Why Use 30M Ohm Resistors in Optical Slave Trigger Circuits?

No pictures or schematics were attached, so I can't comment on the specifics, but one thing did cross my mind: in ancient times (the 1930s and '40s) resistors were sometimes labeled with "M" meaning 1000, as used in some other industries. This got replaced with "K" generally in the US in the 1950s, but it might've hung on with old timers or perhaps foreign engineers. It's a stretch.

Otherwise, they may have been (attempting to) use them in a current to voltage converter from a low-output light sensor. With only a few nanoamps to work with, you need big resistors to get any workable voltage. That seems like a stretch, too, since it'd be hard to maintain that kind of resistance in most environments.

Just my guesses.

Generally they used high resistance in the trigger circuit to force the voltage to build in the capacitor. Once a certain voltage is reached the trigger circuit closes the main circuit and allows the high voltage to discharge through the lamp. Variations on this theme were abundant. Strobe lights and engine ignition systems being two examples. There were plenty of kit strobes available in the past so a search of schematics would probably give you a few example circuits.

Why a circuit like these would use resistors of such high values? In the orange one you can see two 30M in series...



(the picture is clickable for a bigger view)


The two circuits in the picture are exactly the same, just one is built in the hotshoe adapter while the other has a plug at the back to be attached to a strobe or an hotshoe. I posted both because in the orange one you can see the resistors in series and on the builtin the components are more visible without reflections of the epoxy.

Thanks Mark, if you click the thumbnail it will open a bigger picture there you can see the components: one photodiode something probably like a BPW34, i know those are usually the ones more common, one unknown SCR (cant read the values on that), one capacitor 400V 0.047, one resistor 4.7k Ω (yellow, violet, red) and two resistors in series 30M Ω (orange, black,blue).....
The original design of this optical trigger were kind of old but not THAT old (pre-digital) lol and the resistors are normal 4 band ones and i cant understand what they do .
I will attach the picture directly picture anyways if someone cant see the other one

Thanks Chuck, this isnt a whole strobe, its just an "eye", you attach it to the hotshoe or sync contact of a strobe and it simply shorts the negative rail and the positive pin of the strobe to trigger it to flash when it sees the "master flash" going off (the one on the camera typically), exactly the same thing the camera does when the strobe is mounted on it. While older strobes can discharge over 300V at the hotshoe or sync that happens when the flash discharges, the circuit might have to be able to stand that voltage but to work it uses the way lower voltage always present at the hotshoe, newer strobes have the hotshoe and other sync plugs isolated from the flash and stay always at 6V or lower, thats why you cant use older flash units on digital cameras.
I really cant figure this particular circuit out

Sorry I missed the photodiode in it. I know some units used a "smart" circuit the limit how much flash you actually needed so that's my current guess. A circuit to limit the flash output so you don't end up with "washed out" photos. It could divert some energy to the high resistance instead of letting it pass to the flash. I had a top of the line Nikon flash that had that built in about 2000. If this is true I don't see how they could expect it to work consistently with all brands as you could expect similar trip levels, surely there was some variations between manufacturers

not sure, i thought it would be cut off from the workings of the strobe itself, its essentially a switch to trip the positive to ground. Most strobes need to be put on manual settings when optical triggers are used...

i drew the circuit using R1 and R2 for the two 30M resistors and R3 for the 4.7K resistor but it just doesnt seem right... Even a 10K resistor in place of R1+R2 would make it so sensitive it would false trigger all the time, let alone 30M.... for a moment i thought they could be those inductors that look like resistors but the 3rd band blue then isnt possible...

And btw, I really appreciate someone helping me to figure this out

One possibility: the manufacturer's paint band colours on some resistors are very variable - orange and brown and red can be similar, and violet or blue can be so pale that they look grey. Can you poke a pin through the resin and check the actual values ? (After perhaps a fine drill ?)

