Build a (sort of) photocoupler using a photoresistor and a LED

Believe it or not... It is possible to build a resistive opto-isolator!

Parts to continue the Desktop Defender mod are on their way. We will use a IC contianing 4 opto-isolators... And we need 5 "digital switches"...
Why do not use a really low cost device to trigger the missile launch?

Necessary parts? A LED and a photoresistor.
The photoresistor I found in my box reaches 520Ω when the LED is glowing and acts as open circuit when the LED is off.
To be sure to not close the circuit with some environmental light, we have to isolate the led and the resistor using some isolating tape or some plasticine. The best is to put some aluminium foil, so that light will reflect and will redirect rays to the resistor.

Remember to put a marker to remember which is the side of the LED!