Let's Measure the Speed of Light (Part 2): The DIY Edition 

Did you see the wavelength of light one id did a couple of months ago using a laser printed diffraction grating? Here it is ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-WwQXQrm33JM.html

Hay can u do a test with 2 6gauge stranded wire connected with an ideal red wire nuts up to 50amp please

@@syitiger9072 I just looked at the UL approved wires combination for the Ideal 76B which I think is the one you are referring to. It seems to be too small to be approved for two of #6 so probably not a good idea to test or use. I would be concerned that not all strands get properly secured. You probably should consider a larger wire nut or other connector type....

What brilliant idea! Just did some googling. Came across this great app note : google ST AN5073 - I cant seem to get a simple link to it. Looks like symbol (pulse) rates of 15MHz rates are possible so much like my 100ns pulse. Down side is max length is 10m (30 feet) so probably a 40ns trip time (due to slower speed in a fiber). They say limitation is due to attenuation from the cheap fiber. May also be due to the thick fiber being multi-mode. That is, multiple zig-zag reflection paths inside the fiber end to end which will smear the pulse as distance increases. Must be within a few 10s of ns at 10m distance for the system to work. Digikey has two potential transmitters and receivers in stock: Toshiba TOTX1350(F) (Transmitter) CLIFF Electronics FCR684205R (Receiver) Less than $20 for the both. Swapping in a short and long cable as you suggest with a similar experimental setup as mine would be a great classroom demo. I also wonder if one could (without the fiber) use some lenses like I did to focus and concentrate the light on each each end and do an across room demo. As I recall from my own AV gear, the Toslink emitters are quite bright. I might try this in the fall sometime. Would be great if someone reading this tried it and told us how it went! Note: your comment came though twice - I will repost this as an answer both in case something weird went on with RU-vid!

Thanks so much! It was a fun thing to do!

Well thank you so much!

Speed and noise was my problem too - found that many lasers and photodiodes don't have their speed listed and and not optimized for the speeds needed. At the top of the comments for this video is a pinned comment with parts I used. Is your photodiode a pin diode? That with a significant reverse voltage (9V or 18V) will drop the capacitance across the diode and give you the speeds needed. I was getting so much noise it was unusable until I soldered the photodiode leads right to the resistor and battery. I originally had 20cm long wires from the photodiode to the resistor and battery. Noise being picked up was unbelievable - all the switching power supplies etc sure make homes a noisy RF environment these days. Are you getting enough laser light on the diode? Without the magnifying glass concentrating the light, I did not get enough signal. You should be abler to measure light on/off voltage across the resistor with a multimeter as a low speed test to confirm you are able to detect the laser light. Hope this helps! Good luck!

And hopefully an amp driven by the diode. I was hoping that by showing the image of the device someone in the US or elsewhere might recognize it as being very similar or the same as a local store brand in their country and do some experimenting. On sale I got that one for $25.

Just looked - very nicely done and nicely mechanical in the true Fizeau condition. Well worth looking at for anyone reading this! I'm probably not going to do it myself maybe inspiration for someone else to duplicate or use as a starting point. Thanks for pointing that out. The link to his video is here in case anyone wants to watch: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-YMO9uUsjXaI.html

I looked at that - couldn't find an easy combination of things that did this as well as pulsed laser. If you find a good way of ding this, I would be interested in hearing how you do it.

Fabulous - what a neat retirement project! Did you see the comment below about using toslink parts? If you can figure out how to ditch the laser that would be great!

A very insightful thought! A lot of modern electronics does have the speed and capability of doing what we need - just not easily accessible as probably is the case in the phone camera. I think a lot of them handle line data in parallel these days to reduce the digitization speeds. Are you also thinking of the individual pixel sensors being Ghz fast? Unfortunately they are typically setup to store up charge during the period not being actually read to increase sensitivity or for shorter (but still long for our purposes) times for higher shutter speeds. Anyone have more insight as to how modern cell phone cameras work?

The problem is the phone camera has a much longer total, exposure time over which it accumulates light/charge, so it won’t be able to discriminate times much shorter than 1/120s. The pixel frequency is just the clock speed of the readout from the shift register at the edge of the array.

