This LED Optocoupler circuit uses two ordinary LEDs as the optocoupler element. When a voltage of 2V is fed to the input, LED 2 will generate a voltage that is strong enough to switch on transistor T5. The upper frequency limit is around 38 kHz. Very low pulse frequencies are almost meaningless for the circuit.
Eventhough the LEDs are quite intensive to stray light, it is a good idea to enclose them with an opaque box. Resistor R2 must be selected basing on the power supply voltage. It can be between 680Ω and 1KΩ. Resistor R11 must not be lower than 220Ω.
Transistor T6 has an open-collecter character.