Bar Graph Display
An electronic bar graph display uses a series of LEDs to indicate an analogue property, such as a sound level or electrical charge value.
In some applications, it may be appropriate to use different colour LEDs to indicate high and low values.
As with individual LEDs, resistors should normally be placed in series with each LED to limit the current flowing to the required value, as given in the manufacturer's data sheet.
Worked Example 1
Calculate an appropriate current limiting resistor value for a bar graph display intended to work from a 5 V power supply, if the diode forward voltage Vf = 2 V and the forward current If = 10 mA. In the event that your result is not a 'standard' value, select the most appropriate resistor value from the E12 resistor series.
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The voltage across the current limiting resistor is given by Vs − Vf.
From Ohm's law, the current limiting resistance is:
R = (Vs − Vf) / If
= (5 − 2) / 10 × 10-3
= 300 Ω
This is not a standard value in the E12 series, so the next largest value of 330Ω should be chosen, in order to keep the current below the maximum given in the data sheet – hence extending the life of the display.
LM3914 Display Driver IC
The Texas Instruments LM3914 display driver IC (see data sheet) may be used to control a bar graph display, as shown in the example circuit below:
Full design details are available in the data sheet, but the main points are:
- The analogue input is in the range 0−5 V, with this input range set by the values of R1 and R2. The full scale reference value is given by Vref_out = 1.25 × (1 + R2 / R1). Omitting R2 will give a 1.25 V input range.
- LED outputs are open collector and current regulated, so individual current limiting series resistors are not required. The series current is controlled by the chosen value of R1 and is approximately equal to 12.5 / R1. If a display brightness control is required then consider replacing R1 with a resistor and potentiometer series combination.
- Switch SW1 selects a bar or single dot display.
- A 2.2 µF tantalum capacitor (or 10 µF electrolytic capacitor) is required if the display leads are 15 cm (6") or more in length.