I'm trying to design a P-channel MOSFET switch with LED indicator. When the motor will turn on at same time a led indicate the the MOSFET switch is turned on. I could not find such a circuit. Can you verify this circuit is a good design, or can you recommend how a LED indicator should be? Optocoupler is LTV-817. MOSFET is FDT434P.

Note that I want to separate the LED from the Arduino. I don't want to use an Arduino PIN for indicator purpose. enter image description here

  • \$\begingroup\$ What purpose do R5 have? \$\endgroup\$
    – MiNiMe
    Nov 9, 2023 at 13:27

1 Answer 1


The schematic needs to be revised.

  • The maximum source-gate voltage for the selected MOSFET is 8V. When you turn the MOSFET on using the optocoupler the source-gate voltage will be around 12V so the MOSFET may break down. So R2-R3 divider should be revised in order to bring the maximum source-gate voltage to somewhere less than 8V. A Zener might be used but if the input voltage is constant (e.g. 12V ± 10%) then may not be needed.
  • I couldn't understand the purpose of R5 which is in parallel with LED-R4 block. Can be removed.
  • The connection of indicator looks correct – it'll light up when the motor's top-side terminal sees 12V through PMOS' on-resistance. One thing I noticed is that, although I don't know what LED you're going to use, the current therefore intensity/visibility could be low with 10k limiter resistor (R4). For example, for a red LED the forward drop is about 2V, this makes the LED current 1 mA. This may or may not be enough depending on the LED itself. This will get even lower with other LEDs such as green or blue/white. So you may need to play with R4 but without exceeding the LED's and limiter resistor's limits.

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