OVER TEMPERATURE ALARM
101％ working project hello friends today I am gonna telling you about some easy and valuable electronics projects for school and college practical
This circuit can be used as a warning device which triggers alarm whenever the temperature exceeds a preset level. It can also be used as a door - alert or general warning signal.
The circuit can be divided into three part .
1. Control section
This section enables disables the pulse oscillator de pending at the temperature produced near the thermistor, which is used as a sensor. Thermistor used here is a heat sensitive device that acts as a variable resistor. Around normal room temperature, its resistance remains high. but decreases with increase in temperature. The wiper of potmeter VR1 is connected to the base of transistor TI.
The collector of transistor T2 goes high when thermistor's temperature becomes higher than preset temperature level. High voltage of collector of T2 is used to enable the pulse oscillator.
2. Pulse oscillator section
This section is built around gates N1 and N2. The NAND gate N1 has one of its inputs (pin 1) used for control. It oscillates when controlling section provides high voltage at pin 1 of gate N1. On the other hand it does not oscillate when control input pin 1 of gate N1 is held at logic low. The timing capacitor and resistor values of the pulse oscillator can be calculated as follows: f = 1 / (2.2 R9xC1)
A very low - frequency (pulsed oscillator) output is available at pin 4 of gate N2 used to enable / disable the audio oscillator .
3.Audio oscillator section
This section is built around gates N3 and N4 The output of the pulse oscillator is fed to input pin 9 of gate N3. When output of pulse oscillator goes high audio oscillator starts oscillating. This frequency is given to the base of transistor T3 to drive buzzer PZI, which starts producing an intermittent beep sound. The beep rate depends on the values of capacitor C2 and resistor R11
Fig1; circuit diagram
At normal condition, thermistor THI keeps the base of transistor T1 low. Transistor T1 is cut off and T2 conducts, keeping collector voltage of T1 high while collector voltage of 12 is low. As a result both the oscillators silent are disabled and buzzer remains Silent
On the other hand, when the temperature of thermistor goes high, the base of transistor T1 also goes high. Transistor T1 conducts and T2 cuts off, resulting in collector voltages of T1 as low and T2 high. Both oscillators oscillate and buzzer produces intermittent sounds.
Assemble the circuit on a general purpose PCB and enclose it in a suitable cabinet Connect the thermistor TH1 using two wires and preset on the front panel. Mount the buzzer firmly on the enclosure. Use 6V battery or a 6V adapter to operate the circuit. Alternatively, you can use a 9V PP3 type battery also ..
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