+/– Voltage On Bargraph Display


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’ elsewhere in this website.
A set of transistor switches (MOSFETs) controls the direction in which the LEDs
light. When the control voltage is high (+6V, according to the
schematics, but any voltage that is at least 3V higher than reference
voltage will do), T1 and T4 are switched on, while the other two MOSFETs
are off. In this way, the LM3194 is configured in the usual manner with
the top end of the resistor network connected to the internal voltage
reference and the low end connected to ground.

As the input voltage rises, the comparators inside the LM3914 will cause the indicator LEDs
to be switched on one by one, starting with D3. When the control
voltage is lower than about –3V, T2 and T3 are switched on while T1 and
T4 are off. Consequently, the ends of the resistor network are connected
the other way around: the top end goes to ground and the low end, to
the reference voltage. The first LED to be switched on will then be D12; i.e., the LEDs
that forms the bargraph display light in the opposite direction.
Although not documented by the manufacturer of the LM3914, this option
works well, but only in bar mode (in dot mode, internal logic disables
any lower-numbered LEDs when a higher-numbered LED s on, which obviously conflicts with our purposes).

To achieve good symmetry, an adjustable resistor is added to the voltage divider in the LM3914. Using a DVM,
adjust the preset until the voltage across P1+R4 equals 1/11th part of
Urefout. Sensitivity is determined with the ratio of resistors R5 and
P2. If, for example, the reference voltage is set to 2.2 V by means of
P2, there will be a voltage drop of 200 mV per resistor in the ladder
network (including R4-P1). So, the first LED
will switch on when the input voltage exceeds 200 mV, the second, at 400
mV, and so on, and the whole display will be on at 2 V. The circuit
draws about 100 mA when all LEDs are switched on.


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