Battery Back-up Circuit

This Battery Back-up circuit can be added to surveillance systems or control like alarms to power the circuit during Mains power failure.the battery back-up will immediatly take up the load without any delay
figure 1 show you circuit diagram

the circuit is simple to construct .Regulator IC 7809 volts regulated DC for powering the circuit as well as to charge the rechargeable battery. LED indicates the power on status , when the mains power is available, diode D1 forward biases and passes current into the battery through R2. Value of  R2 is selected to give 90mA current (9/100=0.09A)for slow charching . when the mains power failure D1 reverse baises and D2 forward biases and buck-up the circuit . the same circuit can be used in circuits having 6 volt 7.5 Ah battery.for 12 volt battery , use 7814 regulator IC and 14 volt input