Working:

At this end we firstly use the different types of sensors according to the system to be monitored & the o/p of sensors is applied to multiplexer IC CD4051 which is an 8×1 analog multiplexer but because we are using only four sensors , we ground the third selection line to make the mux 4×1.
The selection of mux line is done by micro controllers pin 21& 22 which are the two least significant bits of port 2 of micro controller as it provides the selection signals to mux the mux connects the particular sensors o/p to ADC 0804 pin6 through a resistor (1k) & zener (5v), the resistor & zener protect the ADC by limiting the maximum voltage upto 5v because a voltage greater than this can cause permanent damage to ADC 0804.

Because ADC 0804 works on successive approximation the weightage of MSB defined by biasing of pin no.9 which is adjustable by 2.2k, hence we can adjust ADC to convert the analog signal from zero to any defined level upto 5v. To perform the conversion IC also requires a clock which is generated here by a 10k resistor & 150 pf capacitor thus providing a clock of approximately 600 KHz.
To convert the analog signals available at pin 6 of the IC we require a WR pulse at the pin 3 of IC & this pulse is provided by pin 23 (p2.2) of micro controller. After getting the WR pulse IC needs some time to (approx.100?s) convert analog signal into digital. After conversion ADC produces the 8 bit digital o/p which is collected by micro controller by its port zero.

Now micro controller matches the data with programme written in assembly language & if it detects the abnormal reading it starts the 3 min. timer designed by 555 IC. The 555 IC is working here in mono stable mode & produces the high o/p at its pin 3 for 3 mins. & this high signal drives the tr. BC548B which operates the relay. Since relay consists of a coil hence to protect the transistor during switching a free wheeling diode is connected across the coil.

If micro controller detects that sensors reading is crossing the dangerous limits it also operates the relays to turn off the unit.

Circuit Diagram:

Circuit Diagram

Component list:

COMPONENT LIST

1. IC’s
AT89C51 – 1
ADC0804 – 1
CD4051    - 1
2402    – 3
LM567   – 2
LM7805  – 2
lm35    – 1

2. Transistors
BC548B – 2

3. Light Emitting Diodes (LED’s)
Red – 1

4. Diodes
1N4007 – 9

5. Zener Diodes
5V – 3

6. Crystal
12 MHz – 1

7.  Electrolytic Capacitors
1000uF/25V    - 1
22uF/25V    – 1
10uF/25V    - 1

8. Ceramic Capacitors
103pf – 3
150pf – 1
33pf  – 2

9. Variable Resistors
Preset 2k – 3

10.  Carbon Resistors (0.25W)
10K – 9
2.2k – 2
1K    -  10
220E -  2
100E -  2

11. Relays
12V/200E – 1

12.  Transformers
9/0/9 500mA    - 3

13. PCB
6” X 9”    - 1

14. Display
LCD – 1

15.  Meslenious
IC Base (40 pin) – 1
IC Base (20 pin) – 1
IC Base (16 pin) – 1
IC Base (8 pin) – 2
Mains Cable – 2
Ferric Cloride – 100gms.
Soldering Wire – 20gms.
Connecting wires – 2mtrs.
Soldering Paste – 10gms.

Reference:

REFERENCE FOR TECHNICAL INFORMATION FROM FOLLOWING BOOKS:

1.    Micro Processor Architecture by Ramesh S. Gaonkar .
2.    Communication System by Tob & Shilling.
3.    Micro controller  by K. J. Ayala.
4.    Integrated Electronics by Millman & Hawlkiwas.
5.    Let us C by Yashwan Kanitker.