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Klaas Robers wrote:Harry, running an LED from an adjustable voltage is not the way to do it. Much better is an adjustable current (mA or even more uA). That is easily obtained by circuiting a resistor in series with the LED. Then a potentiometer is an adjustable resistor, so you can go into all directions. The big advantage of a resistor is too that you may use a square wave generator that gives say 5 volts and use that with the adjustable resistor (potentiometer) in series to have a fast switching LED. You can see that on the oscilloscope.
Klaas Robers wrote:I would prefer the circuit called "Transmitter Circuit". That one really sends a modulated CURRENT into the LED. That is what you should like.
Klaas Robers wrote:I think the PMT is saturated, far too much light. make the 100 ohm resistor in the emitter lead much higher, 1k or 10k. Then you get (much) less light out of the LED. PMTs have to work in the shade, or even better in the dark.
Klaas Robers wrote:This looks already much better, however I see some lag, something that looks like afterglow of a phosfor. But there is no phosfor, so, what can it be? Is this with the PMT on 300 volt? May be that is the problem? Are PMTs getting much slower if they are on a lower voltage?
[/quote]I would first look with the oscilloscope on the base of the transistor and on the emitter of the transistor. There should be wave forms comparable, if not identical to the input wave form. If that is the case, the LED output will be Ok.
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