Cordless Phone Backup


Normally the base of a
cordless phone has an adaptor and the handset has Ni-Cd cells for its
operation. The base unit becomes in-operative 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
cel ls 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.


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