Building the Greenhouse Temperature Controller

Temperature in a greenhouse  can differ from outside temperature up to 40F. In the winter time this can be a good thing. In the summer time this means creating a cooler environment to lower or match the outside temperature without giving up the benefits of shielding plants from rain, winds and animals as well as maintaining a humid environment.

It was in the 80’s yesterday and it’s climbing up there today too. The greenhouse still reached 100F yesterday despite turning on the fan when it was 95F inside. The plants are doing well despite the intense heat and colder nights. According to the temperature monitor, the daily greenhouse temperature can range from 42 to 120!

Today we are going to install a temperature controller. The controller will work by monitoring temperature and activating systems when certain conditions are met. The controller will:

  • Turn the fan on when the temperature reaches X and turn off when the temperature reaches X  (speed of fan is manually controlled).
  • Control Fan Speed according to increased cooling demands @ temperature Y ( Y > X)
  • Turn on the sprinklers for the evaporative cooler when it reaches X and turn off when temperature reaches X.

We initially installed a big fan  pulled out of an old AC unit. The fan is rated for about 2500 cfm on its highest setting , and unknown on its lower settings. It runs on 110-120V AC. The fan was hooked up to a power cord, a few wire nuts were added to the connections and this was plugged in  every morning to exhaust the(100F) hot air building up and allow (80F on avg) cooler air to enter through the back side, evap cooler was not on. This was enough to help while we were waiting for the parts to build the rest of the environment control system.

The picture below shows the major components of the temperature controller. Which is composed of a PID temperature controller, Solid State Relay, Single Pole Double Throw SPDT relay, and terminal strips.

To control the speed of the fan, a SPDT relay (acts like a splitter or y valve) was used in combination with a manual switch to select speeds. The lowest speed was ignored, leaving only 3 speeds to deal with(medium-low, medium-high and high). Medium-low was selected as the default operating speed (drawing about 4.7A @118VAC). When the Alarm setting is reached (temp high), the fan is kicked into a higher speed (medium-high or high). Which higher speed is determined by the position of the manual switch located just before the fan. This gives us the ability to control 3 speeds; 2 automatic and 1 manual. Being able to vary the fan between Med-High (5.5A @ 118VAC) and High (6.8A @119VAC) means that, depending on the cooling needed, there is a proportional amount of fan speed and energy available. Why use too much if you can use just enough?

BE CAREFUL, CURRENTLY THE DESIGN SHOWN HERE IS PLUGGED INTO A GFCI PROTECTED SOCKET, IF YOU DON’T HAVE ONE, YOU SHOULD CONSIDER ADDING THIS AS PART OF YOUR CIRCUIT PROTECTION. Later we will be adding some individual line breakers for each of the components, and provide individual breakers on the controller, saving a trip to the breaker box, GCFI, and making troubleshooting easier.

Main Components
-MYPIN TA-4 SNR PID Temperature Controller, single alarm + K-thermoprobe ($40)
-Omron SPDT relay 15A@125VAC rating ($7)
-FOTEK SSR-25 DA Solid State Relay, DC input, AC output ($12)

Other Components
-Toro sprinkler control box empty ($15)
-Spade connectors, insulated and uninsulated ($8)
-120V Green and Yellow Panel Bulbs ($5)
-Terminal strips, need enough space for all circuits, usually 6-10 connectors if possible, use multiples, nothing wrong, ($8)
-electrical tape ($2) Buy another roll anyway, can’t have “too much”
-8-32 or 6-32 machine nuts, bolts, and washers ($3)
-Various Wire (lying around the house? $5-15 if you had to buy a new spool or two)

Tools Needed
-Something to cut panel hole (I used hole saw and filed the square into it, took about 15min)
-The usual suspects of electrical tools:
-Wire Strippers
-Crimpers
-Needle-nose Pliers
-Flat and Phillips Screwdrivers, large and precision
-Lineman’s Scissors
-Power-drill and ½” drill bit for panel indicator lamps, 1/8” for electrical components
-Heat-shrink tubing once things are pretty final.

The system is designed so that once the fan is on, the pump for the evaporative (swamp) cooler is also turned on. Thus it is connected alongside the other output on the solid state relay.

The Power Input was screwed in and SILICONE  was used to seal the outside, helping to protect from possible water damage. The hole in the other area is for drainage in case of condensation, this unit is not supposed to be 100% watertight, only water resistant, at the same time making something watertight can also mean accidentally locking in moisture and then allowing it to condense and revaporize on a daily basis.

