help getting started

First time to anything electronic. I just need resources on setting up what I'm trying to do.

I am willing to learn and read.

What I want to accomplish is having a room whose humidity is controlled by a dehumidifier. I want this humidity to be slowly decreased over a one month time period (to dry foods slowly).

I currently have a dehumidifier (5amp, 110vac) that has only on/off functionality. I also have a love-controls.com humidity switch. This has a humidity probe to sense the %rh in the room. It then switches on/off via a 2 setting (NO NC) relay. The dehumidifier's power is connected to this relay, so that when the humidity drops or increases it is switched on or off. The love-controls humidity switch has a digital readout.

What I want to accomplish is somehow altering the setpoint of the humidity switch. For example:

Day 1 - 90% Day 3 - 85% Day 4 - 80% Day 7 - 75% etc.....

What would my best option be?

  1. To somehow hack into the humidity sensor and decrease the set point?

  2. Someone suggested I put a potentiometer between the humidity sensor and the control switch, and then use the arduino to control the potentiometer, and have it slowly increase/decrease the voltage sent by the sensor. This would be to make it seem as if the humidity in the room was at a different point.

  3. Should I ditch the love-controls.com unit and get a humidity sensor attached to the input of the arduino, and then have a relay connected as well that would switch the dehumidifier on and off? Basically accomplish everything in software?

I think the software part I could figure out. What I need help with is finding some resources on the actual electrical components...where I would need a resistor, voltages and amperages...i don't want to fry anything.

thanks!

  1. To somehow hack into the humidity sensor and decrease the set point?

Personally I wouldn't do it, unless willing to break it. Although if you could interface with it (maybe simulate pressing keys), the 110V AC part of the problem is done.

  1. Someone suggested I put a potentiometer between the humidity sensor and the control switch, and then use the arduino to control the potentiometer, and have it slowly increase/decrease the voltage sent by the sensor. This would be to make it seem as if the humidity in the room was at a different point.

This way you could only reduce the sensor's voltage. As I'm not into humidity senors, I don't know if this is what you need for your humidity ramp.

  1. Should I ditch the love-controls.com unit and get a humidity sensor attached to the input of the arduino, and then have a relay connected as well that would switch the dehumidifier on and off? Basically accomplish everything in software?

I think the most versatile/safest (no messing with 110V relays)/least change of breaking things would be this, maybe not the easiest though:

  • get some data on the sensor itself and hook it up to the arduino
  • obtain/measure a calibration curve, voltage vs. RH
  • use the arduino as the brains and let it turn on/off the "old" controller as needed by simulating the sensor's voltage.

This is what I would do. This way you could keep your present 110V setup, which I hope is safe :-)

Probe Type: 0 to 3V, 0 to 1V or 4-20 mA (depending on model) humidity probe (not included).

The probe I have is a 3V.

How would I go about figuring out the voltage/humidity curve? Is there some way I could interface this with the arduino and figure out the voltages it is putting out?

The probe has three wires: VCC, IN, GND

Forgive my ignorance, but these won’t fit in the input slots in the arduino…what should i do?

Ok, I guess you have the HS311/312 switch.

The first thing to do is to measure what supply voltage the humidity probe gets from your switch-box. That should be the voltage between GND and VCC. The voltage between GND and IN should be zero if it is the switch’s sensor input. If VCC is 5V all is fine, if it’s above that the arduino won’t be able to supply it directly. But that’s not a show stopper, just use a little DC power supply + voltage regulator.

A calibration curve can be obtained like this:

VCC

|
R1
| V2 = 3V !
POT. 1 ------------
| | V3 = 0-3V
R2 |<--------------------- IN / Multimeter
| | POT. 2 (~50k)

GND

I’d take a 10k pot for (R1+R2). Given that only little current is drawn into the IN port, and R2 is considerably smaller than 50kOhms all voltages will vary linearly with potentiometer angle. Adjust potentiometer 1 to 3V output. Then just measure V3 with a multimeter and make a table of the humidity values your controller displays vs. voltage. Done. If you’re lucky this is just a more or less straight line when plotted.

With the voltage/humidity data your arduino board can measure relative humidity ( analogRead() ) and calculate an appropriate output voltage and feed that into your switch-box ( analogWrite() ). Strictly speaking the arduino generates a pulse width modulated 5V square wave at 500Hz (hope that’s still valid), but that can be smoothed with a low pass filter to look like a DC voltage. For that you need an RC filter with a time constant greater than say 10* 1/500Hz = 20ms = R*C.
You could choose a 10k resistor and 2 µF capacitor for that.

Vin, AC

|
10k
|
------- Vout, “DC”
|
2µF (watch out for elco polarity, otherwise prepare for a little explosion and olfactory harassment)
|

GND

If there’s too much ripple left on the voltage (no steady humidity value on the display), just increase R*C. This will increase the response time of it as well, but I think you won’t need to change it every ms or so :slight_smile: