Seeing PWM

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jeff51's picture
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Location: Middle of Texas
Seeing PWM

After reading the excellent review of various diodes (etc.) for measuring PWM by Terry Oregon:

I decided to see if I could get hold of the OSRAM BPW34S
I found this on Amazon:
Which is currently unavailable (Hruumph!)

5 of them for $10 for a BPW34 (no S on the end – S for surface mount?). No need to wait for China, hopefully they would be similar to the one tested.

The envelope arrived and I thought it was empty. Inside was a second envelope with the BPW34. No documentation at all.
Geeze these things are small!

I had been using a cheap solar cell to look at PWM

At about $3, this along with some sort of oscilloscope will give you a look at what the light is doing PWM wise.

Anyhow – I hooked it up as per Terry’s example. Terry thanks for the shot telling us which is the negative side.

I built this setup to test PWM
A tube to hold the sensor, a battery holder, and a spot to hook the scope across the resistor array. With hot glue to hold it all (naturally).
Later I added a lid to hold a filter.
Be sure to put the scope leads on the correct way or you will get a very funny looking output. I had the invert button pushed on the scope and was mighty confused until I did a dual trace with an overhead light.

I found I needed at least 5 10K resistors to get a big enough voltage to see a trace from the overhead florescent lights in my office using my old BK scope. I guess I’m losing some rise time with this setup. If I built this again I might use a POT instead like Terry.

How’s it look?
Take a look at the picture of the florescent lights in my office.
The top trace is from a cheap solar cell I have been using to look at PWM.
The bottom trace is using the BPW34. Quite a resolution difference!

The second picture is a pair of traces from a flashlight. See how the solar cell has a slow V drop compared to the BPW34.

I do notice a negative (ring?) on the test setup. Anybody have a clue what may be going on?
Perhaps my lights just suck and that results in a negative voltage.

It tried it with a few flashlights. I was getting odd looking waveforms. I believe the lights were so bright the signal was clipping and not showing anything above the full pass through voltage.

So I built a lid to hold the flashlight that has a compartment with slide in filters to cut the light in the chamber to something the system can handle. The filters are real high tech. I cut them out of a set of those dark film glasses you get at the eye doc to make the trip home after you get dilated.

Now that I’ve got this nifty toy, it’s time to upgrade my ancient oscilloscope to something that can do a screen capture and give me a frequency (without me having to think too much (which is always a good thing)).
Any of you have any positive experience with USB scopes? Not looking to spend a lot since this will be basically a toy. I’ve heard good things about the PICO scopes being better supported than the no-name brands.

Since I got 5 in the package, I’ll keep an extra (just in case) and as a thank you to you tech savvy BLFers, I’d like to give away the other three.

Let me know and send me a PM with a (US only please) mailing address and I’ll mail the BPW34 and some resistors for you to play with.
All the best,

MRsDNF's picture
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Nice work Jeff.
Love what you have built. Well done and have fun with it. Thumbs Up


djozz quotes, "it came with chinese lettering that is chinese to me".

                      "My man mousehole needs one too"

old4570 said "I'm not an expert , so don't suffer from any such technical restrictions".

Old-Lumens. Highly admired and cherished member of Budget Light Forum. 11.5.2011 - 20.12.16. RIP.


Lightbringer's picture
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Someone here had a long-ass thread showing the effects of like a dozen types of photodiodes, photocells, active, passive, you name it, and the waveshapes, bandwidth, etc.

Hell if I know who did it, though.

Ah, here ya are. Go crazy…

Aaaaand of course, that’s the same thread you listed above. Facepalm

Wellp, ya got a good start. LOL

09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

Terry Oregon
Terry Oregon's picture
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Location: Beaverton Oregon

Yes, I'm the one who posted the "long-ass thread" . Smile

Glad to know someone got some use out of that thread.

jeff51, you wound up doing what I did, mounting the circuitry to something stable - although I took a slightly different rout, using a small plastic hobby board.  And yes, you can never have enough hot glue.



Coincidentally I have a PicoScope. They will do fine for what you're doing - even the cheapest 10mhz bandwidth model should be OK (about $140).  If you want a stand-alone scope, Siglent and Rigol are very popular with the electronics hobby crowd - their lower end models are in the $250-$300 range.


As far as the negative ringing on your scope, that could be caused by several things, which would be difficult to determine here.


And yes, all the sensors I tested will saturate if the light is too close and too bright - which I illustrated in this YouTube video HERE.  All my testing was done in a darkened room, I then moved the flashlight away from the sensor until I got stable waveforms.


My reviews: , My personal collection of lights LINK,  J5 Tactical V1 Pro review LINK,  Thirteen Optical Sensors review LINKZebralight SC700d review LINK,  Ray-O-Vac Super Power Sportsman review LINK,  Convoy S2+ color combos LINK,  How To flash D4V2 LINK.

jeff51's picture
Last seen: 1 day 16 hours ago
Joined: 03/26/2019 - 17:36
Posts: 91
Location: Middle of Texas

Enjoyed your article.
Those little critters taxed my fine soldering skills (or lack thereof).
I’m leaning toward the PICO since it’s easy to get screen captures etc. right onto the computer.
If I was actually going to do something useful I’d be looking at one of the stand alone scopes.
It’s amazing what’s out there compared to 40 years ago.

A reminder to those who may want to play with this stuff,
I’ve got 3 of these diodes to give away…
All the best,

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jeff51's picture
Last seen: 1 day 16 hours ago
Joined: 03/26/2019 - 17:36
Posts: 91
Location: Middle of Texas

LOL, And you know I still put my foot in my mouth more often than not (Metaphorically speaking)…

Hellie112's picture
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Location: The Netherlands

Cool project. It reminded me of a video..
Seeing hearing light.

Lexel's picture
Last seen: 8 hours 15 min ago
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Posts: 5358
Location: Germany

nothing new there any IR light barrier uses modulated light and narrow bandwith filters

Lasers get used to pick up noise from inside buildings spying on the window vibrations

also 100Gbit laser communication systems are used between buildings

ToyKeeper's picture
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Location: (469219) 2016 HO3

It’s interesting that the problem with photo diodes is a matter of light sources being too bright. With LEDs as sensors, I have the opposite problem. It can be difficult to measure moon modes, even after collecting more light with a reflector. The high end is no problem though; I can measure turbo point-blank from an Emisar D4 with no issues.

A scope of some sort would definitely make the response a lot more clear though. If I ever actually get into circuit design, I’ll have to grab one of those PicoScopes. For now though, I think I have all the gear I need. Smile

jeff51's picture
Last seen: 1 day 16 hours ago
Joined: 03/26/2019 - 17:36
Posts: 91
Location: Middle of Texas

I did a post about using Audio software to measure PWM Frequency: