Many home owners with greenhouses or other temperature-sensitive areas on their property are often looking for a cost-effective method of receiving an alert should the temperature fall or rise beyond their specified thresholds. This project solves that!

I got my parts at Absolute Automation.

Project Steps

First you need some sort of mechanical temperature sensor. Old-style thermostats work very well. Or, you can purchase the inexpensive unit pictured here from

With these thermostats you can easily select your high and low temperature thresholds. There is a metal coil in the temperature sensor which expands and contracts when the temperature rises and falls. When it increases or decreases to your marked threshold points the coil makes contact with the pins.

There are many types of wireless transmitters on the market which are designed for use with what are called “dry contact relays.” Essentially these are just normally-open or -closed switches. Guess what! Your mechanical temperature sensor has an open contact between the temperature sensor coil and the pins which you’ve marked your temperature thresholds with. This “open” contact “closes” when the coil touches a pin!

So now just use three small wires (telephone wire works great) and connect one wire to the temperature sensor’s coil and the other two to the two pins. Then connect the wire from the coil to the “COMM” or common terminal on your wireless transmitter and connect the ends from both of your pin wires to the “NO” or Normally Open terminal. Power up your transmitter and whichever type of receiver it came with and you have an inexpensive wireless temperature alert!