Here is a Dual power source for the Home Security system which has to be switched on continuously. It is a power saving circuit that uses a Solar panel for providing power during day time and a transformer power supply during night. It has a battery backup and the power to the security system is derived from the 12 Volt 4.5 Ah battery. So that even if the day light is not sufficient in clody days, the security system will work normally.
About the Solar Power source
During day time, the security system gets 12 volt power supply from the battery or the Solar Panel. If the amount of power from the solar panel is not sufficient, battery will compensate it. Typically a single solar cell generates 0.5 volt and around 50 mA current in bright light. A single solar cell is rated at 45 mW. So in the 12 volt solar panel, there will be 24 solar cells connected serially. It can give 1 Ampere current in day light. Typically a 15 W Solar panel gives 1 Ampere current in bright light. Here a 12 V 5W Solar panel is used in the circuit to give power to the security system and charging current to the battery. It gives around 300 mA current.
About the DC power source
Night charging is attained by the 12 volt DC obtained from a 0-12 volt 1 Amps step down transformer and the fullwave rectifier comprising D3 through D6. Capacitor C2 removes ripples from DC to give clean power to the security system and for the battery.Resistor R4 gives 80 mA current during night for Trickle charging the battery.At the same time, power to the security system will be available from the battery. The trickle charging provision save AC power and protect battery from over charging since the battery will get sufficient charge during day time.
Battery status indicator
This part of the circuit indicates the voltage level in the battery as well as the voltage from the solar panel.Zenerdiode ZD conducts only when the battery holds more than 11.6 volts. When ZD conducts Red LED illuminates.When the battery voltage goes below 11 volts ,LED turns off.If the LED is not lighting, it indicates that battery is not attaining full charge or the solar panel is not giving 12 volts.
This is used to switch on the DC power from the transformer power supply at night. LDR offers low resistance in day light so that it gives more current to the base of T1.It is a PNP transistor and it remains off during day time due high voltage in its base. T2 also remains off due the lack of base bias. During night,resistance of LDR decreases, and the base of T1 becomes negative. It then turns on. T2 also turns on and charging current is available from the emitter of T2. Resistor R4 reduces the charging current to 80 mA for trickle charging the battery. Adjust VR1 to set the sensitivity of LDR.
Setting of the Dual Power source circuit
Assemble the circuit on a General purpose PCB and enclose in a suitable case. LDR should be mounted on the case to get sunlight.Solar panel should be placed in a place where bright sunlight is available.Before connecting the battery, fully charge it using a charger.