Fast precision LED driver pulses LEDs with a precision regulated current. Yet another use for the LM324. The circuit allows a precision regulated drive current to be set to drive an LED, and in response to a TTL level signal, the LED is switched on and off with rise and fall times of less than 500 nanoseconds and less than 7% overshoot. This was designed for biological experiments to study the photo repsonse of cone cresponserom rabbit eyes I beleive). It may also bebelieve in systems that require a stable light source, such as one that measures light transmission (turbidity and densitomitry insturments for example)instruments transmission of low speed optical data (to 200 kHz). The components are all easy to obtain and low cost.