I did the same with mine. I have an IR photodiode mounted in a plastic plate and I place the camera with the back open onto this which places the diode at the film plane (or close enough). I use it on my kitchen worktop directly under a light fitting and I have found it works just as well with or without a lens fitted (35mm SLRs). It works fine with in shutter lenses too.
Originally Posted by BetterSense
Mine has a 7 digit (7 segment LED) counter and a 100kHz oscilator which is gated by the photodiode. It gives the shutter time in milliseconds to the nearest one hundreth of a millisecond.