| A bargraph display is usually used to give a visual indication of an analogue voltage signal. |
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How does it operate?
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Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
A bargraph display consists of 10 LEDs, packaged in a 20 pin DIL IC.
It is usually used with a LM3914 bargraph display driver (as shown in the circuit on the left). The supply voltage Vs should be in the range of 3V to 20V.
If an analogue signal is fed into the display driver the number of LEDs lit increases as the analogue signal voltage increases.
If the analogue input signal voltage is 0V then no LEDs are on.
If the voltage on the ‘SIG’ input pin (pin 5) of the LM3914 increases to 1.25V the bargraph display shows ‘full scale’ (all 10 LEDs on). |
In the circuit diagram R1 and R2 form a potential divider, to scale down the Input signal by a factor of about four. So, an Input signal voltage of 5V gives a voltage at pin 5 of the driver IC of 1.25V, producing ‘full scale’ on the LEDs. To give a ‘full scale’ reading for a different Input signal voltage the values of R1 and R2 would need to be changed.
The resistor R3 regulates the current in the LEDs. The value of 1k used gives a LED current of about 12mA.
Possible applications
Showing variations in analogue quantities, such as:
- temperature;
- speed;
- light level;
- sound volume.
Making
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Pin diagram for the LM3914 Bargraph Display Driver
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Pin diagram for the Rapid 55-0190 Bargraph Display |
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Pin 1 is identified by small flat on the corner of the IC package, not by the usual dot or notch |
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The PCB for a driver unit and bargraph display might look similar to the example on the left.
Small IC pads and narrow tracks have been used to allow the tracks to be run between the pads. This reduces the number of wire links needed but needs careful soldering.
The driver IC and the bargraph display are both mounted with pin 1 at the bottom right (rather than the top left, as in the usual arrangement). This is done so that the first LED that lights is at the bottom, and to simplify the tracks. |
Build and test the unit that will provide the analogue input signal before adding the bargraph driver and display.
Testing
Check that:
- the voltage on pins 2, 4 and 8 of the driver IC is at 0V
- the voltage on pins 3 and 9 of the driver IC is at +Vs
- the voltage on pins 1 to 10 of the bargraph display is at +Vs
- as the analogue Input signal is varied, the voltage on pin 5 of the driver IC is about a quarter of the analogue Input signal
- as the analogue Input signal is varied, the number of illuminated LEDs on the bargraph display changes
Fault finding
If there is a fault, check all the connections and soldering.
Alternatives
- The LM3915 and the LM3916 are very similar to the LM3914, but they are more suitable for displaying sound volumes (like the familiar VU meters on hi-fi systems) because they show the information on a logarithmic scale
- An analogue voltage meter can be used to display voltage levels but this is probably more expensive.
- Separate LEDs (perhaps in a different arrangement) could be used in place of the bargraph display, but this has the disadvantage of making the wiring more complicated.
- Larger (12 LED) displays and drivers are available.
- A PIC could be used in place of the driver IC, giving more control over the display. An 18 pin PIC usually has 8 output signals, so this could not control all 10 LEDs.
Web links
- Manufacturer’s data sheet for LM3914N
Return to list of datasheets
