Central to this circuit is a precise integrated temperature sensor known as LM35 (IC1), delivering an exact linear and directly proportional output in millivolts across the temperature range from zero to +155 degrees Celsius. The LM35 generates an output voltage of 10 millivolts per Kelvin change in the measured temperature. Engineered to consume minimal current, the LM35 exhibits minimal self-heating in stagnant air. In this setup, the LM35’s output is connected to the non-inverting input of a comparator, which is constructed using a CA3130 operational amplifier (opamp).
Setting the Temperature Threshold
A voltage divider network consisting of R3 and P1 establishes the threshold voltage at the inverting input of the op-amp. This threshold voltage defines the customizable temperature trip level at which the circuit becomes active. When the measured temperature surpasses this user-defined level, the comparator shifts its output high, reaching approximately 2.2 V. This action instantaneously forward-biases transistor T1 and activates T2. Consequently, the oscillator circuit around IC3 receives ample voltage to initiate its operation.
Buzzer Alert and Sound Rhythm Control
The 555 timer, configured in astable mode, directly drives the active piezoelectric buzzer Bil, creating a distinct audible alert. The on/off rhythm of the sounder is regulated by components R7, R8, and C4. Alternatively, a relay driver utilizing transistors can be triggered from the emitter of Ti (TIM). By substituting the piezo sounder with an appropriate relay, high-power devices like flashers, sirens, or horns operating on AC mains supply can be controlled effectively.
LM35 Temperature Sensor Principles
The LM35 temperature sensor operates on the fundamental principle of a diode. As temperature increases, the voltage across the diode rises at a known rate. By accurately amplifying this voltage change, it becomes straightforward to generate an analog signal directly proportional to the temperature. The LM35 sensor is highly efficient, drawing only 60 µA from its supply, resulting in minimal self-heating (less than 0.1˚C in still air). Its operational range varies: the LM35 spans from -55˚C to +150˚C, while the LM35C functions within a -40˚C to +110˚C range (with improved accuracy down to -10˚C).