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Suppose we need to control power of a device (12v, 500mA).

I tried to use TLP281 board, which works, except that it inverts the output: device is powered when there is no control signal.

For this reason, the TLP281 board is not useful in my project.

This is picture and schematics for TLP281 board:

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I need an optoisolator board which works analogous to normally open mechanical relay: when there is no signal, device must not be powered; and when there is signal, device must be powered.

I consider to use ULN2003 arduino board instead of TLP281 board. Is ULN2003 supposed to be used as a relay at all?

NOTE: workaround by inverting the control signal in microcontroller firmware is not permitted, as the device must not be powered when microcontroller is off

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  • \$\begingroup\$ Are you using a P-Channel MOSFET/PNP BJT? I am asking this since I had a look at the TLP281 schematics and it does not seems to be an inverting unit. And could you post a schematic of the circuit you want to control? It would be much easier to see what it is needed in order to it to work according to your needs. \$\endgroup\$ – Daniele Tampieri Feb 25 at 8:25
  • \$\begingroup\$ @DanieleTampieri done \$\endgroup\$ – Igor Liferenko Feb 25 at 9:14
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The board has additional discrete transistors to invert logic (signal on the input pins is reflected on output pins), stand alone TLP-281 sans discrete transistors may solve your problem....More curious about your load, thought current was limited to 50mA, you imply it works with 500mA?

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  • \$\begingroup\$ ...nevermind....answer looks like the discrete transistor handles the current also. The opto by itself would not likely handle your load. \$\endgroup\$ – Steve Mead Feb 26 at 18:13
  • \$\begingroup\$ Do you know use cases of optocoupler (used in TLP281) vs field-effect transistor (used in ULN2003)? It looks like both allow to switch load on/off, but how to choose between them? \$\endgroup\$ – Igor Liferenko Feb 27 at 1:15

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