Electronics > Projects, Designs, and Technical Stuff
Brainstorm please! High speed (>20krpm) IR reflective optical trigger
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snarkysparky:
BF862 jfet amp with bipolar cascode load is pretty fast for photodiode detector.


http://www.ti.com/lit/an/snoa620/snoa620.pdf

perhaps a phase lock loop to smooth out jitter ?
max_torque:
Ok, that's one 10 to 50 nanosecond high intensity pulsed IR source knocked up:



Looks like a right dogs dinner, but works a charm! 

It should allow me to build and test the detector stage and get an idea of the responsive and delays inherent in that side of the device.

One thing i've been wondering, is how to drive the signal data up the cable and into the counter unit.  It will probably end up a couple of feet away from the IR head.  I guess a fast buffer driving into a coax is the answer, but i wonder if a balanced (differential) twisted pair might be better?  Or perhaps a low impedance current loop?  The environment will be electrically noisy, and i need to get the nice sharp electrical signal out of the sensing head and up to the measurement unit without falling foul of noise or reflections etc.  Any suggestions anyone?
StillTrying:
"10 to 50 nanosecond high intensity"

You'll be lucky! Does a mobile phone camera see anything of the IR pulses.
max_torque:
nope, not even close, i can't find a CCD device that sees the output at all.

a fast photodiode with 30V backwards across it with a low gain transimpedance amp  repeatably shows 10ns pulses, which match the pulse time (with about 3ns of latency) of the trigger pulse.  There's a bit of ringing for around 5 to 10 ns on the off egde, because i am using a inductor to provide a quick reverse bias to the LED when the fet switches off
StillTrying:
10ns pulses and 3ns latency will just be noise, real light pulses will be 10X slower than that.
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