Electronics > Projects, Designs, and Technical Stuff
Visible Light Oscilloscope Probe
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graybeard:


I describe the design and breadboard construction of a visible light probe for an oscilloscope that is useful for measuring light waveforms such as flicker from LEDs, florescent, and incandescent lights.  It can measure fast switching light waveforms from LED lights and photo strobes (flashes).  It as also useful for measuring the output waveforms of infrared remote controls.

I go over my initial design specifications, how I chose the key components, and the details of how the circuit works.  I go over the input offset sampling noise  of zero-drift operational amplifiers (op-amps) and show how that effects circuit performance.

I demonstrate measurement of the fast switching light waveform from an underwater flashlight, the measurement of a IR remote control, and the flicker waveforms from a LED and florescent light.

You can download a copy of the slides here
NiHaoMike:
So basically a higher bandwidth version of the "light to sound" kit.

I wonder if a USB version could be made with a direct sampling RTLSDR. Could be useful when combined with a smartphone to evaluate flicker of lighting. (A really cheap version would just be a phototransistor or photodiode connected to a headphone jack, but smartphones are slowly phasing that out...)
james_s:
That's neat! I think I might build one of those. In the past I have just used a small photovoltaic panel connected directly to the scope and it worked well to look at the flicker waveforms of various lamps but I have no idea what the frequency response is like or how linear it is. Sure would be nice if that noise could be reduced without needing to filter the output though.
not1xor1:

--- Quote from: graybeard on November 30, 2019, 02:52:24 am ---You can download a copy of the slides here

--- End quote ---

interesting project, but I can't imagine anything more printer unfriendly than a black baground PDF...
You must be paid by the printer ink lobby  >:D
graybeard:

--- Quote from: james_s on November 30, 2019, 05:42:00 am ---That's neat! I think I might build one of those. In the past I have just used a small photovoltaic panel connected directly to the scope and it worked well to look at the flicker waveforms of various lamps but I have no idea what the frequency response is like or how linear it is. Sure would be nice if that noise could be reduced without needing to filter the output though.

--- End quote ---

You can use a more standard high speed op-amp instead of a zero-drift op-amp, but then you will need to add an offset adjustment to compensate for the input voltage and current offset of the op-amp.  You may also need to add some resistors in series with the non-inverting inputs to compensate for input bias currents of bipolar op-amps.   The only reason I did not do that was my desire to build a unit that did not require offset adjustment.
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