D401
88
can you tell me if there is any way i can test the optic device before i install it?
D401 is a typical photo diode emitter-receiver pair.
It will have one LED diode, and one photo transistor.
There should be one terminal on the part that will be a common ground.
There should be another terminal that will be the LED power terminal, which requires +5 V DC. If your DMM has an LED test function, you should be able to use it, connecting it between the LED power terminal, and the D401 ground terminal.
Testing an LED just using a resistor or continuity tester is not always effective. You really need to use an LED test function, if you have one that is available.
The photo transistor is essentially an NPN transistor, with the emitter connected to the part's ground terminal. The collector of the photo NPN should have its own terminal someplace on D401. The base of the photo transistor will be optically activated by the light beam of the LED emitter, and also switched by the rotation of the brass disk on the mechanical shaft.
I'm not completely sure, but if you have an hfe transistor test function on your DMM, you might be able to connect the ground terminal of the D401 part to the emitter connection of the hfe test port, and the collector terminal of the D401 to the collector of the hfe port. The base of the D401 would be controlled by rotating the shaft on the part.
For the transistor test to work, the LED would also have to be powered by +5 V DC, at its respective terminal.
If it looks like too much effort to test the photo transistor, then just test the LED, using the LED tester of the DMM, since the LED is a little more likely to fail than the photo-transistor.