I need to convert 12v inputs to a 3.3v electric imp005, i have a bunch of TLP521 optocouplers i bought to repair something (i've never used one in a circuit before).

From the data sheet it looks like the recommended forward current for the LED is 16ma at 1.15v so i think i need a 680 ohm resistor to power it from 12v

as for the detector resistor, is this just a pull up to stop the output floating?

does this seem right?

enter image description here

Thanks guys and sorry for the noob question.


  • \$\begingroup\$ The 1.15 V is specified for 10 mA, but the current it's actually designed for is the one the CTR is specified for, i.e., 5 mA. \$\endgroup\$
    – CL.
    Commented Sep 17, 2017 at 19:17

1 Answer 1


The 680 Ohm is fine.

The 10-k Ohm resistor on the detector may or may not be fine, depending on what is the speed you want to achieve. Larger resistor values will yield a smaller bandwidth. Smaller resistor values will yield higher speed, but they might increase the low-level voltage when the detector is on.

See figure from the datasheet:

enter image description here

  • \$\begingroup\$ switching times are irrelevant (as long as they are under say 500ms) \$\endgroup\$ Commented Sep 17, 2017 at 17:14
  • \$\begingroup\$ So 10k Ohm are adequate. \$\endgroup\$
    – next-hack
    Commented Sep 17, 2017 at 17:15

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