To find out what a heat detector is, here is the definition of a heat detector. Heat detector is a type of detector that has two types of metal elements that can detect (consisting of two thick strips and a thin strip). Thin strips are more sensitive to temperature rises than thick trips. If there is a sudden rise in temperature, then the thin strip responds faster than the thick strip, the two strips can come into contact with each other. During the normal temperature rise the two strips will experience the same deflection and thus do not show any touch and reaction. Usually if the rate of increase is less than 10oC in half an hour, the detector will not give an alarm. But if the level rises to 75oC or more, the two strips will touch because of deflection, triggering an alarm.
A simple heat detector circuit is shown in the picture above. In the figure, it can be seen that the potential divider circuit is connected to a series of thermistors and resistors with a strong current of 100 Ohms. If the NTC (negative temperature coefficient) used is the type of the thermistor, then the resistance of the thermistor will decrease after experiencing heating. Thus, more current flows through the potential divider circuit formed by the thermistor. Therefore, there is more voltage between the thermistor and the resistor. Let’s look at a 110 Ohm thermistor that drops to 90 ohms after heating. Then, according to the potential of the circuit against the concept of pervading the voltage divider. the voltage of one resistor and the ratio of resistor values and the amount of resistance when multiplied by the series combination voltage are the same. The heat detector circuit is designed using a thermistor, not only using transistors and buzzers, but also using SCR and LED. SCR is connected in series with the LED.
LEDs are used as warning elements. RED-LEDs are connected in circuits that are activated to show significant changes in heat which will then be felt by the thermistor. Generally, thermistors offer a very high resistance of about 100KΩ at room temperature. Because of the very high resistance, practically no current will flow. Therefore, no trigger pulse can be given to the SCR terminal gate. However, if a large amount of heat is felt by the thermistor, the resistance of the thermistor will be significantly reduced. Thus, sufficient current will flow through the SCR circuit and terminal gate. Therefore, the LED is connected in series with the SCR as a warning indicating changes in heat.
Examples of practical applications of heat detectors are fire extinguishers that are controlled using RF transmitters and RF receivers. This circuit consists of a heat detector (thermistor) which is connected to the microcontroller of the receiver block which is connected to the motor driver. Under room temperature, the heat detector will not give a signal to the microcontroller so the pump stays off. When the heat detector detects a significant change in heat it sends a signal to the microcontroller. Next, the microcontroller sends a signal to the pump through a relay to activate and extinguish the fire (if any). Thus, the heat detector can be a solution to overcome the fire.