ESCOL ES-46 Temperature Controlled Relay / HeatAlarm

INTRODUCTION : In this project we use a NTC thermistor to detect temperature change & switch a relay On/Off according to a preset condition. NTC thermistor is a temperature dependent resistor with Negative Temperature Coefficient meaning its resistance changes inversely with temperature. The higher the temperature, the lower its resistance. A sample randomly selected from a batch of MF52E thermistors gives the following characteristic chart   NOTE: the LED serves as an indicator for the relay’s activity. The LED turns on when the relay is actuated. It is otherwise off   To test the operation of the circuit, please follow these steps:   (1) Connect the ciruit to a stable DC 9V supply. Or you can temporarily use a 9V battery for testing purpose   (2) At this moment, the LED should be on. If not, then adjust the preset (trim pot) VR1 in anti-clockwise until the LED comes on   (3) Next, very slowly, turn the preset VR1 in the clockwise direction until the LED just turns OFF. This is the trim pot setting corresponding to the current ambient temperature   (4) Now, let the thermistor sense a temperature rise by placing something hot near the thermistor (eg. a soldering iron, the flame of a lighter,..). The LED should turn on instantly (relay actuated). Remove the heat source and wait a minute or so and LED will be off again