Multiplexing Five 7-Segment Displays -- Appear Dim


I am using 5 seven segment displays, with an Arduino Mega. They are common anode. Each anode is hooked up to the emitter of a NPN transistor, the collector powered by 5 volts, and the base is powered by a High/Low from the Arduino. I have them multiplexing correctly to display what I need, but the problem is the displays are very faint.

Is this a problem of the current draw being too much, or is it a result of the actual display? I don't want to invest in different displays unless I know that they will be bright enough. Do different displays have different brightness's?

Thank you.

Where is the current limiting resistor? If you only have one in the anode that will cause them to be dim. You need one in each cathode.

Each anode is hooked up to the emitter of a NPN transistor,

That is not the way to use an NPN transistor the load should be in the collector.

How about drawing what you have?

Currently, the resistors (270ohms each) are in series with each cathode. Will adding another to the anode do anything? Or should I reduce the value of the resistor?

Will adding another to the anode do anything?

It will only make things worse.

should I reduce the value of the resistor?

Not at the moment get the transistor right first that is limiting the brightness at the moment.

Thanks. I'll give it a try.

dpd814: Thanks. I'll give it a try.

OK but give what to try, draw the circuit you are going to use. NPN transistors only pull down. If you are going to pull up you need PNP transistors.

Its a similar configuration for each display. only the base pin changes.

As I said in reply #1 that is not the way to drive things with an NPN transistor.
Change it to a PNP transistor and swap the connection between emitter and collector.

What is Vcc? Is it 5V?
As you have drawn it you will only ever get 4.3V maximum on the anode of that display. Which is why it is dim.

As Mike says, you need to use PNP transistors in the common emitter configuration instead of NPN transistors to switch the anodes - and then you will need to drive the corresponding pin LOW instead of HIGH to enable the corresponding anode. However, your 270 ohm resistors are probably a little on the high side. Check the display datasheet to see the recommended current (probably about 20mA) and forward voltage at the current, subtract that forward voltage plus 0.1V (for the PNP transistor) from 5V, and use Ohm's Law to calculate the series resistor value. It will probably come to about 100 ohms, maybe a little more if the display is red.

Some seven segment displays have two or three LEDs in seriese. This means they need the full 5V to turn on the LEDs check your data sheet.