Thanks to all of you who have responded with ideas and or circuits ,and advise.

The process of growing vegetables in a greenhouse outdoors in a climate which is more somewhat consistent year round is complicated enough with the following items needing to be addressed to enable consistent production...

.adaquate sun exposure and or timed artificial light cycles
.manual or automated shade coverings on higher solar radiation days
.timed automated watering cycles dependent upon sunlight and heat per plants
.preventing excess humidity within the greenhouse through automated venting
.timed,cycled, or automated ventilation for cooling or heating depending on need
.green house enclosure material and insulating R factors

Take all of that into consideration ,but then add in the fact that I live in a very
non-consistent climate where rain is common for most days ,6 to 7 months of
the year. Temperature can vary 20-30 degrees F between daily high and nightly low, excessive intermittent clouds ,and the sun when shining is usually very intense .humidity is usually very very high and may be cool or warm depending.
I also do not use a high end greenhouse with a preferrable insulating effect
due to being cost prohibitive.

The use of a thermostat has been tried in previous years for cooling and heating but the constant cycling and or lag between cycling on or off due to the inconsistent conditions has resulted in crop failure due to stressing the plants.
Plants love consistent conditions ,and low stress.

The idea of using just temperature to control the environment needs to be more linear in fashion ,ramping upward and downward, not on and off.
The idea of using the sunlight and the LDR to achieve this could maybe solve some of the problems or at least lessen them .
If some of you have a link or a circuit or an idea that would control with temperature and be more linear in fashion ,I would love to see it ,but be sure to share it, as more discussion about which method is better or worse does not help much I think.
thanks all !

 

Welcome to the forums.

The process of growing vegetables in a greenhouse outdoors in a climate which is more somewhat consistent year round is complicated enough with the following items needing to be addressed to enable consistent production...

.adaquate sun exposure and or timed artificial light cycles
.manual or automated shade coverings on higher solar radiation days
.timed automated watering cycles dependent upon sunlight and heat per plants
.preventing excess humidity within the greenhouse through automated venting
.timed,cycled, or automated ventilation for cooling or heating depending on need
.green house enclosure material and insulating R factors

Yes, I am old enough to have several of those Forrest M. Mims III Notebooks circa early 80s. Many of the circuits mentioned using discreet components can be assembled easy enough. Me? I just completed 74 trips around the sun a few days ago. Everything mentioned above involves process control, that's really what a greenhouse is all about. Figure we were doing process control to maintain a controlled environment decades ago and we are still doing it today. Can everything you mentioned above be done using discrete components? Absolutely just like it was done circa early 80s and long before. All of that said I can't help but believe that today if Forrest M. Mims wanted fan speed control based on ambient light or temperature he would opt for a micro-controller like an Arduino flavor or similar. When Forrest wrote and designed these circuits there were no simple micro-controllers around yet. I am not seeing any new designs using a 555 to generate PWM for fan speed control.

I can grab an Arduino Uno Rev 3 and a DHT 22 temperature / humidity sensor and using heating and cooling maintain a temperature and a humidity. I can move my air to prevent stratification layers. Light? I can sense ambient light using a simple photocell or get fancy and run with a better sensor measuring ambient light in Lux. There are common modules for sensing about any unit you have an interest in sensing. Yep, I am willing to bet if Forrest wrote those notebooks today they would have micro-controllers and much lower parts counts. I can also control louvered windows between 0 and 100% open/close.

Ron

 

The idea of using just temperature to control the environment needs to be more linear in fashion ,ramping upward and downward, not on and off.

well if it is temperature you are controlling it certainly seems to follow that‘s what you should be sensing. If there are problems with using a single sensor and not accounting for the behavior of the actual ambient temperature then those things need to be addressed, surely, but it is still temperature you are concerned with.

It is common for a complex structure to have hot and cold spots, and for this to cause trouble when there is a thermostat with a single sensor, and a single source of heat (or cooing), or single zone. The solution to this, already well worked out, is to use multiple sensors and zones. This is done for buildings including houses.

I have to say that it feels like poking around in the dark to abandon temperature sensing because of a problem that certainly doesn’t seem traceable to measuring temperature as a way to control… temperature.

The idea of using the sunlight and the LDR to achieve this could maybe solve some of the problems or at least lessen them

Maybe? Do you have a theory about why that would work? Is there a clear correlation between light levels and the temperature in the greenhouse? Does it somehow account for the uneven air circulation—or whatever other effects are causing the singe sensor thermostat to be ineffective?

Has there been any research on this? If so, what iis the hypothesis?

This could be very sensible and an excellent idea but I can’t find a good theory that would suggest it. In any case, good luck with the project, I would recommend you investigate using one or more microcontrollers and possibly a wireless network of sensor nodes because that’s what is done nowadays, and it is neither expensive nor particularly hard—and the value of potentially lost crops sure can justify the time and money invested.