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Cordless phone backup Click here for the circuit diagram
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.
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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