basic features of the Menu are:

• Enter the current date and time
• View the stored watering schedule
• Create and modify the schedule
  • Resolution is 1 minute
  • Any combination of zones may be activated
  • Once the schedule for a day is entered it can be copied to other days
• Store the schedule in flash

A Relay Board was initially developed to switch the control voltage to the sprinkler solenoids, see Schematic 5.  The board contains essentially only two circuits: 5V switching power supply and eight relays.  It is designed to use a “wall wart” (from a Rabbit product) which has two 24VAC secondaries at 800ma each.  The relays have 5VDC coils and are driven directly by the open drain outputs of the MiniSBC.

There are four connectors on the board:

• 6 pin socket for power
• 3 pin header to supply +5V to the MiniSBC
• 10 pin socket which mates to the output header of the MiniSBC
• 12 point screw terminal for connections to the sprinkler solenoids

The board is designed such that the output pin header of the MiniSBC can plug directly into the input socket of the relay board.  This allows for stacking the two boards without requiring a cable.  Since the RCM plugs directly onto the MiniSBC this yields a fairly small package with three stacked printed circuit boards.  The dimensions, in inches, are: 3.5 x 2.25 x 1.5.

The MiniSBC can be seen inside with a Wi-Fi RCM5400W inserted.

I decided to use a switching regulator on this board because I wanted higher efficiency than a series regulator would offer.  The RCM5400W draws about 600ma at 3.3V when transmitting.  That is almost two watts.  The LM2576 has about 85% efficiency which means that it will dissipate about 0.3 watts.  This means that no heatsink is required so I simply soldered the TO220 case to a copper pour.  If I had used a series regulator it would have had to dissipate over 14 watts (based on having a 24VAC input) between the regulator IC and a series resistor.  In this case the additional complexity is well worth the effort!

As it turned out, this board was not successful in switching the solenoids.  It took only a few on-off cycles to cause the relays contacts to fuse!  The relays are rated at 200VAC and 0.5 amps so I am a bit puzzled as to why they did not work.  I installed an R-C suppression network on each output to see if that would help but it did not.  All I can think of is that there must be quite a surge current when first applying the 24VAC to the solenoid through the relay contact.

I then decided to design a Triac based board, see schematic 6.  The Triacs I selected are the same ones as are in the original controller that Kevin has.  They are “Sensitive Gate” devices and work in all four quadrants.  They are rated at 600VAC and 4amps with a guaranteed maximum I_GT of 5ma.

You can see from the schematic that the power supply is identical to that of the relay board as are the input and output connectors.  The 74HCT540 is needed to invert the signal coming from the MiniSBC board which has normally off, open drain outputs.  As such, the output voltage is normally high which would cause the Triacs to turn on.

The outputs of the Triac circuits switch essentially opposite to those of the Relay Board.  The Relay Board is a “high side” switch in that the solenoids are connected between the relay outputs and ground.  The relays connect 24VAC to one side of the solenoids which would turn them on.  The Triacs are a “low side” switches in that the solenoids are connected between 24VAC and the Triac which completes the circuit to ground when turned on.

The size of the Triac board is same as the Relay Board and the connectors are in exactly the same places.  This allows the MiniSBC to use either one interchangeably.  Even though the Relay board did not work out in this application there are many applications where it would be a better solution.  One of which is that it can switch both AC and DC while the Triac Board can only switch AC.