in upper part of pic:

It has a fast HP4360 or 6N137 opto-coupler in it (center right). It has also a 7805 stabilizer to get a 5V output for the opto-coupler and for the 5V->15V DC/DC converter; the latter (center) gives an isolated 15V supply for feeding the PWM unit. The idea was to get isolated 15V and 5V supplies from one 15V feed; this setup reflects the specific parts I had and should be adapted appropriately; 12V instead of 15V works equally well here. In particular one could use an independent second supply, thereby avoiding the DC/DC converter.