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Picture of Light Probe MK I

The first prototype of any circuit doesn’t always work almost never works 100% like expected and therefore troubleshooting is an essential part of the engineering process. This usually involves taking measurements of the circuit and tweaking things as needed. Commonly this is done with an oscilloscope, just probe the voltage of any signal and see how it changes over time.

However in some (rare) situations the light generated by a circuit is the "signal" you care about. Measuring the drive voltage/ current is not always possible and is also inaccurate. Personally I am curious about how the typical 0.96" OLED screen works, especially how the configuration settings, the panel voltage and the segment current effect the light output. During the design process I came across gerrit’s µs photography flash, which is used to "freeze" fast moving objects on photographs and he would also like to measure the light directly.

Obviously there are plenty of light sensors available, but not a single, public available design came even close to the speed I needed. Thus I created the Light Probe, a very fast light sensor which can be connected to any oscilloscope and used like any other probe! The original bandwidth target was around 10MHz, but the final design can now be used for at least 50 MHz and therefore matches or even exceeds the bandwidth of most entry level oscilloscopes, including the popular DS1054Z.


This instructable contains not only step-by-step build instructions, but also contains optional information why certain design decisions have been made, how it was tested and what optimisations were made. Feel free to jump to any chapter that interests you.

Step 1: Features & Specifications

The light probe behaves like an normal oscilloscope probe, but measures light instead of voltage. The probe contains all circuitry to convert the light into an electrical signal your scope can measure.


Features:

  • probe-shaped for easy handlin
  • fastest DIY light sensor to date!
  • SMA signal output connector suitable for any SMA cable
  • powered from any USB port, even the one of your scope
  • maker friendly
    • common components suitable for hand soldering
    • cost effective 2 layer PCB
    • customizable design
    • input power protection

The performance depends on many aspects, including part selection and the usage environment. Use the give values as a guideline only.


Specification:

  • bandwidth: 50MHz (typ.)
  • dynamic range: 4V (typ.)
  • zero light voltage: 92.5mV ±5mV

Warning: Although the power input is protected, the signal input is not protected. Any protection would add additional capacitance and thus slow down the probe. Do not expose the probe to very bright flashes of light, such as holding it directly in front of a xeon-flash. When in doubt, measure the light indirectly first by bouncing it off the wall or another diffuser.

softuard15 days ago
Great job.
shjalili1516 days ago
Really excellent work thanks
sh. j
jwpa19 days ago
Very cool design and excellent documentation. FWIW, in the old days, batteries were used in instrumentation - like old electrometers - when power supply noise was an issue. And you might consider encasing the sensor in a grounded metal shield. Again, very nice work.
nqtronix (author)  jwpa18 days ago
Thanks! I actually planned on adding a case, but didn't get around to try it in time for the contest. Right now one must pay attention to keep enough distance to other elctronic devices.

I try to avoid batteries for non-portable devices because of the added complexity and size. If I'd do this again I'd spend more time on the design before prototyping it, but I simply did not expect noise to be an issue.
Bverysharp19 days ago
Great instructable. Well written. Thanks.
orlovnik19 days ago
Really excellent work!
kjhall1020 days ago
Excellent work, a lot of engineering went into this project.
satkwcam20 days ago
really cool, it is amazing, wow, keep going my bro.
JJ Slabbert22 days ago
I like the fact that you build your own sensor. The other entries in the sensor contest only show how other sensors is used.