temperature sensor

[farzana]

I would like to know how to construct a temperature sensor which senses the temperature and a humidity sensor which senses the humidity inside a green house .

I had also posted a request for the construction of automatic sensor tap.By automatic sensor tap I meant the watertaps which works automatically when hand is placed underneath it.

 

[Oznog]

Do you have a microcontroller? How do you intend to use the output?

Temp sensing is done by thermistor, precision temp sensor (zener diode based), or digital device such as DS18S20. The thermistor and zener temp sensor will give you a voltage output, digital ones require a microcontroller.

Humidity is a bit more tricky- the sensors are a capacitor and the capacitance varies with humidity. Now Humirel is totally the way to go here, they give detailed specifications on how to use a 555 to turn the output of their HS1100 sensor into a frequency output a microcontroller can read with great accuracy.

Humirel also makes the HS1500 sensor pkg. It's a cased unit with an HS1100 inside which already converts humidity to a simple voltage output, essential if you do not use a microcontroller. It naturally costs more than the HS1100.

I believe the automatic water tap uses an infrared motion sensor. Or it could have an infrared emitter and senses the amount reflected back. You may be able to find the unit on the surplus market, or just buy one retail. Frankly, constructing a sealed sensor and getting the right valve is absolutely not worth constructing if you want it to do the exact same job as an off-the-shelf unit. I'd even say if you wanted to do something a bit different like a dog water bowl, I'd still start by taking apart the manufactured unit.