AUTOMATIC CHANGEOVER CIRCUIT

CIRCUIT DESCRIPTION FOR SWITCH OVER CIRCUIT USING LDR:

                                                  Figure: Switch over Circuit

To design the switch over circuit we are using easily available components in the local market. It is primary based on an OPAMP LM358, a NPN transistor 2N2222, a relay and a LDR.

To power off the system we are taking the power supply from the battery(+12V). The OPAMP we are using in our circuit is LM358 which is having dual comparator with 8pins DIP package. It can operate with a wide range of power supply but as we are using in differential mode we are providing regulated +5V otherwise it can operate from 3V-32V(dual or single power). The voltage regulator 7805 is a 5V regulator which is already used in the inverter circuit also to power up CD4049 but we have isolate both the inverter and switch over circuit hence we have used another 7805 Voltage Regulator. This 7805 voltage regulator is a 3pin, pin 1 is the input which is getting power supply from the 12V battery. Pin 2 is ground and Pin 3 is Regulated Output. Here also we have connected two cemaric capacitor of value 100nF across pin 1-2 and 2-3. To design the switch over circuit we are using easily available components in the local market. It is primary based on an OPAMP LM358, a NPN transistor 2N2222, a relay and a LDR. To power off the system we are taking the power supply from the battery(+12V). The OPAMP we are using in our circuit is LM358 which is having dual comparator with 8pins DIP package. It can operate with a wide range of power supply but as we are using in differential mode we are providing regulated +5V otherwise it can operate from 3V-32V(dual or single power).

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To design the switch over circuit we are using easily available components in the local market. It is primary based on an OPAMP LM358, a NPN transistor 2N2222, a relay and a LDR. To power off the system we are taking the power supply from the battery(+12V). The OPAMP we are using in our circuit is LM358 which is having dual comparator with 8pins DIP package. It can operate with a wide range of power supply but as we are using in differential mode we are providing regulated +5V otherwise it can operate from 3V-32V(dual or single power). The voltage regulator 7805 is a 5V regulator which is already used in the inverter circuit also to power up CD4049 but we have isolate both the inverter and switch over circuit hence we have used another 7805 Voltage Regulator. This 7805 voltage regulator is a 3pin, pin 1 is the input which is getting power supply from the 12V battery. Pin 2 is ground and Pin 3 is Regulated Output. Here also we have connected two cemaric capacitor of value 100nF across pin 1-2 and 2-3.

LM358 is having 8pin,pin 1 is non inverting output of 1^st comparator, pin 2 is inverting input of 1^st comparator, and pin 3 is non inverting input of 1^st   comparator, pin 4 is ground ,pin 5,6,7 are having the 2^nd comparator they are non inverting input, inverting input and non inverting output respectively. As we are using only one comparator, this three pins are unused. Pin 8 is V_cc which is connected to the output of 7805 voltage regulator.

To detect the environmental light we are using a LDR at non inverting input of the 1^st comparator that is pin 3. The LDR controls the flow of current through it depending on light falling on its surface. When there is no light the resistance becomes high and does not allow to pass the current but when light fall on LDR surface the internal resistance drops and allow to pass the current through it. In our circuit one terminal to the LDR is connected to pin3 of LM358 and other terminal is connected to 5V. We are using the OPAMP in differential mode where we have set a threshold value at inverting input of first comparator (pin2) with the help of 10k variable resistance. When we set the threshold value the comparator keep monitoring the input signal and if the input signal cross the threshold value pin1 that is non-inverting output goes high. Using the potentiometer we are adjust the intensity of light, at which circuit will switch over between charging mode to operational mode. We must have to add a pull down resistance at non inverting input (pin3).  Otherwise once it is activated it will detect the signal. To indicate the status of the sensor we are using a LED at pin1 of LM358.

The output of the 1^st comparator is fetched to a relay driver circuit. The relay driver circuit is based on NPN switching transistor 2N2222 from flip; we are using emitter follower circuit.  The emitter is connected to ground and collector is connected to one terminal of the relay coil where other terminal is connected to 12V battery. When the base is low the internal resistance between emitter and collector goes high and doesn’t allow to pass the current, hence the relay coil will be deactivated but when the base get a positive volt from the OPAMP output through a CD4049   resistance, the internal resistance of emitter and collector drop and allow to pass current through it which will couple the circuit and activate the relay coil. When the relay coil is energised it pull a lever to switch over between one terminal to another. We are using a SPDT relay electromagnetic device like relay or motor gives reverse current, which may damage the driver circuit that is 2N2222 in our case . We have to protect from reverse current. Hence a freewheel diode (IN4007) is connected across the relay coil terminal in reverse direction. At the same time we have connect a LED through a 470Ω resistance to indicate whether the relay is on or off (charging mode or inverting mode).

COMPONENT: 

I.	OPAMP	LM358
II.	Light dependent  resistor	10K Ω
III.	Voltage Regulator IC7805	IC7805
IV.	Diode (LED RED)	Standard
v.	Resistor	470 Ω, 10K Ω
vi.	Capacitor	0.22 µF
vii.	Variable Resistor	10K Ω
Viii	Transistor	2N2222
Ix	SPDT Relay	12 Volt