Bicolor LED array driver circuit?

[blapcb]

led array driver

Hi guys. I have 8 bicolor LEDs that I plan to drive from 8x2 ports of the micro. The LEDs are common cathode. Basic idea is to ground the cathode and connect a NPN transistor before every anode and connect the base of the transistor to the micro IO pin. No, this will of course work, if the collectors of the NPNs are connected to VCC (5v) via some resistor.

But here is my question....

In an attempt to get a high brightness out of the LEDs, I would like to use some form of PWM to pulse a higher current through them. So my question is, instead of connecting all the NPNs collectors to VCC, can I use one more transistor as a switch, and turn it on and off using some 555 circuit?

Again basically I guess the idea is correct, but, what I don't know is will the current be uniform across all LEDs (given that any combination may be glowing at any given point in time) or will the LEDs intensity vary with the number of LEDs on? How do I get them to glow nice and bright in a uniform way?

Thanks!

Oh, attached a circuit that sort of gives the idea... (excuse the messy drawing!)

 

[FvM]

bicolor led circuit

connect the base of the transistor to the micro IO pin

Doesn't work without base resistors.

 

[blapcb]

bicolor led driver

Doesn't work without base resistors.

Yes, true. Forgot those (was using an array before and it was internal). But otherwise? Will this method work?

 

[FvM]

bicolor led

Basically yes.

 

[blapcb]

bi color led driver

I am not sure that "Basically" will cut it for my application.... :-(

Sorry, I am not that great with analog calculations and stuff. Will the brightness of the LEDs change significantly based on how many of them are turned on? Do I need another resistor between the upper transistor's collector and Vcc? (and if so, how do I determine it's value?).

Another approach - what about ICs such as this one?
https://www.onsemi.com/pub_link/Collateral/NCP5608-D.PDF
(though I don't really have any I2C to spare on my micro....)

Thanks

 

[Audioguru]

led array driver circuit

Pulsing the LEDs with a high current pulse does not make them any brighter than their average unpulsed continuous current. Since they are turned off for most of the time then they will look dim.

A light dimmer uses PWM to dim, not to brighten.

 

[blapcb]

bi-color led circuit

Pulsing the LEDs with a high current pulse does not make them any brighter than their average unpulsed continuous current. Since they are turned off for most of the time then they will look dim.

A light dimmer uses PWM to dim, not to brighten.

OK, so, how do I get a uniform good brightness for all my 8 LEDs with the ability to switch them individually? What am I doing wrong?

 

[FvM]

pulsing led circuit

There's nothing wrong. Audioguru wanted to mention, that PWM can only dim the LEDs, not increase the intensity compared to static operation. I guess, you already know this. To avoid a brightness variation with the number of switched-on LEDs, the common PWM switch must have a low voltage drop respictively on-resistance. Another common resistor would be completely wrong.

As a problem with your circuit, a NPN transistor never turns on completely, reducing the regulation effect of individual LED series resistors. Thus, a PMOS transistor, controlled by an active low PWM signal would be a much better solution. A small SOT23 transistor as SI2301 can drive up to 2A, the below 100 mA supply demand of your circuit would cause about 10 mV voltage drop only. Alternatively, a PNP BJT can be used.

 

[sukel]

Re: led array driver

Hi guys. I have 8 bicolor LEDs that I plan to drive from 8x2 ports of the micro. The LEDs are common cathode. Basic idea is to ground the cathode and connect a NPN transistor before every anode and connect the base of the transistor to the micro IO pin. No, this will of course work, if the collectors of the NPNs are connected to VCC (5v) via some resistor.

But here is my question....

In an attempt to get a high brightness out of the LEDs, I would like to use some form of PWM to pulse a higher current through them. So my question is, instead of connecting all the NPNs collectors to VCC, can I use one more transistor as a switch, and turn it on and off using some 555 circuit?

Again basically I guess the idea is correct, but, what I don't know is will the current be uniform across all LEDs (given that any combination may be glowing at any given point in time) or will the LEDs intensity vary with the number of LEDs on? How do I get them to glow nice and bright in a uniform way?

Thanks!

Oh, attached a circuit that sort of gives the idea... (excuse the messy drawing!)

what it uses an active low input?