8051 Micro Controller

• .Pins No. 1 to 8 are Port 1. Each of these pins can be configured as input or output.To configure a Port pin as Input one must write 1 to it by programming and to configure a port pin as output one must write 0 to its latch using programming.


• Pin N0. 9 is reset, a high on this pin will reset the microcontroller.On applying Zero to this pin microcontroller starts executing the program from the beginning.


• Pins no. 10 to 17 constitute port 3. These pins have alternate functions as denoted in the pin diagram.These are:


• RXD,Pin 10: This pin is used as Serial asynchronous Communication Input or Serial Synchronous Communication Output.
• TXD,Pin11: This pin is used as Serial asynchronous Communication Output or Serial Synchronous Communication Clock Output.
• INT0,Pin12: This pin is external interrupt 0 input. It is active low.
• INT1,Pin13: This pin is external interrupt 1 input. It is active low.
• T0,Pin 14: Counter 0 clock input.
• T1,Pin15: Counter 1 clock input.
• WR,Pin16:Write to external RAM.
• RD,Pin17:Read from external RAM.
  


• Pins 18 and 19 are for connecting crystal oscillator.


• Pin 20 is ground.


• Pins 21 to 28 are Port 2 lines.These pins are used for addressing external memory ,when used as they also serve as higher order address lines.Microcontroller is capable of addressing 64K external memory as it has 16 address line. It is not necessary that always all the 16 address lines are used.


• Pin 29 is Program Store Enable(PSEN). If external ROM is used then a logic 0 appears on it every time the microcontroller wants to read from external memory.


• Pin 30 is ALE(Address Latch Enable). It is used to latch the lower order address.


• Pin no. 31 is External Access(EA). On applying Logic zero to this pin, microcontroller ignores the internal code memory and execute the program from external memory only. When a high is applied to it, microcontroller first uses the internal ROM and then External ROM(if available).


• Pins 32 to 39 constitute Port 0 and lower order Address Bus when ALE is 1 or Data Bus when ALE=0.


• Pin no. 40 is Vcc(5V).
  

Power Supply Failure Alarm

Most of the power supply failure indicator circuits need a separate power supply for themselves. But the alarm circuit presented here needs no additional supply source. It employs an electrolytic capacitor to store adequate charge, to feed power to the alarm circuit which sounds an alarm for a reasonable duration when the supply fails.
This circuit can be used as an alarm for power supplies in the range of 5V to 15V.
To calibrate the circuit, first connect the power supply (5 to 15V) then vary the potentiometer VR1 until the buzzer goes from on to off.
Whenever the supply fails, resistor R2 pulls the base of transistor low and saturates it, turning the buzzer ON.

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Theft Alarm

This circuit utilising a 555 timer IC can be used as an alarm system to prevent the theft of your luggage, burglars breaking into your house etc. The alarms goes ON when a thin wire, usually as thin as a hair is broken.
The circuit is straightforward. It uses a 555 IC wired as an astable multivibrator to produce a tone of frequency of about 1kHz which gives out a shrill noise to scare away the burglar.
The wire used to set off the alarm can be made of a thin copper wire like SWG 36 or higher.
You can even use single strands of copper form a power cable.

The circuit operates on a wide range of voltages from 5V to 15V.
The speaker and the circuit could be housed inside a tin can with holes drilled on the speaker side for the sound to come out.

Rain Alarm

This circuit gives out an alarm when its sensor is wetted by water.
A 555 astable multivibrator is used here which gives a tone of about 1kHz upon detecting water.
The sensor when wetted by water completes the circuit and makes the 555 oscillate at about 1kHz.

The sensor is also shown in the circuit diagram.
It has to placed making an angle of about 30 - 45 degrees to the ground. This makes the rain water to flow through it to the ground and prevents the alarm from going on due to the stored water on the sensor.
The metal used to make the sensor has to be aluminum and not copper. This is because copper forms a blue oxide on its layer on prolonged exposure to moisture and has to be cleaned regularly.
The aluminum foils may be secured to the wooden / plastic board via epoxy adhesive or small screws.
The contact X and Y from the sensor may be obtained by small crocodile clips or you may use screws.