I'm currently making a project with multiple series circuits to PWM control multiple LEDs. I have figured out how to make it work with a transistor, however I was curious if this way would work as well. Thanks for your help!


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ How much current do you need for your LEDs? How much current can your Arduino pin supply without exceeding its limits? \$\endgroup\$
    – Andy aka
    Dec 4, 2018 at 12:55
  • \$\begingroup\$ why did you put two resistors? \$\endgroup\$ Dec 4, 2018 at 12:57
  • \$\begingroup\$ Which untermniated wire at the right is going to "To 5 volt pwm..." ? Where's the other wire going? \$\endgroup\$
    – mike65535
    Dec 4, 2018 at 13:00
  • 1
    \$\begingroup\$ Ideally, current would be (7.5V-2-2-2)/132ohm =12.2mA. However, VLo from Arduino will not be 0, but something a little higher, somewhere between 0.6 to 0.9V per page 365 of the 2018 Atmega328P datasheet. So (7.5-2-2-2-0.6)/132 = 6.8mA. What voltage does the pin see when it is an input put? Will it see 7.5V clamped to 5V by the internal clamping diodes, which are suggested to be limited to 1mA of current? \$\endgroup\$
    – CrossRoads
    Dec 4, 2018 at 14:01
  • 1
    \$\begingroup\$ This is just a bad idea... even if you get away with it as the LED drops are likely sufficient (if and only if the 7.5v is never for an instant present without the Arduino's supply), you save just a few cents on a transistor or FET. \$\endgroup\$ Dec 4, 2018 at 15:25

1 Answer 1


Use the Arduino's 3.3V rather than a 7.5V supply.
Power each LED separately each with its own resistor rather than all three in series. Wire them as common cathode.
Use LEDs with high luminous efficiency to reduce the current.
Just because an LED is rated at 20 mA does not mean you have to run it at 20 mA.
With a high luminous intensity LED you can likely reduce the current to 1 mA and still have a fairly bright LED.

Use a 22,450 mcd Cree C503B at 1 mA you will still have over 1,100 mcd and the ATmega only has to sink 3 mA.

With three LEDs wired common cathode the ATmega could sink 6 mA per LED at 6,700 mcd.
If 6,700 mcd is not enough, double them the up (six LEDs) and use the 5V supply.

If you can use surface mount LEDs use Luxeon Color C, OSRAM Oslon SSL 80, or Cree XP-E.


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