I brought up this question in my previous question on Connecting a Camera Hot Shoe to a homemade strobe, but there was no answer to this specific point.

Is there such thing as a "dark on" opto-isolator, that allows current to flow through pins 5 and 4 when no current flows trough pins 1 and 2, and blocks current through pins 5 and 4 when a current flows trough pins 1 and 2?

This will be like a normal opto-isolator, but it "turns on" when the LED is turned off, instead of the opposite.

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    \$\begingroup\$ If you add an inverter to the output of the opto-isolator you will achieve the result you're after. There may be a device that has the inverter integrated into the opto-isolator. However, there is definitely not such a device that consists of just an optical transistor and an LED based on the junction physics (i.e. the junction is turned on by the light) that will turn on when no light is applied to the junction. \$\endgroup\$ – Captainj2001 Jun 27 '16 at 12:55
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    \$\begingroup\$ @Captainj2001 you should make that your answer. It's concise and correct. \$\endgroup\$ – Brian Onn Jun 27 '16 at 13:00
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    \$\begingroup\$ @captainj2001 you'd be wrong then. There are reverse optos. See dupe. \$\endgroup\$ – Passerby Jun 27 '16 at 13:10
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    \$\begingroup\$ A MAJOR problem in most cases is the OP (that's you) asking for ways to achieve an already arrived at solution to a problem INSTEAD of explaining the actual problem and asking for answers. If you are an EGU (electronics guru extroadinaire) it may be that you need exactly what you are asking for. However, if you are not an EGU then it would be a REALLY good idea to explain what you wish to achieve and see what the EGUs here have to say. The "solution" that you are talking of has its place but is seldom a good enough solution to be worth using due to its several disadvantages. \$\endgroup\$ – Russell McMahon Jun 27 '16 at 13:18
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    \$\begingroup\$ Having to reference another question and its competent but a bit long to take in at a few bites description makes answering harder. | An "easy" solution would be to charge a capacitor from the source voltage prior to the lash being tripped and use this capacitor as an energy source to send a pulse via an optocoupler when the input is shorted. | This requires eg a diode to charge a capacitor, perhaps a series resistor to limit charge rate (maybe not) and a comparator to detect when Vin falls to zero and pulse the optocoupler LED from the capacitor. A MOSFET could probably be all that was ... \$\endgroup\$ – Russell McMahon Jun 27 '16 at 15:19

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