Normally-Open opto-coupler board I'm looking for a Normally-Open optocoupler board for arduino. I need to control power of a device (DC 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 (i.e., it is Normally-Closed).

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

This is picture and schematics for TLP281 board:







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.

NOTE: workaround by inverting the control signal in microcontroller firmware is not permitted, as the device must not be powered when microcontroller is off
I don't think there is a need for a specific "normally-open" opto-coupler. Same result can be easily achieved by using "normally-closed" opto-coupler and making a slightly modified "NOT" logic gate using three resistors and NPN transistor.



1. When INPUT is LOW:

Base of Q1 is shorted to ground through R1 so Q1 not letting the magic pixies go through. This makes OUTPUT show 5V and be in HIGH logic state.

2. When INPUT is HIGH:

Base of Q1 is powered by voltage divider R/R1. R needs to be much lower than R1 to keep Q1 saturated. This makes Q1 to be passing current and setting OUTPUT to LOW state by creating a voltage divider R2/Q1(collector-emitter resistance) This means that value of R2 should be much higher then c-e resistance of Q1.

3. When INPUT is floating (microcontroller is turned off):

Any voltage on INPUT caused by imperfections of the hardware, E-M noise induction, static electricity, etc. should be discharged to ground by resistors R+R1 keeping INPUT in LOW state. (This means that R+R1 cannot have to high value or the discharge speed might be to slow)