I suppose this is a "stupid newbie question", for which I apologise, but I've honestly done quite a lot of searching, but I think I'm looking at this the wrong way and I'm a bit stuck.
My ultimate goal is to have a thermostat, powered by Arduino, but I'm trying to start really small.
I discovered that the wiring behind the thermostat is surprisingly simple:
If I connect the ends of 2 wires, the heating is on, if I "disconnect" them, heating goes off
As an additional test, I hooked them up to a breadboard with a button in between and once again, when I push the button, it goes on! So far, so good
Now, my question is, what type of electronic item do I need to programatically connect / disconnect those 2 wires? Or another way to say it: I need a "software-controlled button", where I can tell through programming the Arduino: Connect the wires or disconnect the wires.
This seems very basic to me, but I can't seem to find the solution. In my search, the best I've found are relays but these seem to be used only for high voltage things. So, is a relay what I need here (I have this one, can that be used?) or is there some other obvious solution which i'm missing?
Is the cable you are talking about a power cable or a signal cable, do you know? If its the power supply, then I would use the relay, if its simply a signal cable, I would use a transistor.
Not knowing anything else about the device you are hacking, I really cant give better feedback.
If you post some images or a datasheet or a schematic, I'd be happy to take a look.
Standard "American" home Heat/Air Condition thermostats are 24V switch closures. Meaning... 24 Volts are in the line. This, to me, qualifies as "higher than TTL Signal level" and a relay is justified as a "switch closure" device.
fkeel:
Is the cable you are talking about a power cable or a signal cable, do you know? If its the power supply, then I would use the relay, if its simply a signal cable, I would use a transistor.
Thank you for the reply!
As I understand it, it's just a signal cable. I've uploaded a few images here:
The wires I'm talking about , which I also think are just signal cables, are 2 & 3; As I understand it, 4 & 5 are the power supply, but I'm staying away from those. 1 is not connected.
I believe it's usually AC, and that would also favor a relay. (A single transistor won't work with AC). In the thermostat, there is either a relay (maybe more than one) or in an older thermostat without a timer or battery, there is a thermally-activiated mechanical switch.
In any case, a relay is simply an electrically activated switch... It "makes" or "breaks" a connection (or multiple connections). And, the contacts on a relay are electrically isolated from the coil. A relay is the most foolproof solution.
A relay has voltage/current ratings for the coil ("input"), and voltage/current ratings for the contacts ("output"). The coil voltage has to match... To drive a relay directly from the Arduino, it needs a 5V coil. The coil current rating is what it will draw from the Arduino. (The Arduno can supply no more than 40mA, so the coil current has to be 40mA or less.)
On the other hand, the contact ratings are maximum ratings. i.e. It's OK to use a relay with 220V/10A rated contacts in a 12 or 24V low-current application.
The downside to a "big" relay (besides the bulk & higher cost) is that it usually requires more coil current, and that can mean shorter battery life in battery-operated devices, and if there the contacts get corroded over time, you might not have enough current/voltage to "burn-off" the corrosion, so an over-rated relay might have a shorter life.
Unless it's some proprietary signalling method (which is rare), it'll be 24VAC. Use the relay. It shouldn't have to carry significant current -- most of the transformers that supply the 24VAC are tiny and only good for a couple hundred mA tops. Pretty much just enough to close a bigger relay (or a similar purpose) in the appliance itself.
I would suggest using a transistor to drive the relay coil, rather than directly from the AVR. Also, use two diodes -- one in series between the transistor and relay coil, the other "across" the relay coil, backwards, as an inductive spike shunt.
