Your child’s battery toy
has failed and you have to fix it. Once you have managed to get it
apart, the battery compartment is not likely to be connected to the
works or the batteries might have gone flat anyway. The solution is this
switchable supply which is designed to replace from one to six dry
cells. It is not intended to replace the batteries on a permanent basis,
as in most cases this is not practical. The heart of the supply is an
LM317T adjustable 3-terminal regulator and six trimpots selected by
switch S1b. The other pole of the switch, S1a, is used to select taps on
the transformer secondary, to minimize power dissipation in the LM317T.
The table shows the trimpot settings for the six voltage outputs. Diode
D1 and the 10µF capacitor and the LED provide power indication. This has the advantage of constant brightness which would not be obtained if the LED was run from the unregulated switchable DC.
Author: Philip Button
Copyright: Silicon Chip Electronics