I am hoping to hook up two servos and a high power (1 W) LED to an Arduino to control them. Due to my inexperience, I thought it would be best to check here to see if this circuit will work.

My main uncertainties about this circuit are:

  • Is the LM317 transistor the right component?
  • Is the 100 Ω Resistor the right size for the 1 W (3.3 V, 0.33 A) LED?

If someone is able to give some advice for this set up that would be amazing. I apologise in advance for the non-standard diagram (image attached).

Circuit Diagram for Arduino with 2 servos and 1 high power LED

Circuit Lab (UPDATED) circuit below:


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ You have no pin numbers or functions on the LM317, etc., so it's difficult to know. It appears that you are trying to power the LM317 from D3 which will have a 20 to 40 mA max current so that won't work. The LM317 is a voltage regulator (not a "transistor). There are many transistors inside it. There's a CircuitLab schematic button on the editor toolbar. It's easy to use and saves an editable schematic in-line in your question. There's a custom component which you can modify for any odd-bod bits. Double-click to set parameters, pins, etc. 'R', 'H' and 'V' for rotate, horiz-flip and vert-flip. \$\endgroup\$
    – Transistor
    Dec 26, 2018 at 0:25
  • \$\begingroup\$ For the LED, the 100 ohm resistor powered by 5 volts limits the 3.3-volt LED to less than about one twentieth of a Watt, which is probably not what you want. Oh, yes... and don't forget to heat sink the LED when you do get closer to 1 Watt. \$\endgroup\$ Dec 26, 2018 at 1:29
  • \$\begingroup\$ It's normally best to use a current controller or LDO regulator to drive high power leds rather than a resistor. You can either use the arduino to perform current regulation or find a current controller for your LED that has a logic level input. \$\endgroup\$
    – K H
    Dec 26, 2018 at 4:47
  • \$\begingroup\$ Thank you all for your help in this matter! It seems I may have been way off the mark in terms of the LED circuitry. I have updated the post with a CircuitLab circuit, with some minor changes. Please do advise if there are any more errors! @Transistor thanks for your help. Regards the Voltage Regulator LM317 (thanks for the clarification) I wasn't trying to power from D3, it should have been powering from the (5V step down), taking control from D3, and outputting to the LED. \$\endgroup\$
    – BaseThree
    Dec 26, 2018 at 4:59
  • \$\begingroup\$ @MicroservicesOnDDD cheers for letting me know! What size resistor would limit the voltage appropriately? \$\endgroup\$
    – BaseThree
    Dec 26, 2018 at 4:59

1 Answer 1


You should use a constant current driver for the LED such as this


See page 23 https://www.infineon.com/dgdl/Infineon-BCR420U-BCR421U-DS-v02_01-EN.pdf?fileId=5546d4626102d35a01617524f09e061d

The driver prevents the current from increasing as the LED heats up and tries to allow even more current to flow, leading to an overheating/burnout condition.

  • 1
    \$\begingroup\$ This chip's maximum current is 150 mA, less than half of what a 1 watt LED likely requires. \$\endgroup\$ Dec 26, 2018 at 17:53
  • \$\begingroup\$ 5V x 200mA = 1W, yes? 150 x 5V = 750mW. How are you getting < 0,5W? \$\endgroup\$
    – CrossRoads
    Dec 27, 2018 at 4:05
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    \$\begingroup\$ The question is by quite explicit statement about supplying 330 mA across an LED drop of approximately 3.3v. You can't calculate LED power from the supply voltage to the driver, but only from the drop actually across the LED excluding that across the current regulator. And the chip is designed for only 150 mA. \$\endgroup\$ Dec 27, 2018 at 4:10
  • \$\begingroup\$ You are right, I missed that. I saw the 12V to 5V converter going into the LED and was thinking 5V. There are other similar current controllers for 3.3V and higher current. Power them with 12V and let the controller output the correct current for the resulting Vf of the LED. \$\endgroup\$
    – CrossRoads
    Dec 27, 2018 at 4:14
  • \$\begingroup\$ With the kind of headroom of 12v in hopefully a switching regulator, or more dies in series, otherwise 1 watt into the LED means nearly 3 up in heat in a linear regulator. \$\endgroup\$ Dec 27, 2018 at 4:18

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