To be used in greenhouses, heating or drying systems, this device activates a relay when the temperature in the sensor drops below a predetermined value. The trigger point adjustment is made in the trimpot, according to the resistance presented by the sensor (an NTC) at the temperature at which the trigger is desired. The complete circuit of the device is shown in the figure. The relay is for general use for 12 V. The supply in this circuit does not have to be symmetrical. The transistor is for general use PNP, BC557 or its equivalents, such as BC558, BD136, BD138, etc. can be used. The sensor must have a resistance of the order of 500 to 1 000 ohms at the temperature at which the circuit is to be triggered, for greater ease of adjustment and accuracy of operation. Changing the position of the NTC with the trimpot changes the mode of action of the circuit, which will operate as an overtemperature alarm, ie, triggering when the temperature rises above a certain value.
