The Power Module

This page outlines two different kinds of power supply. One is for DC power (this is to control low-voltage things like the water valve). The other is for AC power (this is to control standard, houshold voltage and is more dangerous to work with).

Local circuit

For this how-to, we're going to start with the local circuit. The reason is that this circuit will allow you to control any source voltage (as long as it's within the rating of the relay and wiring).

The circuit is  very simple. It consists mainly of the relay which controls the source voltage. There is an indicator light (LED and 1k resistor) so you know if the power is on.

This circuit really needs to be built on a proto-board to make any sense, so there is no example of it on your breadboard.


Here is a view of a possible assembly of the components on a 1 inch proto-board.
Here is a back view. Notice the screw-terminal -- this is where you will attach the two ends of wire that you are controlling.
And this is a top view. Notice that there is a small piece of paper wedged between the resistor and the RCA plug -- this was particular to this specific board because the resistor was too close to the RCA plug and could have caused an electrical short.
This is the underside of the proto-board. If you use short pieces of wire as jumpers on the bottom side like this, you can fit more stuff on a smaller board.

DC power

To get nice, clean, well-regulated DC power, we will be using an old computer power supply, also known as an ATX power supply. That little box that used to power your old pc can deliver a range of good clean voltages for all sorts of electronic projects. And most importantly, you can also combine the +12v and the -12v to get a total of 24v DC -- enough for your Orbit valve or other garden hardware.

Here is a link to an article about How to Convert a Computer ATX Power Supply to a Lab Power Supply. There are other turoials out there on the web.

It is VERY worth while to do this conversion. These old ATX power supplies serve as reliable bench-top workhorses for years to come.

AC power 

CAUTION: The AC power module makes use of household AC power which is 120 volts in the US (even more dangerous if your voltage is higher). This voltage can kill you if handled irresponsibly. Please slow down, take your time, and take proper precautions. Above all, never work on electrical wiring while it is plugged in.

(see the parts page)
  • small relay
  • screw terminal
  • RCA jack
  • LED
  • 150ohm (approx.) resistor
  • 1 in proto-board
  • heat-shrink
  • film canister (or similar small plastic enclosure)
  • sturdy, AC rated cable
  • male plug end (x2), and female plug end (x1) -- see note

Note: the extra male and the single female plug ends are to make the AC power unit free-standing. You could combine this module with the water-valve module if you don't feel like putting on the extra plug ends for something that will likely stay plugged in all the time anyway. Personally, I like the free-standing unit.


Please note that this particular build is on kind of a tight scale -- the components are very close together. If you are not comfortable working this tight, you might consider using a larger proto-board which also means you will need to select a larger plastic enclosure.

This is the film canister with the heavy-duty AC cable run through the bottom of it already (sorry there are not photos of the assembly of that).

The cable has a crimp connector for the wire that runs straight through. Then the other wire (the two wires sticking out of the film canister in the photo) is long enough for you to remove the board for service.
Now the wires have been locked down in the screw-terminals on the board. All we have to do is shove the board into the canister so that the wires bunch up neatly behind it.
Once the board is in, you will want to secure it with a bolt or any solution that won't let the board move around when you plug the signal wire in -- i.e. the RCA cable.
This is the finished AC power module. You can use this inline anywhere you want to control AC power (up to the rating on the relay) with a 5v signal (such as Arduino).

about how-to parts contact GardenBot home forum