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At some point this year I noticed that my camera was not working. (Skype, Google, Facetime)
I tried this https://support.apple.com/en-us/HT2090
But under devices for Camera it says no devices found.
Mac OS 10.10.4
It might be some VDCAssistant programs using it.
Try this ,Open a terminal window and type the command below.
sudo killall VDCAssistant
This will kill all other programs and you will be able to use your camera again.