Fire AlarmSecurity & SafteySensors - Tranducers Circuits

Overheat Detector Alarm/Switch Schematic Circuit Diagram

At the heart of this circuit is a precision integrated temperature sensor type LM35 (IC1), which provides an accurate linear and directly proportional output in mV, over the zero to +155 degrees C temperature range. The LM35 develops an output voltage of 10 mV/K change in measured temperature. Designed to draw a minimal current of its own, the LM35 has very low self-heating in still air. Here the output of the LM35 is applied to the non-inverting input of a comparator wired around a CA3130 opamp (10).

Overheat Detector Alarm Switch Schematic Circuit DiagramA voltage divider network R3-P1 sets the threshold voltage, at the inverting input of the opamp. The threshold voltage determines the adjustable temperature trip level at which the circuit is activated. When the measured temperature exceeds the user-defined level, the comparator pulls its output High to approx. 2.2 V causes transistor T1 to be forward-biased instantly. T2 is also switched on, supplying the oscillator circuit around IC3 with sufficient voltage to start working. The 555 set up in astable mode directly drives active piezoelectric buzzer Bil to raise a bud alert. Components R7, R8, and C4 determine the on/off rhythm of the sounder. A transistor-based relay driver may be driven off the emitter of Ti (TIM Similarly, replacing the piezo sounder with a suitable relay allows the switching of high-power flashers, sirens, or horns working on the AC mains supply.

The LM35 device is rated to operate over a −55°C to 150°C temperature range, while the LM35C device is rated for a −40°C to 110°C range (−10° with improved accuracy).
LM35 sensor uses the basic principle of a diode, when the temperature increases, the voltage across a diode increases at a known rate. By precisely amplifying the voltage change, it is easy to generate an analog signal that is directly proportional to temperature.
As it draws only 60 µA from its supply, it has very low self-heating, less than 0.1˚C in still air. The LM35 is rated to operate over a −55˚ to +150˚C temperature range, while the LM35C is rated for a −40˚ to +110˚C range (−10˚ with improved accuracy).

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