Upcoming video in that! Its because the light is coherent meaning that all the waves are synchronized with each other. Think of the waves from a single drop of water in a pond. The glitter you see is the constructive (bright areas) and destructive (dark areas) interference patterns. Look carefully at the glitters and move left or right - they generally dont move in the same way other things do at the distance they appear to be at. White light - or incoherent colored light is like the waves from millions of tiny drop falling in a pond. The waves form each individual drop forms the same sort of interference patters, but the patterns will peak in different places for the waves from each drop. All it all together and you dont see the interference patterns - they all smear together.

I have never tried a LED as a photodiode - I would be interesting to see how effective they are compared to a PIN diode optimized for detection. Have you tried it? How good was it compared to a purpose built one? Great idea about using the distance to an aperture to make the pulse short. I would be a bit afraid to of the mirror flying off at high speed if it came loose. Its funny you mentioned the spinning mirror. I was thinking about thinks like that yesterday and it dawned on me that the spinning scanner mirror from a junked laser printer could a great mechanism for that sort of thing. And its balanced and includes the motor. If anyone plans to try it, be aware that some laser printers (if not most these days) use a linear array of LEDs rather than a laser + mechanical scanner to draw on the photoconductor drum.

@@ElectromagneticVideos It is fast, but not very sensitive.

@@byronwatkins2565 Interesting!

Thanks! What a wonderful thing you are doing! I havnt had a chance to do any more with it - just getting back to videos right now (half way though a halloween video -Jacob's ladders and and that sort of thing). I think the main issue with distance measurers is that they shut down after a few minutes. You might need to open one to attach wires to an external switch that can be used to periodically turn it on. Haven't experimented with any others. So far people who have been doing the experiment essentially have duplicated what I did. What about a mirrored laser scanner variant of the experiment? I guess you challenge is space - you could send the light down a long fiber. I will be trying that with the 500m of fiber I purchased this summer Email me if you get a chance - its hidden in the Electromagnetic Videos About page - maybe we can toss some ideas back and form - I'd be thrilled to help if I can!

@@ElectromagneticVideos thanks so much for your response and offer to collaborate. I will email you (assuming I can find it) 😀

@@dougmodlin2032 You found it!

Just checked - your right - in the same settings section as pulse. Funny - I never noticed it! Thanks!

wow, I just watched a @zachstar video on the smith chart and how+why impedance matching helps for this reason

You know, there are a lot of easily available 50 Ohm LNAs that go from close to DC to 2GHz or 5Ghz or more. I wonder if they would would - they should have the BW for the speed.

Certainly - one needs to eliminate reflections with a 50 Ohm amp setup. Some scopes like my digital one do have built-in 50 Ohm resistors ...

Something like that would certainly be the way to do it if one was going for accuracy. A couple of commenters to the first "measure the seed of light" video I did mentioned it - one had done that in an undergrad physics lab in university. I wonder if there is some way to do it using minimal expensive equipment?

Yes - the analog scope I used was the PM3215 which is the 50 Mhz version. Your 25Mhz should be fine. Sounds like you are close to doing the experiment - if you do, email some photos and the your results!

So you are connecting a signal generator or similar to the scope with a cable with BNC connectors on each end (or BNC on one end and clips on the other). Those types of voltages should be OK with most scopes as long as the input of the scope is high impedance which is true for most old er scopes. Newer scopes sometimes offer the option of the scope input being in either high impedance or 75 or 50 Ohms. If you have the scope set to those low input impedance, keep the voltage in to 3 or 4 volts. Note that normally scopes are used with the inputs set to high impedance.

That should work well for it!

I used the Advanced Photonix PDB-C156. I put a parts list at the top of the comments for this video in case you need other part numbers. Just looked at SFH 206K - it looks very similar to the PDB-C156 so it should work.

