controlling a humidistat

I am a rookie at this. (that seems to be a popular opening line)

I just got an arduino board and have figured out the leds and a couple other sensors and now I have a plan to put this to use. I have a wall controller for my Bryant HRV system. Long story short, during winter when it get really cold out here in Montana there is a humidity sensor that activates and brings in fresh, dry air from outside so the windows don't have puddles of condensation on them.

The problem is that the humidity sensor is a manual nylon thread that expands and contracts with the humidity and this trips the switch. It is very inaccurate, +- 8%. The switch is a separate component in the wall unit.

My vision is to disconnect the two wires that go to the switch unit and use an arduino and a digital humidity sensor to take the place of the switch.

Here is what I have found so far. The voltage across the two wires leading to the switch is 5V. The whole wall unit runs on 12VDC. If I short a wire across the two wires it will turn the system on. When tested with a ammeter across the two wires it draws .4 mA.

I went out on a limb and tried to use the basic "blink" code with the delay set to about 20 secs and connect grd and pin 13 to the two wires and could get the system to turn on and off every 20 seconds. Interestingly enough when the led went out the system came on. Maybe I had the wires backwards. This seemed encouraging.

Before I go any farther I figured I better check with the pros before I melt something and my wife rolls her eyes at me.

Questions: Can this work? Could I run the arduino off power coming into the wall unit? It has 4 wires coming into the box. I have the wiring diagram for the whole machine in hardcopy.

The ultimate goal would be to receive the temp from my 433MHz LaCrosse weather sensor and adjust the indoor humidity based on the outdoor temp. Like off of Practical Arduino.

Feel free to throw out any ideas.

I have the wiring diagram for the whole machine in hardcopy.

It sounds like the system could very well be controlled by an Arduino. However there are always details that only proper documentation can answer. If there is a wiring or schematic type drawing(s) in that manual and you can somehow scan or copy and post, then advice, ideas, and recommendations are bound to be more useful.

PS: My wife was raised in Montant. I met her while serving in the Air Force at Great Falls. Even though forty years has passed, I still recall how friggin cold it was that winter. :P

Lefty

Especially when you are new, it is wise to "divide" to conquer.

If the humidistat you have at the moment is, essentially, a switch, you can start by replacing that with a relay. Be sure you understand about "the diode" (for snubing reverse voltage spikes).

See...

http://www.arunet.co.uk/tkboyd/ec/ec1relay.htm

The unit which needs turning on would connect to the relay's contacts, on the right in the illustration at the page above. Get yourself a "double throw" relay, and you can switch "things" around easily, so you can have "dry air injector on" when the relay is energised OR when the relay is NOT energised. (The relay in the illustration is only single throw)

Don't abandon your "nylon thread" humidity "thing" too soon. Set things up so that the switch in that can turn the relay on and off.... get one bit... the relay... working at a time.

When you have the relay added to the circuit, you can then turn to replacing the nylon thread thing with an output from an Arduino which has a humidity sensor input.

Warning: Humidity sensors, in my limited experience, are very temperature sensitive. I.e. the "humidity" they report changes if the temperature in the room changes. This can be dealt with, in software, but is "a joy" ahead of you. Unless, of course, the air you are concerned about is always at the same temperature. But you probably have (or at least plan) "set back" provision, so the house can be cool when you are out, or in the middle part of the night?

Humidity sensor: Good news at last...

At the Arduino Playground, there are notes on humidity sensors for the Arduino...

http://www.arduino.cc/playground/Main/InterfacingWithHardware#envhum

I like...

http://sheepdogguides.com/arduino/ar3ne1humDHT11.htm

... at least as a starting point. There are some excellent humitity sensors interfaced to 1-Wire MicroLans, but they cost more, and involve fancier... not impossible, but fancier... software. But if you master the extra software issues, a relatively small bit of learning opens up a huge new vista, the world of 1-Wire....

http://www.arunet.co.uk/tkboyd/e1dhw.htm

Since the switch is DC and low amps I don't think I need a large relay that can control a toaster. Also it sounds like the arduino can't control a big relay with its own power. I was looking at something like this:

http://www.sparkfun.com/products/524

Would that work?

Thanks for all the advice already.

Warning: Humidity sensors, in my limited experience, are very temperature sensitive. I.e. the “humidity” they report changes if the temperature in the room changes.

As it should, what you are describing is the different measurements of absolute humidity Vs relative humidity. Both are valid measurements of different things involving mosture content of air.

Lefty

I don't think I need a large relay that can control a toaster. Also it sounds like the arduino can't control a big relay

You need to distinguish between the two sides of the relay, and understand the reason I suggested using one... it wasn't, primarily, about power.

The coil is turned on and off by the Arduino. It would probably be best to drive it with the unregulated voltage (9v? 12V? Whatever) you are "feeding" your Arduino's voltage regulator with. And "switich it" (turn it on/off) with a transistor.

The other side of the relay, the contacts, is what you connect to your existing humidifier circuits.

The beauty of using a relay is that "problems" can't jump past the "gap" between the contacts and the coil.