pnp transistor array with common emitter

Hello, i am making a project that uses 7 rgb leds, that are common cathode, which means i need to use pnp transistors to control them, so i am looking for a chip that would have at least 8 pnp transistors on it, and a common emitter would be ideal, so far i have trouble finding this kind of chip.

Any help would be greatly appreciated

I think you are confused.
Use NPN, or N-channel MOSFETs, to connect common cathode to Gnd. Source current into the discrete anodes.

ULN2803 with open collector NPN output as a buffer to Arduino output, or as a buffer to 74HC595 shift register, or TPIC6B595 shift register which has high current open drain output, are commonly used.

see attached

ca3096.pdf (133 KB)

CrossRoads:
I think you are confused.
Use NPN, or N-channel MOSFETs, to connect common cathode to Gnd. Source current into the discrete anodes.

ULN2803 with open collector NPN output as a buffer to Arduino output, or as a buffer to 74HC595 shift register, or TPIC6B595 shift register which has high current open drain output, are commonly used.

I have RGB leds that have one ground pin, and three + pins for each color, so that would mean that i need to use a transistor before the load, so i would need to use a pnp transistor, no?

raschemmel:
see attached

that one has both pnp and npn transistors, and only 5 of them at that?

This, or draw a picture of what you think you are doing:

When you decide what you want we can look for a chip.

I want to control each of the color individually, without drawing current from the arduino.

I was thinking of doing it as in the attached picture.

Ask Crossroads to redesign it. He has way more experience than you.

I would be very grateful if someone could redesign it, as i see no other choice. I know there are things like led drivers that work through I2C, but i want to drive it from the arduino, and not get into learning how to use I2C devices and such right now.

You need current to drive PNP base, you need current to drive the LED anode.
I don't see the need to add the transistors.
You can if you want to, be sure to add a 180 ohm resistor between the Arduino pin and the PNP base. Drive the base Low to turn on the PNP.

CrossRoads:
You need current to drive PNP base, you need current to drive the LED anode.
I don't see the need to add the transistors.
You can if you want to, be sure to add a 180 ohm resistor between the Arduino pin and the PNP base. Drive the base Low to turn on the PNP.

I see, what if i use a 10k resistor for the base? Is that too much?

Doubt you'll get much current flow with 10K. I wouldn't go much above 1K.

Would this be suitable?

The forward voltage of the blue/green leds is likely to be around 3.2V, but with 5V supply, even if the udn drops a volt, it should be ok with a low value current limiting resistor?

Yes, but you can't get them anymore.

I was thinking of doing it as in the attached picture.

Even with series base resistors, the Arduino would never completely turn off the LEDs. The problem is that logic HIGH is only at 4.2V.

Control is still possible, but a pull-up resistor at each base would also be needed. The resistor network values would need to be calculated. My guess would be 470 ohm pull-up at the base and 1 K series resistor to the Arduino pin.

dlloyd:

I was thinking of doing it as in the attached picture.

Even with series base resistors, the Arduino would never completely turn off the LEDs. The problem is that logic HIGH is only at 4.2V.

Control is still possible, but a pull-up resistor at each base would also be needed. The resistor network values would need to be calculated. My guess would be 470 ohm pull-up at the base and 1 K series resistor to the Arduino pin.

I was told that the arduino output pins do not need pullup resistors, as they are actively driven to both high and low states, does that still mean i need pullup resistors?

Also, what if i would use p channel mosfets? would that be a better idea?

I was told that the arduino output pins do not need pullup resistors, as they are actively driven to both high and low states, does that still mean i need pullup resistors?

The pull-ups would not be for the arduino, but for the base of the PNP transistors. With the collectors connected to 5V and the base at i.e. 4.1V, the transistor would still be conducting.

With a voltage divider, as shown below, when the Arduino pin is at 4.1V, the voltage at the resistor divider junction would now be 4.7V, so the transistor is completely turned off. When the Arduino output goes low, the divider tries to bring the voltage at the divider down to 3.3V and it sinks enough base current to completely turn on the transistor.

dlloyd:

I was told that the arduino output pins do not need pullup resistors, as they are actively driven to both high and low states, does that still mean i need pullup resistors?

The pull-ups would not be for the arduino, but for the base of the PNP transistors. With the collectors connected to 5V and the base at i.e. 4.1V, the transistor would still be conducting.

With a voltage divider, as shown below, when the Arduino pin is at 4.1V, the voltage at the resistor divider junction would now be 4.7V, so the transistor is completely turned off. When the Arduino output goes low, the divider tries to bring the voltage at the divider down to 3.3V and it sinks enough base current to completely turn on the transistor.

Thank you.

Would i keep the same resistors with the same values if i were to use a p channel mosfet?

Would i keep the same resistors with the same values if i were to use a p channel mosfet?

No - mosfets are essentially voltage driven. If you decide to use mosfets, make sure they're logic level type.

Would a BS250 mosfet be fine? What kind of resistors would i need then?