Automatic Electronic Fire Extinuisher

This was my senior year project at the University of Mumbai. As the name suggests, it was a fire extinguisher that with the aid of electronic circuitry, automatically extinguishes fire by turning on the water pump connected to the sprinkler system. So, basically it was fire sprinkler system.

We designed the circuit, tested various components and built a fire extinguisher using operational amplifiers, temperature sensors, optocoupler, etc. It involved a lot of patience and testing during each stage. But, we got through it great. In the end, we built a miniature house model and had our sensors inside it. Also connected was the sprinkler system, which was basically a pipe from the water pump.

There were three main stages:
1. Sensor circuit: It senses increase in temperature using LM 334, which is basically a current source used a temperature sensor; its output voltage is directly proportional to the rise in temperature.
2. Transducer circuit: This consists of an inverting amplifier, error & hysteresis controller and a triggering circuit, in that order. The output of the last circuit goes to the water pump, which draws water from a water tank when turned on.
3. Water tank level indicator: This circuit indicates the water level at all times.

We also added a fire alarm circuit that would alarm everyone. Below, I’ve only provided information on the sensor and transducer circuits, along with other things that I thought I should include.

The temperature sensor (LM 334) is given below.
This is the testing circuit for the sensor and the results from the test follow.

As can be seen, the output of the sensor is directly proportional to the rise in the temperature. The output of the sensor was given to an inverting amplifier circuit. The gain of the inverting amplifier is given as:

Av = -Rf / Rin

From the values in the circuit given below, it turns out to be -10. Therefore, the output is:

Vo = -10 x Vin

Above, values beyond 0.55V are extrapolated values.

After the inverting amplifier, there is an error amplifier. The circuit and the results are below:

We decided that 0.51V ≈ 65 C ≈ 149 F will be our trigger point i.e. the point when we will start the water pump. Accordingly, in the above circuit, the reference voltage was selected to be 5V. This results in the output as shown.

Now, the hysteresis controller takes this output and converts the irregular waveform into a square wave. The output of this circuit goes into negative saturation when the input exceeds a preset limit i.e. the upper trigger point (UTP). It is calculated below with the values from the circuit that follows:

V(UTP) =     R1       x Vsat
     R1 + R2
                     =        100          x 10
              100 + 4900
= 0.2V

Therefore, according to the calculations shown, the output goes from 10V to –Vsat at 0.2V; however because of the 10V zener diode, it goes from 10V to 0V.

The lower trigger point (LTP) explains the table above. Calculations given below:

V(LTP) = R1 x -Vsat
R1 + R2
= 0V

This is because of the use of the zener again; the output is 10V when the input is 0V or below it, as shown.

From the discussion before, we know that the output of the hysteresis controller is either 10V or 0V. The optocoupler above is an optically isolated TRIAC driver. It consists of a gallium-arsenide infra emitting diode, optically coupled to a silicon bilateral switch and is designed for isolated TRIAC triggering applications like ours. It also provides low current isolated AC switching and high electrical isolation apart from being small in size and low cost.

As the figure above shows, for the infrared LED to activate, pin #2 needs to be grounded or 0V. So, when the hysteresis controller switches from 10V to 0V, the LED activates and as a result, the silicon bilateral switch (TRIAC) closes. The green and red LEDs and the BJT transistor are present just as an indication of the output of the hysteresis controller. The green indicates output is 0V and the red indicates 10V.

The next phase of the circuit is given below:

The optocoupler is used to fire the SCR and in effect, the water pump turns ON. The transducer & water level indicator circuit PCB diagrams, which were created in popular PCB software like Cadence, are given below.