A closeup of the terminal strips. We see that one strip is dedicated to easily connecting outputs , this way they can always be modified depending on later applications down the line. The other terminal strip allows for easy access when swapping components in case of failure or upgrade. These also make troubleshooting a real snap and means anyone can use what you have built safely. Murphey’s Law says when something does fail, it’ll be when you’re on vacation and your buddy is taking care of your plants. This way, troubleshooting is simple for someone not even familiar with circuits.

Internally most of the exposed metal was covered in electrical tape to help insure everything stays insulated.

Shows labels on terminal strips, allowing for easy installation and troubleshooting.

With that, the system was mounted in the greenhouse, and we began plugging parts into the system. The orange cord on the left is actually the pump output socket. It’s also good to remind folks that most of the bare metal on the inside is conducting 120Volt Alternating Current, so it is advised to be careful in here and not just stick hands around. On the bright side, since there are no capacitors in the circuit, once the plug is pulled, there is no charge stored, so work can begin immediately.

Soon to be encased: A DPDT manual switch to choose the speed of the alarm triggered output (remember our fan has 4 speeds, lowed eliminated, medium selected by default, the alarm switches to the last 2 choices, which is made manually) This way there is a setting between a wintertime “high” and a summertime “high”.  (Circuit will be enclosed soon for safety , just good to test before you put it all together and then scratch the finish)

The Logic Ladder behind this is:

Cooling ON =>  ALARM RELAY ON  => Medium  => FAN
OFF => MANUAL SWITCH  “HI”   => FAN
“Med Hi” => FAN

The power basically has to travel through another selector switch that allows the user to manually select how fast the “Alarm” or “High” setting of the fan actually goes, saving power in times like winter when too much cold air may be brought in. The power will always get to the fan, but which switches are on create a logic path for the electricity to follow to power the fan correctly.

Pump output socket for the evaporative cooler. Output is started anytime the fan is turned on. It is only temperature controlled, not humidity dependent.

Output from the sprayers back to the tank is a little slow, raising it should help. Currently it uses a 600 GPH pond/fountain pump with 8.3’ of head, pumping to 6’. Raising this 12-18 inches will help water flow to the cooling pads a lot. Along with that, will also soon come an auto-fill capability, but all in due time.

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7 Responses to Building the Greenhouse Temperature Controller

  1. Jim says:

    Hi I too purchasec the TA 4 Controller but cannot change the Temp to read in deg. F. How do you program the controller from reading in deg C to F??? Thanks…jim

    • The set button needs to be held for a few seconds until the unit is in the “Menu” area. Once there, press the set button firmly to go through the menus. The temperature display unit selection is one of the last setting before the menu loops around. If you still have your manual, you will see the order it is in. After you get to it (yours will have a small C in the bottom display) Press the <–/AT button until the C flashes, and then use the arrows up and down to select F. Then press Set one last time, and the info should be saved. You can follow most of this procedure for setting other things like Integration and temperature hysteresis.

  2. Jim says:

    Thank you, I did find everything once I started reading the instructions. I reset my PID today to give a gentle cycle. I set “I” to .75 and “P” to 0.0 and left “D” set as received from the factory. Now I ramp up in temp rather fast but start slowing down about 20 degrees before target. Then a three degree cycle at set temp. Want to thank you for your responses. This little Controller is something else. It works as promised. My oven is for vacuum forming large parts. I test ran the oven today and WOW! Worked just as our plans showed it should…jim

    • The solid state relay is so much better suited for heating applications, not so much for cooling, but the level of math the little controller performs is amazing. It also learns your system after a while and after it tunes, (you can set some basic settings in, and run the auto-tune for a while and see how it does). Useful little bugger.

  3. Sam says:

    Even though I am going to be using my TA4-SNR+K in a powder coating oven, your setup gave me some very valuable information on wiring the PID. The pictures are great! Since the PID was shipped with a K -TC I will be using that configuration. I have a question. I can see that you put the red connector on pin 7 and the blue connector on pin 8, but am wondering about pin 9. Did you use a jumper between 8 and 9 with the K-TC as shown for the RTD probe in the instruction sheet? Any users instructions you can give will be greatly appreciated as the instruction sheet is bordering on very poor. 🙂

    • Hello, yeah the temp controller instructions are pretty lacking, although eventually the broken English makes sense once you start plugging things in. For the K thermocouple we have, I didn’t use a jumper to the other pins, for either heating or cooling.

      We actually use another one of these controllers and relays in a heating application for our seedling mat (in the winter, summertime is plenty warm for them) the Solid State relay is awesome when it pulses to maintain perfect temperature, but even that didn’t require the pins jumped. If the temp seems off, or rather it shows a lower temperature as things warm up, you may have the pins backwards, it happens.

      • Sam says:

        Thanks for the quick reply. I will wire mine the way you have and see how it goes. I’m feeling better about this PID after seeing your website.

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