RAIN DETECTOR USING ASTABLE MULTIVIBRATOR

Here we going to make a rain detector using astable multivibrator and simlpe conitinuouity tester.
These circuits are we already learned in my previous post. We all know water is a precious thing in earth. Also  it is less in earth so we want to protect and reduce the use of water.
Use of this circuit we can save the rain water whenever the raining occur.

ASTABLE MULTIVIBRATOR USING NE555

In this am going to explaining about astable multivibrator using ne555. Am already explained the astable multivibrator using BC107 and OPAMP in my previous post.




The working of astable using ne555 is starting by the capacitor C.
Initially capacitor C starts charging through Ra and Rb towards Vcc with a time constant (Ra+Rb)C. Durring this time R=0, S=1, Q*=0 and ouput is high. When capacitor voltage equals (2/3)Vcc the upper comparator triggers the control flip flop so that Q*(Q bar)=1 . This makes transistor Q1 ON and the capacitor C starts discharging towards ground through Rb and transistor Q1 with a time constant RbC.


Durring thr discharge of the timing capacitor C, a it reaches Vcc/3 the lower comparator is triggerd and at this stage S=1, R=0, which turns Q*=0 . This makes transistor Q1 OFF and again capacitor C starts to charge. Thus the capacitor periodically charges and discharges between (2/3)Vcc . Output amplitude of the square wave swings between 0.3V to applied Vcc.



The charging periode of the capacitor Tc=0.69(Ra+Rb)C . The discharging periode of Td=0.69RbC . If Tc is equal to Td , the duty cycle of the output waveform will be 1/2 . In order to make Tc and Td equal to each other , a diode is connected in parallel with Rb such that capacitor charges through Ra and diode, and discharges through Rb . Another method is using a resistor of same value of Ra and Rb between the pin number 7 and the junction formed by Ra and Rb. The difference we can seen in the output waveform.



Components for wiring this circuit


IC NE555
Ra - 6.8K
Rb - 6.8K
C - 0.1μF
C1 - 0.01μF
Vcc - 10V

FUNCTIONAL BLOCK DIAGRAM OF NE555

In this we are going to study about functional block diagram of ne555. It is also called 555 timer. This IC used for generating accurate time delay or oscillation. The maximum operating frequency is excess of 500kHz. The 555 timer can be used with supply voltage in the range from +5V to +18V and can drive load up to 200mA.




Functional diagram of 555




The 5K intrenal resistance act as a voltage devider network, providing (2/3) Vcc at inverting terminal of upper comparator (UC) and (1/3)Vcc at the noninverting terminal of lower comparator (LC). In the stable state , the Q*(Q bar) output of the control flip flop is high. This makes the output low because of the buffer which basically is an inverter.


Pin 1 - It is ground

Pin 2 (Trigger) - This pin is used to feed the trigger input when the chip is set up as a monostable multivibrator. When a trigger of amplitude greater than (1/3)Vcc is applied to this terminal , circuit switches to quasi stable state.

Pin 3 - This is output

Pin 4 (Reset) - This terminal is used to reset the output of the circuit irrespective of th input. A logic low input will be reset the output. For normal operation this pin is connected to Vcc.

Pin 5 (Control) - Viltage applied to this terminal will control the instant at which the comparator switches and hence the pulse width of the output. When this pin is not used it is bypassed to ground using a 0.01μF capacitor.

Pin 6 (Threshold) - If the voltage applied to the threshold terminal is greater than (2/3)Vcc, upper comparator switches to +Vsat and flip flop output get reset.