They both look like excellent possible candidates for the task: bpw34 : Just looked at the ams OSRAM BPW 34. The Mouser website (Mouse # 720-BPW34) has a datasheet thats seems to show 0.02 μs rise and fall with a 50 Ohm load. The capacitance is high probably due to the large area, but if you use two or three 9V batteries to run it at 18 or 27 volts you can get it down to 10pF or so and you can probably use a 1K resistor like I did for a more an output V easily readable on a scope. The bpw24r has a smaller active area and much lower capacitance. With the lens to focus the incoming light, properly aligned it can have can have a potentially much larger capture area. I would try them both!

See the answer in the comments for that video here ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-L4ML0-OiLu0.htmlsi=oaSZ1J0xHJ8ZPM8-

Is this what you are seeing: the oscilloscope trace does not reach the full extent from left to right on the screen, and seems to move faster right. So that is something wrong with the horizontal deflection system - the oscillator or the amplifier. A sawtooth waveform is used to move the beam left to right, and is often generated by charging a capacitor though a resistor, then emptying the charge and repeating. To keep the waveform looking like a sawtooth, the capacitor is only charged to 10% (approx) of the max applied voltage to the resistor before dumping. It could be that voltage to the resistor is low so you arnt getting full deflection and the top of the sawtooth is getting close to the lower max voltage as becoming rounded. Or something in the amp section also limiting the max voltage to the crt. So in the end, you need to either return it to where you purchased it and get your money back or have someone with electronics experience see if they can fix it. It probably is not worth paying for someones time to fix it unless the cost of that is really low where you are. So I probably would return it. or see if the sell will fix it. Thats really too bad - I know you ave been working on this a long time.

The values on the side of the BNC connector is what that connector/cable/probe is. So that probe has a capacitance of 25pf. Less capacitance is better (faster response time) so 12pf probes would work. The BNC Oscilloscope inputs: You usually can switch between 75 Ohms or infinite input impedance. 75 Ohms should only be used for direct connection to devices with an output impedance of 75 Ohms (ie video out of a VCR or signal generator) with a low voltage output (2 or 3 volts max). Any higher may burn out the 75 Ohm resistor in the scope. For use with probes like the 1Mohm, 25pf ones mentioned above, the oscilloscope should be set to infinite input impedance. The maximum voltage that can be used here depends on the specs of the scope and the probe. If you dont have the specs for both, I would not use more than 50V max for the x1 setting on the probe, and 10 times that for x10 setting. Hope that helps.

The one I used was 20mW. 5mw could work if you too more care with focusing the optics than I did - also many photodiodes do better with red light than green so that would help. Speed is a different story - you need the specs of the diode. The one I used was rated for use at a 100MHz modulation frequency: 1/100MHz = 10ns so its switch on / switch off time is about half that or 5ns. There is really no way of knowing what a devices switching time is without the specs or testing it. The PLT5 522EA_Q laser I used is available from Digikey, Mouser and Newark for about US$15 so it might be easier to order one from them than go though the frustration of trying other ones. You do need a laser not much slower than that speed for the types of distance I was doing the experiment with.

The SFH 206K has a much larger active surface area (7 mm²) so I would suggest you use it. The other other one with only 1mm² would require much (well 7x) the light intensity to achieve the same voltage output.

Dont! Thats way to dangerous.

I would try one with a lower capacitance like the 12pf one, since the capacitance will slow rise or fall times from the diode. All of those speeds are fine - almost anything rated for 100Mhz or more should work. Good luck!

That could be another video :) Very briefly: Diodes can be thought of as two touching chunks of semiconductor. One chunk is doped with impurities to give it an excess of electrons, the other an excess of holes. Where the chunks touch the holes and electrons move interact to form a very thin region - the depletion region - where there are few of either. This sets up an an electrical field across the depletion region and hence a potential between the two chunks of semiconductor. If a photo hits the semiconductor in that region an an electron-hole pair is created and because they have - and plus charges are swept off to opposing sides of the depletion region . That charge movement creates a tiny current though the device. Thats how solar panels work. Add a reverse bias and the extra field from the external voltage widens the depletion region and so lowers the capacitance (the depletion region is like a an insulating layer) and also increases slightly how fast the electron and hole move over to their respective sides.

@@ElectromagneticVideos that’s very helpful and explains why the reverse bias has that effect. Appreciated!

@@5cyndi Great! Its always hard to explain something like that in a few words without diagrams!