Are you certain about the circuit ? Resin is a very pretty way of presenting a circuit (and practical for letting light in), but it does make diagnosis and maintenance difficult.

You could google "flash slave circuit" or something similar, and probably get a variety of circuits with explanations of operation.

Thanks Frank, cant poke in the circuit since I dont have any at hand, the ones I used in the past werent on my equipment and now I dont even know anyone that has it. If I had one I would just take the base off and see how the components are connected and figure something out of that. I really hoped at one point that one color could have been mistaken for something else, maybe even because of color cast by the resin, unfortunately in between the two examples of the pictures the colors are pretty clear. I thought I had it when I found out that there are inductors in tan packages that look exactly like a resistor but the color code (orange black blue gold) doesnt match an inductor and I really looked to be sure the blue band wasnt really a black band

Initially I picked this one in particular as starting point (i will have to add a few things to it) because in between all the ones I had the occasions to try it was the most reliable, I have other 6 schematics for optical slaves that could work, just dont know how reliably, and I wont build all them to try and see which is the best. The general working is a photodiode and an SCR, some use just that but it f false trigger, so to that its added an RC circuit to filter ambient light, other types around the net dont use the RC they just use a resistor between ground and the photodiode to control the sensitivity as a mean to deal with ambient light... all of them arent that good outdoors during the day because they saturate so people tends to shade them or cover them with dark plastic caps to do optically do what its not done by the circuit itself.
But now Im obsessing about this particular one just because I dont understand it, I dont know why it uses those values and so what is going on with the circuit... call it curiosity, being stubborn or pms, I dont know, its a bit like a riddle that drives you nuts until you solve it. And think I do these things for fun....

PS: agree with you for the resin, it makes them basically indestructible but I wont cast mine in it

this is a closeup of the resistors

I don't have any particular experience with this type of circuit, but I would suggest trying some other electronics forums, or searching for photographic forums with an emphasis on DIY electronics.

And as for stubborn ?
Congratulations on your tenacity and deternination to crack the problem !

Idid ask in other electronics forums and had no replies, i even wondered if the idea of "cloning" an existing circuit was tuning people off.... to me thats how i can learn something, im surely not starting a business selling that stuff.
Photography forums: stay on the "its a waste of time, buy them on ebay"... which is of no help and misses totally the point, harder to find someone that could reverse-engineer a circuit looking at a picture of it.
If i paid more attention when i was in school maybe now id get it by myself lol...
You guys are the only ones that tried to help, i appreciate it

If you are in the process (like me) of improving your electronic education, may I offer a few useful sites:

Old (allegedly Chinese) proverb: “A journey of a thousand miles begins with but a single step.”

Many sites have a user forum for asking questions.

http://www.allaboutcircuits.com
series of online textbooks covering electricity and electronics

www.learnabout-electronics.org/index.php
Descriptions of components and principles, good diagrams, animations, & fault-finding techniques

http://www.satcure-focus.com/hobby/index.htm
Useful range of ideas tutorials circuits links etc

http://electronics-for-beginners.com/links.php

http://www.hobby-hour.com/electronics/
Teaching from basic principles, resistor code calculators, projects

http://www.radio-electronics.com
Useful information and tutorials

http://www.electronics-tutorials.ws/index.html
Plus calculators & tools

http://talkingelectronics.com/

http://electronics-diy.com
Many projects - 0.3-1.5V LED Flashlight - LED Torch Using MAX660 - Stepper Motor Controller - Strobe

http://www.kpsec.freeuk.com/index.htm
Electronics Club - 555 and 556 Circuits - Logic Symbols - WMF circuit symbols - Logic IC details - Projects

http://www.circuitstoday.com

http://www.eleinmec.com/index.asp
Electronics in Meccano
- Controlling Motors - PWM &c - sound generator ICs

http://www.ecelab.com/circuit-astable-555.htm
Electronics & Communications Engineering - from basic concepts

Thanks Frank