Normally the base of a cordless phone has an adaptor and the handset has Ni-Cd cells for its operation. The base unit becomes inoperative in case of power failure. Under such conditions, it is better to provide a backup using Ni-Cd cells externally. Here is a simple power supply back-up circuit which can be used with cordless phone SANYO CLT-420 or similar sets.
The working is simple. When AC mains is present, Ni-Cd cells are charged through IC LM317L, which is wired as a current source. Also, diode D3 is reverse-biased, which keeps Ni-Cd cells isolated from positive rail. When AC mains goes off, the Ni-Cd cells provide supply to the cordless phone base unit through diode D3. A green LED is used to indicate the presence of AC mains.
Each Ni-Cd cell costs around Rs 34, and the cost of the backup unit, including the box and cells, would not exceed Rs 300. Hence the circuit is well worth the investment.