I'm an experienced SW engineer, but a noob when it comes to .NETMF & hardware. I've started working on my first project (garage controller that will monitor & control the open state of the garage doors & manage the sprinkler system) and thinking through the organization of where I want stuff.
Right now the sprinkler system controller is on the far end of the garage (about 40' from the door to the house where the garage door openers are). I'd like to put this single controller I'm building with a Netduino Plus 2 but had a concern/question about length of the wires. I wasn't planning to go wireless for cost reasons & because the distance seems a bit too far for Bluetooth reliability.
Thinking about the sensors I'm making to monitor the open state of the garage doors… the wires from the controller will be roughly 35-45' long & the wires to the sprinkler system valves will be even further… 50' or so. I understand I need to use a relay to power the 24V sprinkler valves and was planning to put that in the controller. Are either of these two wires (sensors for the garage door / sprinkler valves) too long or am I concerning myself with something I shouldn't worry about?
Another question about the connections to the controller box. I planned to create a serial connector for the garage door sensors on the exterior of the box to make the connections a bit cleaner, but I'm not sure if serial will work to power 4x 24V sprinkler valves (or if I can have 1x serial connection for each valve). Is that doable? If not, what other options would I have?
Best Answer
For the sensors, pretty much anything will work without issue, as these are low current DC signals.
For the sprinkler valves, you need to make sure the voltage drop across your wire at that distance is not too high such that the valves will fail to actuate.
You need to take into account the total number of valves on a single wire pair to get the total current draw.
Take the current for one valve and multiply it by the number of valves on your wire to get your total current draw at 24V.
Then need to calculate the resistance of the wire, which will be determined by the AWG of the wire. You can look these values up in a table online. Typically given in ohms/1000ft. Keep in mind the total resistance is effectively multiplied by 2, as you have a supply, and return wire.
Take the total current, multiplied by this resistance, and you will have your voltage drop across the cable. If 24V minus the voltage drop results in a value still high enough to operate the valves, you are good to go!