Robonz wrote:Well that rules out PWM. At 128 levels of video levels that would equate to 1Mhz x 128 = 128Mhz PWM frequency.
I'm not quite sure where the working out above comes from. I would have thought that (say) 5MHz of PWM would be adequate. Steve Ostler used PWM at 100kHz in his colour mechanical monitor, the bandwidth of the signal was around 10-15kHz as a guess.
In this case the signal was analogue, not derived from a D-A which (in theory) means there's an infinite number of levels between black and white for an analogue signal.
However, even a 5MHz PWM circuit is not for the faint-hearted! And yes, the LED junction capacitance does rear its ugly head. The same applies to photodiodes, but here it's easier. Just shove the output of the photodiode into a zero-impedance load - a transimpedance amplifier, e.g. a grounded-base transistor being the simplest.
The BPW34 is a good choice, I have used it and others have too. But make sure you get the transparent version with the clear case, there's also an IR version which is black. The pdf for the IR version is attached.
Steve A.