Greenhouse Heater Temperature Control

Standard resistance heaters used for space heaters sometimes have thermostats, but these are not adjustable to the low temperature settings required for winter greens. Instead of purchasing a high end programmable temperature controller, I fabricated this greenhouse heater temperature control project circuit. It cycles an electric heater. It has been operating for two winters now with good results, and I just added the LEDs so I could tell from a distance if it was functioning.

Temperature Control Circuit Schematic

Greenhouse Temperature Control Circuit Schematic

Key components are documented on the schematic – there is no bill of materials.

Power supply

Basic transformer isolated full-wave center-tapped configuration with LM7812 voltage regulator

Temperature probe

This is simply (4) 1N4148 diodes connected in series with a thermal anticipation resistor (R1) heat shrunk together at the end of a (3) wire signal cable—it is visible on some of the photos. The use of a thermal anticipation resistor is an old HVAC thermostat technique that adds negative feedback to the system by immediately heating the temperature sensing device slightly. It forces short cycles and prevents temperature overshoot. Because the amount of power to apply to the thermal anticipation resistor was unknown, I incorporated a power level pot (R6).

I later determined that it works quite well at the maximum setting, so the pot is not required. Although the temperature measurement can be accomplished via a single diode, four diodes in series are used to get the signal “out of the mud.” The inexpensive LM324 op amp has a maximum input offset voltage of 7mV, so the higher level voltage signal helps to improve performance. The diodes are biased at approx 4mA via R2.


U1C is connected as a voltage comparator with positive feedback via R5. Positive feedback prevents relay chatter. C3 is an input noise filter capacitor. As the temperature cools, the voltage drop across the probe diodes increases. When it reaches the set point, the output changes states and the positive feedback through R5 further increases the non-inverting input by 5mV to assure that it remains latched until the temperature increases and the voltage drops below the set point.


In a previous circuit we see that the temperature coefficient of silicon diodes is approx. -2mV /°C. (4) in series boosts it to -8mV /°C.

I dropped the probe in ice water and measured the voltage—2.773V in my case. Then I calculated what the voltage would be at 4.4°C (40°F). Then I adjusted the voltage at the calibration pot R8 to get 2.738V—this is the set point. Proper operation was subsequently determined by placing the probe in and out of ice water to observe cycling of the relay.

Relay driver

Q1 is a simple NPN relay driver. For more information check out this post:
Quencharc RC-1 is connected across the relay contacts to reduce arcing. These are unreasonably expensive, so I recommend using a discrete resistor and capacitor. Polypropylene is the capacitor of choice—just make sure that it has sufficient AC voltage rating;

LED Driver

U1A is another comparator that drives the LEDs. Red indicates power ON and Green indicates OFF.

Greenhouse specifications

For a guide for estimating your power requirements, compare your greenhouse with the following:

  • Dimensions: 8ft wide x 12ft long x 7ft high.
  • North side is insulated by high density foam to minimize heat loss
  • East and West ends have a double layer of plastic sheeting
  • South side and top are single layer greenhouse plastic—UV resistant—long life
  • Latitude is 40oN.


The heater is a DIY rack of surplus power resistors that work out to 500W @ 115VAC—very rugged, large and ugly… Any resistance heater may be used. I recommend one that has a low power setting as most run 1500 to 1800W. Surprisingly, the increase in my energy bill has not been noticeable. The last two years have not been cold, but this year seems more seasonable. As a result, I may have to use a larger heater—at least during 10°F and below temperatures.

Photo gallery of the greenhouse and the box

For the future

Discrete component op amp/comparator

Glossary of undocumented words and idioms (for our ESL friends)

out of the mud –idiom— increase signal strength to above noise level—electronics

high end –idiom— expensive device as opposed to a cheap (low end) device

Preferred components for the serious experimenter

LM7812ACT, +12V Voltage regulator, TO-220 package
DigiKey LM7812ACT-ND, $0.69 each
Also available in the following voltages: 5,6,8,9,10,12,15,18,24V

LM324 Pinout
LM324 General purpose quad single supply op amp
DIP-14 package
DigiKey 497-1579-5-ND, $0.45 each


Join the conversation!

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  • Jim Keith

    I placed the temp sensor inside the greenhouse about 12″ above the ground. It is enclosed in shrink tube and silicon rubber sealant (RTV) that protects it from moisture. Yes, the sun could heat it in the daytime, but in that case the greenhouse is being heated by the sun and not the heater. Wind is a non-issue.

    For checking normal operation, I simply insert the probe into a handful of snow (if snow is available) and observe how quickly it turns on.

    To calibrate, I measured the DC voltage across the diodes while submerged in a water/ice bath –from that point, I simply scaled up the set point pot voltage a little so it was sure to switch on above freezing.

    Since last winter was extremely cold, I had to increase the power of the heater to 1.8kW. Then to prevent stratification of hot air in the top of the greenhouse, I added a small fan to blow the warm air down –it switches on with the heater.

    Had some problems last winter that caused things to freeze up. My lettuce and greens survived, but most of the temperature sensitive plants died. The first time, the electrical power connector got water into it and corroded and burned up. The second time, I accidentally tripped a circuit breaker and did not notice right away, and that was on perhaps the coldest day of the winter.

    What is amazing was that our electric utility stayed on all winter –even though a very severe ice storm that brought lots of trees down. In case of power failure, I can fire up a portable generator. In case of a prolonged power outage, I have a Kerosun heater that I salvaged from a neighbor’s garbage.

  • Rudi


    I’m quite interested in your greenhouse project and have a question for which I can’t seem to get a clean answer.

    Where do you place the temperature sensor, inside the greenhouse? And how do you protect it from the sun, and moisture / wind?
    i.e. how do you know the temperatures reported are accurate, and not affected by the elements?

  • Mircea

    Jim, I really like your articles, you are doing a great job on this website. I want to ask how do I calibrate this heater to turn ON at 16oC and then turn OFF when the temperature reaches 24oC. I want to use it to warm a room with an electric heater.

    • Jim Keith

      Your on/off differential of 24 – 16° is much greater than the 0.625° differential of this circuit. To increase this hysteresis, R5 has to drop in resistance by a factor of 8 /0.625 to about 780K (use a 750K resistor).

      Then, establish your 0° benchmark by measuring probe voltage in ice water bath. Add 16° * 8mV /°C to this voltage and set the output of the calibration potentiometer (R8) to this value. This will get you into the ballpark.

      Better yet would be a simple programmable temperature control project based upon the LM35. It would have two pots to control the on /off temperatures and would be very easy to set using only a multimeter.

      It would be an upgrade of this project: