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Cordless Phone Backup


Animesh Maurya

Jan 1, 1970
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. In such conditions, it is better to provide
a backup using Ni-Cd cells externally. Here is a simple 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.

Circuit is here

My problem is that Iam using Panasonic KX-TC1000BXB which is having a
9V adapter rather than 12V, so what changes should be made in the
circuit for proper operation.

Thanks in advance