The LM3914 is a truly versatile component. Besides LEDs, only a
few other components are needed to make the ‘bidirectional’ bargraph
voltmeter shown here. The circuit is similar to a conventional
bar display, but it offers a possibility to change the direction
in which the LEDs are switched on. This may be useful, for example,
when positive and negative voltages are measured. For a positive
input voltage, the LEDs are switched on in the usual manner,
that is, from D3 to D12, while for negative voltages, the LEDs are
switched on in the opposite direction, from D12 to D3. Obviously,
the negative voltage must be ‘rectified’, i.e. inverted, before the
measurement. A suitable circuit for this purpose is presented in
the article ‘Absolute-value meter with polarity detector’ (reference
number 994020-1) elsewhere in this issue.
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