What really matters is how how sharply it can be switched on or off. With pulse width control you might be able to make a suitably sharp pulse. Since it is more powerful, be really cautious that the light beam gets nowhere near anyones eye!

You got me on that one - I had look at the video at that point. I did apparently say "splitting image." I wish I was smart enough to have made a pun like that but I not. Just a blooper on my part!

The two diode critical specs issues are speed and photosensitive active area size (to capture as might light as possible). I would be surprised if a sensor diode like that has a large active area. If you use better optics than I did, you should be able to concentrate more light in a smaller area and a diode with a smaller area might work. With speed not being critical in mouse applications, no telling if the diodes you are thinking of are fast enough. They may also have some filtering to keep out wavelengths of light they dont normally use. So I would say maybe experiment with them and if they dont work, get the one I put in the parts list sticky comment. Its only $2 so if you are ordering the laser, might as well include a diode. Good luck!

The laser I used is one of Osram's Direct Green Laser Diodes so it does not have that issue. Your comment is good caution for anyone considering using some other green laser type that might have dangerous IR emissions.

Thats too bad. Sorry but I dont want to sell mine. Also shipping from Canada would be so expensive it wouldn't make it worthwhile anyway. Try looking on ebay or better yet, local online classifieds. Scopes like that often show up because everyone wants fancy new digital ones. One thing about the philips scopes - the selector push buttons do fail after a lot of use and are apparently hard or expensive to replace, so be sure to check them on any scope you are buying.

Most "green lasers" are IR lasers pumping a frequency doubler crystal. However, there are also lasers that natively produce green light.

I'm pretty sure this semiconductor one is directly producing green but I could be wrong. Dont the frequency doubler ones generally operate with more input power?

@@ElectromagneticVideos I’m not certain, but they probably do; I don’t recall where I saw but *certain* types were-I think-more of a concern due to the invisible (yet powerful) IR sneaking out at an angle to the main green beam. I also recall that other varieties of green laser did as you mentioned put out direct green light.

@@PsRohrbaugh thanks. Yep I believe I heard of that kind and then the better more direct emitting green diodes which didn’t have that sneaky IR emission.

@@5cyndi There certainly wasn't a warning as such on the spec sheet I looked at. Burt better safe than sorry!

20MHz you mean? Probbaly - I wouldn't go for a bandwidth lower than that though.

What brilliant idea! Just did some googling. Came across this great app note : google ST AN5073 - I cant seem to get a simple link to it. Looks like symbol (pulse) rates of 15MHz rates are possible so much like my 100ns pulse. Down side is max length is 10m (30 feet) so probably a 40ns trip time (due to slower speed in a fiber). They say limitation is due to attenuation from the cheap fiber. May also be due to the thick fiber being multi-mode. That is, multiple zig-zag reflection paths inside the fiber end to end which will smear the pulse as distance increases. Must be within a few 10s of ns at 10m distance for the system to work. Digikey has two potential transmitters and receivers in stock: Toshiba TOTX1350(F) (Transmitter) CLIFF Electronics FCR684205R (Receiver) Less than $20 for the both. Swapping in a short and long cable as you suggest with a similar experimental setup as mine would be a great classroom demo. I also wonder if one could (without the fiber) use some lenses like I did to focus and concentrate the light on each each end and do an across room demo. As I recall from my own AV gear, the Toslink emitters are quite bright. I might try this in the fall sometime. Would be great if someone reading this tried it and told us how it went! Note: your comment came though twice - I will repost this as an answer both in case something weird went on with RU-vid!

@@ElectromagneticVideos Great! My thought was not so silly, I would like to repeat your experiment to my audience in Italy, I have many young people who follow my channel, I am sure they will appreciate it very much. Thank you for this valuable information. PS. The comment is double because I have 2 RU-vid channels, I was uploading a video on the second channel but I commented as the first channel, something went wrong, you can delete one of the two comments.

@@MVVblog It certainly wasn't silly! Let me know if do repeated it and send me the link! Even if its in Italian, I'm sure I can figure out whats going on from the video. Are you going to try the SPDIF/TOSLINK version? No worries about the double comment!