Connecting 3 red LEDs to single pin

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pobo123
 
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Connecting 3 red LEDs to single pin

Post by pobo123 »

Hi. I would like to connect and control 3 red LEDs to a single PCA9685 PWM pin. I am using an external 5V power supply to power the PCA9685 so I have the anode for my LEDs connected to the V+ line / pin and my cathode connected to my PWM pin(8). Can I connect 3 red leads (each has forward voltage 2-2.4v and current 20mA) in parallel or series and control them from a single pin. In parallel I will use 3 resistors but if in series just one. My question is, is this possible or will it damage the board, please?

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Re: Connecting 3 red LEDs to single pin

Post by adafruit_support_bill »

If powering with 5v, the output voltage will also be 5v, so there is not enough voltage to drive the 3 LEDs in series. You could wire them in parallel, but the pins are only rated for 25mA each, so you would not be able to drive all 3 LEDs to max (20mA) output. Also note that there is already a 220 ohm resistor in series with each output pin.

Other options would be to use the pin to drive a transistor which could switch current to the LEDs. A PN2222 can handle up to about 500mA. https://www.adafruit.com/product/756
https://learn.adafruit.com/transistors-101

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pobo123
 
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Re: Connecting 3 red LEDs to single pin

Post by pobo123 »

Thanks for the quick response.

"Also note that there is already a 220 ohm resistor in series with each output pin" - does this mean I don't need to use another resistor for each LED?

I think I may just use a pin for each LED and chain some more PCA9685 boards together. I'm also, considering using a MOSFET with a 12V supply and using the pin as a switch. I haven't used transistors yet so would prefer to use a MOSFET if I can.

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adafruit_support_bill
 
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Re: Connecting 3 red LEDs to single pin

Post by adafruit_support_bill »

Assuming a Vf of 2v for your LEDs, the 220 ohm resistor will limit the total current to about 14mA. So none of the LEDs are going to be in danger of overload. Although none will be close to full brightness either.

A MOSFET designed to work with logic-level signals can be simpler to wire than a BJT, since you don't need a resistor on the gate.

Also check out the TLC5947 PWM drivers. These can directly handle higher voltages, so you could control several LEDs in series on one pin with no external components: https://www.adafruit.com/product/1429

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pobo123
 
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Re: Connecting 3 red LEDs to single pin

Post by pobo123 »

"Assuming a Vf of 2v for your LEDs, the 220 ohm resistor will limit the total current to about 14mA. So none of the LEDs are going to be in danger of overload. Although none will be close to full brightness either." - is this with the internal PCA9685 3.5V power supply and would the lights be brighter if I add an external 5V power supply?

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Re: Connecting 3 red LEDs to single pin

Post by adafruit_support_bill »

That would be with a 5v supply. With a Vf of 2v, there would be a 3v drop across the resistor and 3v / 220 ohms = 13.6mA

Using a 3.5v supply, you would have 1.5v across the resistor and just 6.8mA.

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Re: Connecting 3 red LEDs to single pin

Post by pobo123 »

Thanks for clarifying that :-)

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