Thermostat-controlled soil heating

EE 40 July/August 1986 151thermostat-controlled soil heating Many people with a keen interest in growing plants insist on the fact that many of the more exotic species, such as c6rtain species of orchid and fungi, will only thrive in warm soil and relatively high humidity. Whether or not this is a correct assumption, this circuit offers the possibility to keep the soil tempera- ture in a miniature hot-house at a con- stant, adjustable level. The heating element is made of several loops of plastic covered steel wire, such as used in gardening. The wire used in the prototype had a diameter of 1 mm and a resistivity of about 0.2 Q per metre. The circuit diagram of the soil heater shows that the heating element is temperature controlled by means of a triac, driven by a Type TDA1024 elec- tronic thermostat which gets the necessary information as to the soil temperature from Rs, an NTC type sensor. The circuit is fed from the trans- former secondary by means of recti- fier Di and series...