Making a clock cp for counter, using a photodiode
Try this







simulate this circuit &ndash; Schematic created using CircuitLab

The 7414 is a Schmitt trigger, and will handle even very slow transitions of light level. If you just use an ordinary gate, and the input changes slowly, you may get multiple triggers as the input responds to noise right at the threshold. Conversely, if your light pulse has a very fast turn-on AND turn-off, you may not need the inverter at all.

R1 is used to set sensitivity. But note that a "real" 7414 needs 1.2 mA of photodiode current, plus the resistor current to operate. If your light source is not intense enough, the photodiode won't be able to pull the input low. In that case you can try a 74LS14 which, like all 74LS, only requires 0.4 mA. If that's still too much, go to a 74HC14. It will work just fine driving the original 7400 series. 

And, of course, if you need the clock on the falling edge of the light pulse, just add another inverter.