TESTING AN OPTO COUPLER

Most multimeters cannot test the LED on the input of an opto-coupler because the
ohms range does not have a voltage high enough to activate the LED with at least
2mA.
You need to set-up the test-circuit shown below with a 1k resistor on the input and
1k5 on the output. When the 1k is connected to 12v, the output LED will illuminate.
The opto-coupler should be removed from circuit to perform this test.