Continuous monitoring of
the mains voltage is required in many applications such as manual
voltage stabilisers and motor pumps. An analogue voltmeter, though
cheap, has many disadvantages as it has moving parts and is sensitive to
vibrations. The solidstate voltmeter circuit described here indicates
the mains voltage with a resolution that is comparable to that of a
general-purpose analogue voltmeter. The status of the mains voltage is
available in the form of an LED bar graph.
Presets VR1 through VR16 are used to set the DC voltages corresponding
to the 16 voltage levels over the 50-250V range as marked on LED1
through LED16, respectively, in the figure. The LED
bar graph is multiplexed from the bottom to the top with the help of
ICs CD4067B (16-channel multiplexer) and CD4029B (counter).
The counter clocked by NE555 timer-based astable multivibrator
generates 4-bit binary address for multiplexer-demultiplexer pair of
CD4067B and CD4514B. The voltage from the wipers of presets are
multiplexed by CD4067B and the output from pin 1 of CD4067B is fed to
the non-inverting input of comparator A2 (half of op-amp LM358) after
being buffered by A1 (the other half of IC2). The unregulated voltage
sensed from rectifier output is fed to the inverting input of comparator
A2. The output of comparator A2 is low until the sensed voltage is
greater than the reference input applied at the non-inverting pins of
comparator A2 via buffer A1.
When the sensed voltage goes below the reference voltage, the output
of comparator A2 goes high. The high output from comparator A2 inhibits
the decoder (CD4514) that is used to decode the output of IC4029 and
drive the LEDs. This ensures that the LEDs
of the bar graph are ‘on’ up to the sensed voltage-level proportional
to the mains voltage.The initial adjustment of each of the presets can
be done by feeding a known AC voltage through an auto-transformer and
then adjusting the corresponding preset to ensure that only those LEDs that are up to the applied voltage glow.
Note.
It is advisable to use additional transformer, rectifier, filter, and
regulator arrangements for obtaining a regulated supply for the
functioning of the circuit so that performance of the circuit is not
affected even when the mains voltage falls as low as 50V or goes as high
as 280V. During Lab testing regulated 12-volt supply for circuit
operation was used.)
